Insectivorous Plants
by Charles Darwin · English · Gutenberg #5765 · 949,325 chars · public domain (US)
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Title: Insectivorous Plants
Author: Charles Darwin
Release date: May 1, 2004 [eBook #5765]
Most recently updated: December 28, 2021
Language: English
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*** START OF THE PROJECT GUTENBERG EBOOK INSECTIVOROUS PLANTS ***
INSECTIVOROUS PLANTS
By Charles Darwin
CONTENTS
DETAILED TABLE OF CONTENTS.
INSECTIVOROUS PLANTS.
CHAPTER I. DROSERA ROTUNDIFOLIA, OR THE COMMON SUN-DEW.
CHAPTER II. THE MOVEMENTS OF THE TENTACLES FROM THE CONTACT OF SOLID BODIES.
CHAPTER III. AGGREGATION OF THE PROTOPLASM WITHIN THE CELLS OF THE TENTACLES.
CHAPTER IV. THE EFFECTS OF HEAT ON THE LEAVES.
CHAPTER V. THE EFFECTS OF NON-NITROGENOUS AND NITROGENOUS ORGANIC FLUIDS ON THE LEAVES.
CHAPTER VI. THE DIGESTIVE POWER OF THE SECRETION OF DROSERA.
CHAPTER VII. THE EFFECTS OF SALTS OF AMMONIA.
CHAPTER VIII. THE EFFECTS OF VARIOUS OTHER SALTS AND ACIDS ON THE LEAVES.
CHAPTER IX. THE EFFECTS OF CERTAIN ALKALOID POISONS, OTHER SUBSTANCES AND VAPOURS.
CHAPTER X. ON THE SENSITIVENESS OF THE LEAVES, AND ON THE LINES OF TRANSMISSION OF THE MOTOR IMPULSE.
CHAPTER XI. RECAPITULATION OF THE CHIEF OBSERVATIONS ON DROSERA ROTUNDIFOLIA.
CHAPTER XII. ON THE STRUCTURE AND MOVEMENTS OF SOME OTHER SPECIES OF DROSERA.
CHAPTER XIII. DIONAEA MUSCIPULA.
CHAPTER XIV. ALDROVANDA VESICULOSA.
CHAPTER XV. DROSOPHYLLUM—RORIDULA—BYBLIS—GLANDULAR HAIRS OF OTHER PLANTS—CONCLUDING REMARKS ON THE DROSERACEÆ.
CHAPTER XVI. PINGUICULA.
CHAPTER XVII. UTRICULARIA.
CHAPTER XVIII. UTRICULARIA (continued).
CONCLUSION.
INDEX.
DETAILED TABLE OF CONTENTS.
CHAPTER I.
DROSERA ROTUNDIFOLIA, OR THE COMMON SUN-DEW.
Number of insects captured—Description of the leaves and their
appendages or tentacles— Preliminary sketch of the action of the
various parts, and of the manner in which insects are captured—Duration
of the inflection of the tentacles—Nature of the secretion—Manner in
which insects are carried to the centre of the leaf—Evidence that the
glands have the power of absorption—Small size of the roots.
CHAPTER II.
THE MOVEMENTS OF THE TENTACLES FROM THE CONTACT OF SOLID BODIES.
Inflection of the exterior tentacles owing to the glands of the disc
being excited by repeated touches, or by objects left in contact with
them—Difference in the action of bodies yielding and not yielding
soluble nitrogenous matter—Inflection of the exterior tentacles
directly caused by objects left in contact with their glands—Periods of
commencing inflection and of subsequent re-expansion—Extreme minuteness
of the particles causing inflection—Action under water—Inflection of
the exterior tentacles when their glands are excited by repeated
touches—Falling drops of water do not cause inflection.
CHAPTER III.
AGGREGATION OF THE PROTOPLASM WITHIN THE CELLS OF THE TENTACLES.
Nature of the contents of the cells before aggregation—Various causes
which excite aggregation—The process commences within the glands and
travels down the tentacles— Description of the aggregated masses and of
their spontaneous movements—Currents of protoplasm along the walls of
the cells—Action of carbonate of ammonia—The granules in the protoplasm
which flows along the walls coalesce with the central masses—Minuteness
of the quantity of carbonate of ammonia causing aggregation—Action of
other salts of ammonia—Of other substances, organic fluids, &c.—Of
water—Of heat—Redissolution of the aggregated masses—Proximate causes
of the aggregation of the protoplasm—Summary and concluding
remarks—Supplementary observations on aggregation in the roots of
plants.
CHAPTER IV.
THE EFFECTS OF HEAT ON THE LEAVES.
Nature of the experiments—Effects of boiling water—Warm water causes
rapid inflection— Water at a higher temperature does not cause
immediate inflection, but does not kill the leaves, as shown by their
subsequent re-expansion and by the aggregation of the protoplasm— A
still higher temperature kills the leaves and coagulates the albuminous
contents of the glands.
CHAPTER V.
THE EFFECTS OF NON-NITROGENOUS AND NITROGENOUS ORGANIC FLUIDS ON THE
LEAVES.
Non-nitrogenous fluids—Solutions of gum arabic—Sugar—Starch—Diluted
alcohol—Olive oil— Infusion and decoction of tea—Nitrogenous
fluids—Milk—Urine—Liquid albumen—Infusion of raw meat—Impure
mucus—Saliva—Solution of isinglass—Difference in the action of these
two sets of fluids—Decoction of green peas—Decoction and infusion of
cabbage—Decoction of grass leaves.
CHAPTER VI.
THE DIGESTIVE POWER OF THE SECRETION OF DROSERA.
The secretion rendered acid by the direct and indirect excitement of
the glands—Nature of the acid—Digestible substances—Albumen, its
digestion arrested by alkalies, recommences by the addition of an
acid—Meat—Fibrin—Syntonin—Areolar tissue—Cartilage—Fibro-cartilage—
Bone—Enamel and dentine—Phosphate of lime—Fibrous basis of
bone—Gelatine—Chondrin— Milk, casein and
cheese—Gluten—Legumin—Pollen—Globulin—Haematin—Indigestible
substances—Epidermic productions—Fibro-elastic
tissue—Mucin—Pepsin—Urea—Chitine— Cellulose—Gun-cotton—Chlorophyll—Fat
and oil—Starch—Action of the secretion on living seeds—Summary and
concluding remarks.
CHAPTER VII.
THE EFFECTS OF SALTS OF AMMONIA.
Manner of performing the experiments—Action of distilled water in
comparison with the solutions—Carbonate of ammonia, absorbed by the
roots—The vapour absorbed by the glands—Drops on the disc—Minute drops
applied to separate glands—Leaves immersed in weak solutions—Minuteness
of the doses which induce aggregation of the protoplasm—Nitrate of
ammonia, analogous experiments with—Phosphate of ammonia, analogous
experiments with—Other salts of ammonia—Summary and concluding remarks
on the action of salts of ammonia.
CHAPTER VIII.
THE EFFECTS OF VARIOUS OTHER SALTS, AND ACIDS, ON THE LEAVES.
Salts of sodium, potassium, and other alkaline, earthy, and metallic
salts—Summary on the action of these salts—Various acids—Summary on
their action.
CHAPTER IX.
THE EFFECTS OF CERTAIN ALKALOID POISONS, OTHER SUBSTANCES AND VAPOURS.
Strychnine, salts of—Quinine, sulphate of, does not soon arrest the
movement of the protoplasm—Other salts of
quinine—Digitaline—Nicotine—Atropine—Veratrine— Colchicine—
Theine—Curare—Morphia—Hyoscyamus—Poison of the cobra, apparently
accelerates the movements of the protoplasm—Camphor, a powerful
stimulant, its vapour narcotic—Certain essential oils excite
movement—Glycerine—Water and certain solutions retard or prevent the
subsequent action of phosphate of ammonia—Alcohol innocuous, its vapour
narcotic and poisonous—Chloroform, sulphuric and nitric ether, their
stimulant, poisonous, and narcotic power—Carbonic acid narcotic, not
quickly poisonous—Concluding remarks.
CHAPTER X.
ON THE SENSITIVENESS OF THE LEAVES, AND ON THE LINES OF TRANSMISSION OF
THE MOTOR IMPULSE.
Glands and summits of the tentacles alone sensitive—Transmission of the
motor impulse down the pedicels of the tentacles, and across the blade
of the leaf—Aggregation of the protoplasm, a reflex action—First
discharge of the motor impulse sudden—Direction of the movements of the
tentacles—Motor impulse transmitted through the cellular tissue—
Mechanism of the movements—Nature of the motor impulse—Re-expansion of
the tentacles.
CHAPTER XI.
RECAPITULATION OF THE CHIEF OBSERVATIONS ON DROSERA ROTUNDIFOLIA.
CHAPTER XII.
ON THE STRUCTURE AND MOVEMENTS OF SOME OTHER SPECIES OF DROSERA.
Drosera anglica—Drosera intermedia—Drosera capensis—Drosera
spathulata—Drosera filiformis—Drosera binata—Concluding remarks.
CHAPTER XIII.
DIONAEA MUSCIPULA.
Structure of the leaves—Sensitiveness of the filaments—Rapid movement
of the lobes caused by irritation of the filaments—Glands, their power
of secretion—Slow movement caused by the absorption of animal
matter—Evidence of absorption from the aggregated condition of the
glands—Digestive power of the secretion—Action of chloroform, ether,
and hydrocyanic acid—The manner in which insects are captured—Use of
the marginal spikes—Kinds of insects captured—The transmission of the
motor impulse and mechanism of the movements— Re-expansion of the
lobes.
CHAPTER XIV.
ALDROVANDA VESICULOSA.
Captures crustaceans—Structure of the leaves in comparison with those
of Dionaea—Absorption by the glands, by the quadrifid processes, and
points on the infolded margins—Aldrovanda vesiculosa, var.
australis—Captures prey—Absorption of animal matter—Aldrovanda
vesiculosa, var. verticillata—Concluding remarks.
CHAPTER XV.
DROSOPHYLLUM—RORIDULA—BYBLIS—GLANDULAR HAIRS OF OTHER PLANTS—
CONCLUDING REMARKS ON THE DROSERACEÆ.
Drosophyllum—Structure of leaves—Nature of the secretion—Manner of
catching insects— Power of absorption—Digestion of animal
substances—Summary on Drosophyllum—Roridula—Byblis—Glandular hairs of
other plants, their power of absorption—Saxifraga—Primula—
Pelargonium—Erica—Mirabilis—Nicotiana—Summary on glandular
hairs—Concluding remarks on the Droseraceae.
CHAPTER XVI.
PINGUICULA.
Pinguicula vulgaris—Structure of leaves—Number of insects and other
objects caught—Movement of the margins of the leaves—Uses of this
movement—Secretion, digestion, and absorption—Action of the secretion
on various animal and vegetable substances—The effects of substances
not containing soluble nitrogenous matter on the glands—Pinguicula
grandiflora—Pinguicula lusitanica, catches insects—Movement of the
leaves, secretion and digestion.
CHAPTER XVII.
UTRICULARIA.
Utricularia neglecta—Structure of the bladder—The uses of the several
parts—Number of imprisoned animals—Manner of capture—The bladders
cannot digest animal matter, but absorb the products of its
decay—Experiments on the absorption of certain fluids by the quadrifid
processes—Absorption by the glands—Summary of the observation on
absorption— Development of the bladders—Utricularia
vulgaris—Utricularia minor—Utricularia clandestina.
CHAPTER XVIII.
UTRICULARIA (continued).
Utricularia montana—Description of the bladders on the subterranean
rhizomes—Prey captured by the bladders of plants under culture and in a
state of nature—Absorption by the quadrifid processes and glands—Tubers
serving as reservoirs for water—Various other species of
Utricularia—Polypompholyx—Genlisea, different nature of the trap for
capturing prey— Diversified methods by which plants are nourished.
INSECTIVOROUS PLANTS.
CHAPTER I.
DROSERA ROTUNDIFOLIA, OR THE COMMON SUN-DEW.
Number of insects captured—Description of the leaves and their
appendages or tentacles— Preliminary sketch of the action of the
various parts, and of the manner in which insects are captured—Duration
of the inflection of the tentacles—Nature of the secretion—Manner in
which insects are carried to the centre of the leaf—Evidence that the
glands have the power of absorption—Small size of the roots.
During the summer of 1860, I was surprised by finding how large a
number of insects were caught by the leaves of the common sun-dew
(Drosera rotundifolia) on a heath in Sussex. I had heard that insects
were thus caught, but knew nothing further on the subject.* I
* As Dr. Nitschke has given (‘Bot. Zeitung,’ 1860, p. 229) the
bibliography of Drosera, I need not here go into details. Most of the
notices published before 1860 are brief and unimportant. The oldest
paper seems to have been one of the most valuable, namely, by Dr. Roth,
in 1782. There is also an interesting though short account of the
habits of Drosera by Dr. Milde, in the ‘Bot. Zeitung,’ 1852, p. 540. In
1855, in the ‘Annales des Sc. nat. bot.’ tom. iii. pp. 297 and 304, MM.
Groenland and Trcul each published papers, with figures, on the
structure of the leaves; but M. Trcul went so far as to doubt whether
they possessed any power of movement. Dr. Nitschke’s papers in the
‘Bot. Zeitung’ for 1860 and 1861 are by far the most important ones
which have been published, both on the habits and structure of this
plant; and I shall frequently have occasion to quote from them. His
discussions on several points, for instance on the transmission of an
excitement from one part of the leaf to another, are excellent. On
December 11, 1862, Mr. J. Scott read a paper before the Botanical
Society of Edinburgh, [[page 2]] which was published in the ‘Gardeners’
Chronicle,’ 1863, p. 30. Mr. Scott shows that gentle irritation of the
hairs, as well as insects placed on the disc of the leaf, cause the
hairs to bend inwards. Mr. A.W. Bennett also gave another interesting
account of the movements of the leaves before the British Association
for 1873. In this same year Dr. Warming published an essay, in which he
describes the structure of the so-called hairs, entitled, “Sur la
Diffrence entre les Trichomes,” &c., extracted from the proceedings of
the Soc. d’Hist. Nat. de Copenhague. I shall also have occasion
hereafter to refer to a paper by Mrs. Treat, of New Jersey, on some
American species of Drosera. Dr. Burdon Sanderson delivered a lecture
on Dionaea, before the Royal Institution published in ‘Nature,’ June
14, 1874, in which a short account of my observations on the power of
true digestion possessed by Drosera and Dionaea first appeared. Prof.
Asa Gray has done good service by calling attention to Drosera, and to
other plants having similar habits, in ‘The Nation’ (1874, pp. 261 and
232), and in other publications. Dr. Hooker, also, in his important
address on Carnivorous Plants (Brit. Assoc., Belfast, 1874), has given
a history of the subject. [page 2]
gathered by chance a dozen plants, bearing fifty-six fully expanded
leaves, and on thirty-one of these dead insects or remnants of them
adhered; and, no doubt, many more would have been caught afterwards by
these same leaves, and still more by those as yet not expanded. On one
plant all six leaves had caught their prey; and on several plants very
many leaves had caught more than a single insect. On one large leaf I
found the remains of thirteen distinct insects. Flies (Diptera) are
captured much oftener than other insects. The largest kind which I have
seen caught was a small butterfly (Caenonympha pamphilus); but the Rev.
H.M. Wilkinson informs me that he found a large living dragon-fly with
its body firmly held by two leaves. As this plant is extremely common
in some districts, the number of insects thus annually slaughtered must
be prodigious. Many plants cause the death of insects, for instance the
sticky buds of the horse-chestnut (Aesculus hippocastanum), without
thereby receiving, as far as we can perceive, any advantage; but it was
soon evident that Drosera was [page 3] excellently adapted for the
special purpose of catching insects, so that the subject seemed well
worthy of investigation.
The results have proved highly remarkable; the more important ones
being—firstly, the extraordinary
FIG. 1.* (Drosera rotundifolia.) Leaf viewed from above; enlarged four
times.
sensitiveness of the glands to slight pressure and to minute doses of
certain nitrogenous fluids, as shown by the movements of the so-called
hairs or tentacles;
* The drawings of Drosera and Dionaea, given in this work, were made
for me by my son George Darwin; those of Aldrovanda, and of the several
species of Utricularia, by my son Francis. They have been excellently
reproduced on wood by Mr. Cooper, 188 Strand. [page 4]
secondly, the power possessed by the leaves of rendering soluble or
digesting nitrogenous substances, and of afterwards absorbing them;
thirdly, the changes which take place within the cells of the
tentacles, when the glands are excited in various ways.
It is necessary, in the first place, to describe briefly the plant. It
bears from two or three to five or six leaves, generally extended more
or less horizontally, but sometimes standing vertically upwards. The
shape and general appearance of a leaf is shown, as seen from above, in
fig. 1, and as seen laterally, in fig. 2. The leaves are commonly a
little broader than long,
FIG. 2. (Drosera rotundifolia.) Old leaf viewed laterally; enlarged
about five times.
but this was not the case in the one here figured. The whole upper
surface is covered with gland-bearing filaments, or tentacles, as I
shall call them, from their manner of acting. The glands were counted
on thirty-one leaves, but many of these were of unusually large size,
and the average number was 192; the greatest number being 260, and the
least 130. The glands are each surrounded by large drops of extremely
viscid secretion, which, glittering in the sun, have given rise to the
plant’s poetical name of the sun-dew.
[The tentacles on the central part of the leaf or disc are short and
stand upright, and their pedicels are green. Towards the margin they
become longer and longer and more inclined [page 5] outwards, with
their pedicels of a purple colour. Those on the extreme margin project
in the same plane with the leaf, or more commonly (see fig. 2) are
considerably reflexed. A few tentacles spring from the base of the
footstalk or petiole, and these are the longest of all, being sometimes
nearly 1/4 of an inch in length. On a leaf bearing altogether 252
tentacles, the short ones on the disc, having green pedicels, were in
number to the longer submarginal and marginal tentacles, having purple
pedicels, as nine to sixteen.
A tentacle consists of a thin, straight, hair-like pedicel, carrying a
gland on the summit. The pedicel is somewhat flattened, and is formed
of several rows of elongated cells, filled with purple fluid or
granular matter.* There is, however, a narrow zone close beneath the
glands of the longer tentacles, and a broader zone near their bases, of
a green tint. Spiral vessels, accompanied by simple vascular tissue,
branch off from the vascular bundles in the blade of the leaf, and run
up all the tentacles into the glands.
Several eminent physiologists have discussed the homological nature of
these appendages or tentacles, that is, whether they ought to be
considered as hairs (trichomes) or prolongations of the leaf. Nitschke
has shown that they include all the elements proper to the blade of a
leaf; and the fact of their including vascular tissue was formerly
thought to prove that they were prolongations of the leaf, but it is
now known that vessels sometimes enter true hairs.** The power of
movement which they possess is a strong argument against their being
viewed as hairs. The conclusion which seems to me the most probable
will be given in Chap. XV., namely that they existed primordially as
glandular hairs, or mere epidermic formations, and that their upper
part should still be so considered; but that their lower
* According to Nitschke (‘Bot. Zeitung,’ 1861, p. 224) the purple fluid
results from the metamorphosis of chlorophyll. Mr. Sorby examined the
colouring matter with the spectroscope, and informs me that it consists
of the commonest species of erythrophyll, “which is often met with in
leaves with low vitality, and in parts, like the petioles, which carry
on leaf-functions in a very imperfect manner. All that can be said,
therefore, is that the hairs (or tentacles) are coloured like parts of
a leaf which do not fulfil their proper office.”
** Dr. Nitschke has discussed this subject in ‘Bot. Zeitung,’ 1861, p.
241 &c. See also Dr. Warming (‘Sur la Diffrence entre les Trichomes’
&c., 1873), who gives references to various publications. See also
Groenland and Trcul ‘Annal. des Sc. nat. bot.’ (4th series), tom. iii.
1855, pp. 297 and 303. [page 6]
part, which alone is capable of movement, consists of a prolongation of
the leaf; the spiral vessels being extended from this to the uppermost
part. We shall hereafter see that the terminal tentacles of the divided
leaves of Roridula are still in an intermediate condition.
The glands, with the exception of those borne by the extreme
FIG. 3. (Drosera rotundifolia.) Longitudinal section of a gland;
greatly magnified. From Dr. Warming.
marginal tentacles, are oval, and of nearly uniform size, viz. about
4/500 of an inch in length. Their structure is remarkable, and their
functions complex, for they secrete, absorb, and are acted on by
various stimulants. They consist of an outer layer of small polygonal
cells, containing purple granular matter or fluid, and with the walls
thicker than those of the pedicels. [page 7] Within this layer of cells
there is an inner one of differently shaped ones, likewise filled with
purple fluid, but of a slightly different tint, and differently
affected by chloride of gold. These two layers are sometimes well seen
when a gland has been crushed or boiled in caustic potash. According to
Dr. Warming, there is still another layer of much more elongated cells,
as shown in the accompanying section (fig. 3) copied from his work; but
these cells were not seen by Nitschke, nor by me. In the centre there
is a group of elongated, cylindrical cells of unequal lengths, bluntly
pointed at their upper ends, truncated or rounded at their lower ends,
closely pressed together, and remarkable from being surrounded by a
spiral line, which can be separated as a distinct fibre.
These latter cells are filled with limpid fluid, which after long
immersion in alcohol deposits much brown matter. I presume that they
are actually connected with the spiral vessels which run up the
tentacles, for on several occasions the latter were seen to divide into
two or three excessively thin branches, which could be traced close up
to the spiriferous cells. Their development has been described by Dr.
Warming. Cells of the same kind have been observed in other plants, as
I hear from Dr. Hooker, and were seen by me in the margins of the
leaves of Pinguicula. Whatever their function may be, they are not
necessary for the secretion of a digestive fluid, or for absorption, or
for the communication of a motor impulse to other parts of the leaf, as
we may infer from the structure of the glands in some other genera of
the Droseraceae.
The extreme marginal tentacles differ slightly from the others. Their
bases are broader, and besides their own vessels, they receive a fine
branch from those which enter the tentacles on each side. Their glands
are much elongated, and lie embedded on the upper surface of the
pedicel, instead of standing at the apex. In other respects they do not
differ essentially from the oval ones, and in one specimen I found
every possible transition between the two states. In another specimen
there were no long-headed glands. These marginal tentacles lose their
irritability earlier than the others; and when a stimulus is applied to
the centre of the leaf, they are excited into action after the others.
When cut-off leaves are immersed in water, they alone often become
inflected.
The purple fluid or granular matter which fills the cells of the glands
differs to a certain extent from that within the cells of the pedicels.
For when a leaf is placed in hot water or in certain acids, the glands
become quite white and opaque, whereas [page 8] the cells of the
pedicels are rendered of a bright red, with the exception of those
close beneath the glands. These latter cells lose their pale red tint;
and the green matter which they, as well as the basal cells, contain,
becomes of a brighter green. The petioles bear many multicellular
hairs, some of which near the blade are surmounted, according to
Nitschke, by a few rounded cells, which appear to be rudimentary
glands. Both surfaces of the leaf, the pedicels of the tentacles,
especially the lower sides of the outer ones, and the petioles, are
studded with minute papillae (hairs or trichomes), having a conical
basis, and bearing on their summits two, and occasionally three or even
four, rounded cells, containing much protoplasm. These papillae are
generally colourless, but sometimes include a little purple fluid. They
vary in development, and graduate, as Nitschke* states, and as I
repeatedly observed, into the long multicellular hairs. The latter, as
well as the papillae, are probably rudiments of formerly existing
tentacles.
I may here add, in order not to recur to the papillae, that they do not
secrete, but are easily permeated by various fluids: thus when living
or dead leaves are immersed in a solution of one part of chloride of
gold, or of nitrate of silver, to 437 of water, they are quickly
blackened, and the discoloration soon spreads to the surrounding
tissue. The long multicellular hairs are not so quickly affected. After
a leaf had been left in a weak infusion of raw meat for 10 hours, the
cells of the papillae had evidently absorbed animal matter, for instead
of limpid fluid they now contained small aggregated masses of
protoplasm, which slowly and incessantly changed their forms. A similar
result followed from an immersion of only 15 minutes in a solution of
one part of carbonate of ammonia to 218 of water, and the adjoining
cells of the tentacles, on which the papillae were seated, now likewise
contained aggregated masses of protoplasm. We may therefore conclude
that when a leaf has closely clasped a captured insect in the manner
immediately to be described, the papillae, which project from the upper
surface of the leaf and of the tentacles, probably absorb some of the
animal matter dissolved in the secretion; but this cannot be the case
with the papillae on the backs of the leaves or on the petioles.]
* Nitschke has elaborately described and figured these papillae, ‘Bot.
Zeitung,’ 1861, pp. 234, 253, 254. [page 9]
_Preliminary Sketch of the Action of the several Parts, and of the
Manner in which Insects are Captured._
If a small organic or inorganic object be placed on the glands in the
centre of a leaf, these transmit a motor impulse to the marginal
tentacles. The nearer ones are first affected and slowly bend towards
the centre, and then those farther off, until at last all become
closely inflected over the object. This takes place in from one hour to
four or five or more hours. The difference in the time required depends
on many circumstances; namely on the size of the object and on its
nature, that is, whether it contains soluble matter of the proper kind;
on the vigour and age of the leaf; whether it has lately been in
action; and, according to Nitschke,* on the temperature of the day, as
likewise seemed to me to be the case. A living insect is a more
efficient object than a dead one, as in struggling it presses against
the glands of many tentacles. An insect, such as a fly, with thin
integuments, through which animal matter in solution can readily pass
into the surrounding dense secretion, is more efficient in causing
prolonged inflection than an insect with a thick coat, such as a
beetle. The inflection of the tentacles takes place indifferently in
the light and darkness; and the plant is not subject to any nocturnal
movement of so-called sleep.
If the glands on the disc are repeatedly touched or brushed, although
no object is left on them, the marginal tentacles curve inwards. So
again, if drops of various fluids, for instance of saliva or of a
solution of any salt of ammonia, are placed on the central glands, the
same result quickly follows, sometimes in under half an hour.
* ‘Bot. Zeitung,’ 1860, p. 246. [page 10]
The tentacles in the act of inflection sweep through a wide space; thus
a marginal tentacle, extended in the same plane with the blade, moves
through an angle of 180o; and I have seen the much reflected tentacles
of a leaf which stood upright move through an angle of not less than
270o. The bending part is almost confined to a short space near the
base; but a rather larger portion of the elongated exterior tentacles
FIG. 4. (Drosera rotundifolia.) Leaf (enlarged) with all the tentacles
closely inflected, from immersion in a solution of phosphate of ammonia
(one part to 87,500 of water.)
FIG. 5. (Drosera rotundifolia.) Leaf (enlarged) with the tentacles on
one side inflected over a bit of meat placed on the disc.
becomes slightly incurved; the distal half in all cases remaining
straight. The short tentacles in the centre of the disc when directly
excited, do not become inflected; but they are capable of inflection if
excited by a motor impulse received from other glands at a distance.
Thus, if a leaf is immersed in an infusion of raw meat, or in a weak
solution of ammonia (if the [page 11] solution is at all strong, the
leaf is paralysed), all the exterior tentacles bend inwards (see fig.
4), excepting those near the centre, which remain upright; but these
bend towards any exciting object placed on one side of the disc, as
shown in fig. 5. The glands in fig. 4 may be seen to form a dark ring
round the centre; and this follows from the exterior tentacles
increasing in length in due proportion, as they stand nearer to the
circumference.
The kind of inflection which the tentacles undergo is best shown when
the gland of one of the long exterior
FIG. 6. (Drosera rotundifolia.) Diagram showing one of the exterior
tentacles closely inflected; the two adjoining ones in their ordinary
position.)
tentacles is in any way excited; for the surrounding ones remain
unaffected. In the accompanying outline (fig. 6) we see one tentacle,
on which a particle of meat had been placed, thus bent towards the
centre of the leaf, with two others retaining their original position.
A gland may be excited by being simply touched three or four times, or
by prolonged contact with organic or inorganic objects, and various
fluids. I have distinctly seen, through a lens, a tentacle beginning to
bend in ten seconds, after an object had been [page 12] placed on its
gland; and I have often seen strongly pronounced inflection in under
one minute. It is surprising how minute a particle of any substance,
such as a bit of thread or hair or splinter of glass, if placed in
actual contact with the surface of a gland, suffices to cause the
tentacle to bend. If the object, which has been carried by this
movement to the centre, be not very small, or if it contains soluble
nitrogenous matter, it acts on the central glands; and these transmit a
motor impulse to the exterior tentacles, causing them to bend inwards.
Not only the tentacles, but the blade of the leaf often, but by no
means always, becomes much incurved, when any strongly exciting
substance or fluid is placed on the disc. Drops of milk and of a
solution of nitrate of ammonia or soda are particularly apt to produce
this effect. The blade is thus converted into a little cup. The manner
in which it bends varies greatly. Sometimes the apex alone, sometimes
one side, and sometimes both sides, become incurved. For instance, I
placed bits of hard-boiled egg on three leaves; one had the apex bent
towards the base; the second had both distal margins much incurved, so
that it became almost triangular in outline, and this perhaps is the
commonest case; whilst the third blade was not at all affected, though
the tentacles were as closely inflected as in the two previous cases.
The whole blade also generally rises or bends upwards, and thus forms a
smaller angle with the footstalk than it did before. This appears at
first sight a distinct kind of movement, but it results from the
incurvation of that part of the margin which is attached to the
footstalk, causing the blade, as a whole, to curve or move upwards.
The length of time during which the tentacles as [page 13] well as the
blade remain inflected over an object placed on the disc, depends on
various circumstances; namely on the vigour and age of the leaf, and,
according to Dr. Nitschke, on the temperature, for during cold weather
when the leaves are inactive, they re-expand at an earlier period than
when the weather is warm. But the nature of the object is by far the
most important circumstance; I have repeatedly found that the tentacles
remain clasped for a much longer average time over objects which yield
soluble nitrogenous matter than over those, whether organic or
inorganic, which yield no such matter. After a period varying from one
to seven days, the tentacles and blade re-expand, and are then ready to
act again. I have seen the same leaf inflected three successive times
over insects placed on the disc; and it would probably have acted a
greater number of times.
The secretion from the glands is extremely viscid, so that it can be
drawn out into long threads. It appears colourless, but stains little
balls of paper pale pink. An object of any kind placed on a gland
always causes it, as I believe, to secrete more freely; but the mere
presence of the object renders this difficult to ascertain. In some
cases, however, the effect was strongly marked, as when particles of
sugar were added; but the result in this case is probably due merely to
exosmose. Particles of carbonate and phosphate of ammonia and of some
other salts, for instance sulphate of zinc, likewise increase the
secretion. Immersion in a solution of one part of chloride of gold, or
of some other salts, to 437 of water, excites the glands to largely
increased secretion; on the other hand, tartrate of antimony produces
no such effect. Immersion in many acids (of the strength of one part to
437 of water) likewise causes a wonderful amount of [page 14]
secretion, so that when the leaves are lifted out, long ropes of
extremely viscid fluid hang from them. Some acids, on the other hand,
do not act in this manner. Increased secretion is not necessarily
dependent on the inflection of the tentacle, for particles of sugar and
of sulphate of zinc cause no movement.
It is a much more remarkable fact that when an object, such as a bit of
meat or an insect, is placed on the disc of a leaf, as soon as the
surrounding tentacles become considerably inflected, their glands pour
forth an increased amount of secretion. I ascertained this by selecting
leaves with equal-sized drops on the two sides, and by placing bits of
meat on one side of the disc; and as soon as the tentacles on this side
became much inflected, but before the glands touched the meat, the
drops of secretion became larger. This was repeatedly observed, but a
record was kept of only thirteen cases, in nine of which increased
secretion was plainly observed; the four failures being due either to
the leaves being rather torpid, or to the bits of meat being too small
to cause much inflection. We must therefore conclude that the central
glands, when strongly excited, transmit some influence to the glands of
the circumferential tentacles, causing them to secrete more copiously.
It is a still more important fact (as we shall see more fully when we
treat of the digestive power of the secretion) that when the tentacles
become inflected, owing to the central glands having been stimulated
mechanically, or by contact with animal matter, the secretion not only
increases in quantity, but changes its nature and becomes acid; and
this occurs before the glands have touched the object on the centre of
the leaf. This acid is of a different nature from that contained in the
tissue of the leaves. As long as the [page 15] tentacles remain closely
inflected, the glands continue to secrete, and the secretion is acid;
so that, if neutralised by carbonate of soda, it again becomes acid
after a few hours. I have observed the same leaf with the tentacles
closely inflected over rather indigestible substances, such as
chemically prepared casein, pouring forth acid secretion for eight
successive days, and over bits of bone for ten successive days.
The secretion seems to possess, like the gastric juice of the higher
animals, some antiseptic power. During very warm weather I placed close
together two equal-sized bits of raw meat, one on a leaf of the
Drosera, and the other surrounded by wet moss. They were thus left for
48 hrs., and then examined. The bit on the moss swarmed with infusoria,
and was so much decayed that the transverse striae on the muscular
fibres could no longer be clearly distinguished; whilst the bit on the
leaf, which was bathed by the secretion, was free from infusoria, and
its striae were perfectly distinct in the central and undissolved
portion. In like manner small cubes of albumen and cheese placed on wet
moss became threaded with filaments of mould, and had their surfaces
slightly discoloured and disintegrated; whilst those on the leaves of
Drosera remained clean, the albumen being changed into transparent
fluid.
As soon as tentacles, which have remained closely inflected during
several days over an object, begin to re-expand, their glands secrete
less freely, or cease to secrete, and are left dry. In this state they
are covered with a film of whitish, semi-fibrous matter, which was held
in solution by the secretion. The drying of the glands during the act
of re-expansion is of some little service to the plant; for I have
often observed that objects adhering to the leaves [page 16] could then
be blown away by a breath of air; the leaves being thus left
unencumbered and free for future action. Nevertheless, it often happens
that all the glands do not become completely dry; and in this case
delicate objects, such as fragile insects, are sometimes torn by the
re-expansion of the tentacles into fragments, which remain scattered
all over the leaf. After the re-expansion is complete, the glands
quickly begin to re-secrete, and as soon as full-sized drops are
formed, the tentacles are ready to clasp a new object.
When an insect alights on the central disc, it is instantly entangled
by the viscid secretion, and the surrounding tentacles after a time
begin to bend, and ultimately clasp it on all sides. Insects are
generally killed, according to Dr. Nitschke, in about a quarter of an
hour, owing to their tracheae being closed by the secretion. If an
insect adheres to only a few of the glands of the exterior tentacles,
these soon become inflected and carry their prey to the tentacles next
succeeding them inwards; these then bend inwards, and so onwards; until
the insect is ultimately carried by a curious sort of rolling movement
to the centre of the leaf. Then, after an interval, the tentacles on
all sides become inflected and bathe their prey with their secretion,
in the same manner as if the insect had first alighted on the central
disc. It is surprising how minute an insect suffices to cause this
action: for instance, I have seen one of the smallest species of gnats
(Culex), which had just settled with its excessively delicate feet on
the glands of the outermost tentacles, and these were already beginning
to curve inwards, though not a single gland had as yet touched the body
of the insect. Had I not interfered, this minute gnat would [page 17]
assuredly have been carried to the centre of the leaf and been securely
clasped on all sides. We shall hereafter see what excessively small
doses of certain organic fluids and saline solutions cause strongly
marked inflection.
Whether insects alight on the leaves by mere chance, as a resting
place, or are attracted by the odour of the secretion, I know not. I
suspect from the number of insects caught by the English species of
Drosera, and from what I have observed with some exotic species kept in
my greenhouse, that the odour is attractive. In this latter case the
leaves may be compared with a baited trap; in the former case with a
trap laid in a run frequented by game, but without any bait.
That the glands possess the power of absorption, is shown by their
almost instantaneously becoming dark-coloured when given a minute
quantity of carbonate of ammonia; the change of colour being chiefly or
exclusively due to the rapid aggregation of their contents. When
certain other fluids are added, they become pale-coloured. Their power
of absorption is, however, best shown by the widely different results
which follow, from placing drops of various nitrogenous and
non-nitrogenous fluids of the same density on the glands of the disc,
or on a single marginal gland; and likewise by the very different
lengths of time during which the tentacles remain inflected over
objects, which yield or do not yield soluble nitrogenous matter. This
same conclusion might indeed have been inferred from the structure and
movements of the leaves, which are so admirably adapted for capturing
insects.
The absorption of animal matter from captured insects explains how
Drosera can flourish in extremely poor peaty soil,—in some cases where
nothing but [page 18] sphagnum moss grows, and mosses depend altogether
on the atmosphere for their nourishment. Although the leaves at a hasty
glance do not appear green, owing to the purple colour of the
tentacles, yet the upper and lower surfaces of the blade, the pedicels
of the central tentacles, and the petioles contain chlorophyll, so
that, no doubt, the plant obtains and assimilates carbonic acid from
the air. Nevertheless, considering the nature of the soil where it
grows, the supply of nitrogen would be extremely limited, or quite
deficient, unless the plant had the power of obtaining this important
element from captured insects. We can thus understand how it is that
the roots are so poorly developed. These usually consist of only two or
three slightly divided branches, from half to one inch in length,
furnished with absorbent hairs. It appears, therefore, that the roots
serve only to imbibe water; though, no doubt, they would absorb
nutritious matter if present in the soil; for as we shall hereafter
see, they absorb a weak solution of carbonate of ammonia. A plant of
Drosera, with the edges of its leaves curled inwards, so as to form a
temporary stomach, with the glands of the closely inflected tentacles
pouring forth their acid secretion, which dissolves animal matter,
afterwards to be absorbed, may be said to feed like an animal. But,
differently from an animal, it drinks by means of its roots; and it
must drink largely, so as to retain many drops of viscid fluid round
the glands, sometimes as many as 260, exposed during the whole day to a
glaring sun. [page 19]
CHAPTER II.
THE MOVEMENTS OF THE TENTACLES FROM THE CONTACT OF SOLID BODIES.
Inflection of the exterior tentacles owing to the glands of the disc
being excited by repeated touches, or by objects left in contact with
them—Difference in the action of bodies yielding and not yielding
soluble nitrogenous matter—Inflection of the exterior tentacles
directly caused by objects left in contact with their glands—Periods of
commencing inflection and of subsequent re-expansion—Extreme minuteness
of the particles causing inflection—Action under water—Inflection of
the exterior tentacles when their glands are excited by repeated
touches—Falling drops of water do not cause inflection.
I will give in this and the following chapters some of the many
experiments made, which best illustrate the manner and rate of movement
of the tentacles, when excited in various ways. The glands alone in all
ordinary cases are susceptible to excitement. When excited, they do not
themselves move or change form, but transmit a motor impulse to the
bending part of their own and adjoining tentacles, and are thus carried
towards the centre of the leaf. Strictly speaking, the glands ought to
be called irritable, as the term sensitive generally implies
consciousness; but no one supposes that the Sensitive-plant is
conscious, and as I have found the term convenient, I shall use it
without scruple. I will commence with the movements of the exterior
tentacles, when indirectly excited by stimulants applied to the glands
of the short tentacles on the disc. The exterior tentacles may be said
in this case to be indirectly excited, because their own glands are not
directly acted on. The stimulus proceeding from the glands of the disc
acts on the bending part of the [page 20] exterior tentacles, near
their bases, and does not (as will hereafter be proved) first travel up
the pedicels to the glands, to be then reflected back to the bending
place. Nevertheless, some influence does travel up to the glands,
causing them to secrete more copiously, and the secretion to become
acid. This latter fact is, I believe, quite new in the physiology of
plants; it has indeed only recently been established that in the animal
kingdom an influence can be transmitted along the nerves to glands,
modifying their power of secretion, independently of the state of the
blood-vessels.
The Inflection of the Exterior Tentacles from the Glands of the Disc
being excited by Repeated Touches, or by Objects left in Contact with
them.
The central glands of a leaf were irritated with a small stiff
camel-hair brush, and in 70 m. (minutes) several of the outer tentacles
were inflected; in 5 hrs. (hours) all the sub-marginal tentacles were
inflected; next morning after an interval of about 22 hrs. they were
fully re-expanded. In all the following cases the period is reckoned
from the time of first irritation. Another leaf treated in the same
manner had a few tentacles inflected in 20 m.; in 4 hrs. all the
submarginal and some of the extreme marginal tentacles, as well as the
edge of the leaf itself, were inflected; in 17 hrs. they had recovered
their proper, expanded position. I then put a dead fly in the centre of
the last-mentioned leaf, and next morning it was closely clasped; five
days afterwards the leaf re-expanded, and the tentacles, with their
glands surrounded by secretion, were ready to act again.
Particles of meat, dead flies, bits of paper, wood, dried moss, sponge,
cinders, glass, &c., were repeatedly [page 21] placed on leaves, and
these objects were well embraced in various periods from one hr. to as
long as 24 hrs., and set free again, with the leaf fully re-expanded,
in from one or two, to seven or even ten days, according to the nature
of the object. On a leaf which had naturally caught two flies, and
therefore had already closed and reopened either once or more probably
twice, I put a fresh fly: in 7 hrs. it was moderately, and in 21 hrs.
thoroughly well, clasped, with the edges of the leaf inflected. In two
days and a half the leaf had nearly re-expanded; as the exciting object
was an insect, this unusually short period of inflection was, no doubt,
due to the leaf having recently been in action. Allowing this same leaf
to rest for only a single day, I put on another fly, and it again
closed, but now very slowly; nevertheless, in less than two days it
succeeded in thoroughly clasping the fly.
When a small object is placed on the glands of the disc, on one side of
a leaf, as near as possible to its circumference, the tentacles on this
side are first affected, those on the opposite side much later, or, as
often occurred, not at all. This was repeatedly proved by trials with
bits of meat; but I will here give only the case of a minute fly,
naturally caught and still alive, which I found adhering by its
delicate feet to the glands on the extreme left side of the central
disc. The marginal tentacles on this side closed inwards and killed the
fly, and after a time the edge of the leaf on this side also became
inflected, and thus remained for several days, whilst neither the
tentacles nor the edge on the opposite side were in the least affected.
If young and active leaves are selected, inorganic particles not larger
than the head of a small pin, placed on the central glands, sometimes
cause the [page 22] outer tentacles to bend inwards. But this follows
much more surely and quickly, if the object contains nitrogenous matter
which can be dissolved by the secretion. On one occasion I observed the
following unusual circumstance. Small bits of raw meat (which acts more
energetically than any other substance), of paper, dried moss, and of
the quill of a pen were placed on several leaves, and they were all
embraced equally well in about 2 hrs. On other occasions the
above-named substances, or more commonly particles of glass,
coal-cinder (taken from the fire), stone, gold-leaf, dried grass, cork,
blotting-paper, cotton-wool, and hair rolled up into little balls, were
used, and these substances, though they were sometimes well embraced,
often caused no movement whatever in the outer tentacles, or an
extremely slight and slow movement. Yet these same leaves were proved
to be in an active condition, as they were excited to move by
substances yielding soluble nitrogenous matter, such as bits of raw or
roast meat, the yolk or white of boiled eggs, fragments of insects of
all orders, spiders, &c. I will give only two instances. Minute flies
were placed on the discs of several leaves, and on others balls of
paper, bits of moss and quill of about the same size as the flies, and
the latter were well embraced in a few hours; whereas after 25 hrs.
only a very few tentacles were inflected over the other objects. The
bits of paper, moss, and quill were then removed from these leaves, and
bits of raw meat placed on them; and now all the tentacles were soon
energetically inflected.
Again, particles of coal-cinder (weighing rather more than the flies
used in the last experiment) were placed on the centres of three
leaves: after an interval of 19 hrs. one of the particles was tolerably
well embraced; [page 23] a second by a very few tentacles; and a third
by none. I then removed the particles from the two latter leaves, and
put on them recently killed flies. These were fairly well embraced in 7
1/2 hrs. and thoroughly after 20 1/2 hrs.; the tentacles remaining
inflected for many subsequent days. On the other hand, the one leaf
which had in the course of 19 hrs. embraced the bit of cinder
moderately well, and to which no fly was given, after an additional 33
hrs. (i.e. in 52 hrs. from the time when the cinder was put on) was
completely re-expanded and ready to act again.
From these and numerous other experiments not worth giving, it is
certain that inorganic substances, or such organic substances as are
not attacked by the secretion, act much less quickly and efficiently
than organic substances yielding soluble matter which is absorbed.
Moreover, I have met with very few exceptions to the rule, and these
exceptions apparently depended on the leaf having been too recently in
action, that the tentacles remain clasped for a much longer time over
organic bodies of the nature just specified than over those which are
not acted on by the secretion, or over inorganic objects.*
* Owing to the extraordinary belief held by M. Ziegler (‘Comptes
rendus,’ May 1872, p. 122), that albuminous substances, if held for a
moment between the fingers, acquire the property of making the
tentacles of Drosera contract, whereas, if not thus held, they have no
such power, I tried some experiments with great care, but the results
did not confirm this belief. Red-hot cinders were taken out of the
fire, and bits of glass, cotton-thread, blotting paper and thin slices
of cork were immersed in boiling water; and particles were then placed
(every instrument with which they were touched having been previously
immersed in boiling water) on the glands of several leaves, and they
acted in exactly the same manner as other particles, which had been
purposely handled for some time. Bits of a boiled egg, cut with a knife
which had been washed in boiling water, also acted like any other
animal substance. I breathed on some leaves for above a minute, and
repeated the act two or three times, with my mouth close to [[page 24]]
them, but this produced no effect. I may here add, as showing that the
leaves are not acted on by the odour of nitrogenous substances, that
pieces of raw meat stuck on needles were fixed as close as possible,
without actual contact, to several leaves, but produced no effect
whatever. On the other hand, as we shall hereafter see, the vapours of
certain volatile substances and fluids, such as of carbonate of
ammonia, chloroform, certain essential oils, &c., cause inflection. M.
Ziegler makes still more extraordinary statements with respect to the
power of animal substances, which have been left close to, but not in
contact with, sulphate of quinine. The action of salts of quinine will
be described in a future chapter. Since the appearance of the paper
above referred to, M. Ziegler has published a book on the same subject,
entitled ‘Atonicit et Zoicit,’ 1874.) [page 24]
The Inflection of the Exterior Tentacles as directly caused by Objects
left in Contact with their Glands.
I made a vast number of trials by placing, by means of a fine needle
moistened with distilled water, and with the aid of a lens, particles
of various substances on the viscid secretion surrounding the glands of
the outer tentacles. I experimented on both the oval and long-headed
glands. When a particle is thus placed on a single gland, the movement
of the tentacle is particularly well seen in contrast with the
stationary condition of the surrounding tentacles. (See previous fig.
6.) In four cases small particles of raw meat caused the tentacles to
be greatly inflected in between 5 and 6 m. Another tentacle similarly
treated, and observed with special care, distinctly, though slightly,
changed its position in 10 s. (seconds); and this is the quickest
movement seen by me. In 2 m. 30 s. it had moved through an angle of
about 45o. The movement as seen through a lens resembled that of the
hand of a large clock. In 5 m. it had moved through 90o, and when I
looked again after 10 m., the particle had reached the centre of the
leaf; so that the whole movement was completed in less [page 25] than
17 m. 30 s. In the course of some hours this minute bit of meat, from
having been brought into contact with some of the glands of the central
disc, acted centrifugally on the outer tentacles, which all became
closely inflected. Fragments of flies were placed on the glands of four
of the outer tentacles, extended in the same plane with that of the
blade, and three of these fragments were carried in 35 m. through an
angle of 180o to the centre. The fragment on the fourth tentacle was
very minute, and it was not carried to the centre until 3 hrs. had
elapsed. In three other cases minute flies or portions of larger ones
were carried to the centre in 1 hr. 30 s. In these seven cases, the
fragments or small flies, which had been carried by a single tentacle
to the central glands, were well embraced by the other tentacles after
an interval of from 4 to 10 hrs.
I also placed in the manner just described six small balls of
writing-paper (rolled up by the aid of pincers, so that they were not
touched by my fingers) on the glands of six exterior tentacles on
distinct leaves; three of these were carried to the centre in about 1
hr., and the other three in rather more than 4 hrs.; but after 24 hrs.
only two of the six balls were well embraced by the other tentacles. It
is possible that the secretion may have dissolved a trace of glue or
animalised matter from the balls of paper. Four particles of
coal-cinder were then placed on the glands of four exterior tentacles;
one of these reached the centre in 3 hrs. 40 m.; the second in 9 hrs.;
the third within 24 hrs., but had moved only part of the way in 9 hrs.;
whilst the fourth moved only a very short distance in 24 hrs., and
never moved any farther. Of the above three bits of cinder which were
ultimately carried to the centre, one alone was well embraced by [page
26] many of the other tentacles. We here see clearly that such bodies
as particles of cinder or little balls of paper, after being carried by
the tentacles to the central glands, act very differently from
fragments of flies, in causing the movement of the surrounding
tentacles.
I made, without carefully recording the times of movement, many similar
trials with other substances, such as splinters of white and blue
glass, particles of cork, minute bits of gold-leaf, &c.; and the
proportional number of cases varied much in which the tentacles reached
the centre, or moved only slightly, or not at all. One evening,
particles of glass and cork, rather larger than those usually employed,
were placed on about a dozen glands, and next morning, after 13 hrs.,
every single tentacle had carried its little load to the centre; but
the unusually large size of the particles will account for this result.
In another case 6/7 of the particles of cinder, glass, and thread,
placed on separate glands, were carried towards, or actually to, the
centre; in another case 7/9, in another 7/12, and in the last case only
7/26 were thus carried inwards, the small proportion being here due, at
least in part, to the leaves being rather old and inactive.
Occasionally a gland, with its light load, could be seen through a
strong lens to move an extremely short distance and then stop; this was
especially apt to occur when excessively minute particles, much less
than those of which the measurements will be immediately given, were
placed on glands; so that we here have nearly the limit of any action.
I was so much surprised at the smallness of the particles which caused
the tentacles to become greatly inflected that it seemed worth while
carefully to ascertain how minute a particle would plainly act. [page
27] Accordingly measured lengths of a narrow strip of blotting paper,
of fine cotton-thread, and of a woman’s hair, were carefully weighed
for me by Mr. Trenham Reeks, in an excellent balance, in the laboratory
in Jermyn Street. Short bits of the paper, thread, and hair were then
cut off and measured by a micrometer, so that their weights could be
easily calculated. The bits were placed on the viscid secretion
surrounding the glands of the exterior tentacles, with the precautions
already stated, and I am certain that the gland itself was never
touched; nor indeed would a single touch have produced any effect. A
bit of the blotting-paper, weighing 1/465 of a grain, was placed so as
to rest on three glands together, and all three tentacles slowly curved
inwards; each gland, therefore, supposing the weight to be distributed
equally, could have been pressed on by only 1/1395 of a grain, or .0464
of a milligramme. Five nearly equal bits of cotton-thread were tried,
and all acted. The shortest of these was 1/50 of an inch in length, and
weighed 1/8197 of a grain. The tentacle in this case was considerably
inflected in 1 hr. 30 m., and the bit of thread was carried to the
centre of the leaf in 1 hr. 40 m. Again, two particles of the thinner
end of a woman’s hair, one of these being 18/1000 of an inch in length,
and weighing 1/35714 of a grain, the other 19/1000 of an inch in
length, and weighing of course a little more, were placed on two glands
on opposite sides of the same leaf, and these two tentacles were
inflected halfway towards the centre in 1 hr. 10 m.; all the many other
tentacles round the same leaf remaining motionless. The appearance of
this one leaf showed in an unequivocal manner that these minute
particles sufficed to cause the tentacles to bend. Altogether, ten such
particles of hair were placed on ten glands on several leaves, and
seven of them caused [page 28] the tentacles to move in a conspicuous
manner. The smallest particle which was tried, and which acted plainly,
was only 8/1000 of an inch (.203 millimetre) in length, and weighed the
1/78740 of a grain, or .000822 milligramme. In these several cases, not
only was the inflection of the tentacles conspicuous, but the purple
fluid within their cells became aggregated into little masses of
protoplasm, in the manner to be described in the next chapter; and the
aggregation was so plain that I could, by this clue alone, have readily
picked out under the microscope all the tentacles which had carried
their light loads towards the centre, from the hundreds of other
tentacles on the same leaves which had not thus acted.
My surprise was greatly excited, not only by the minuteness of the
particles which caused movement, but how they could possibly act on the
glands; for it must be remembered that they were laid with the greatest
care on the convex surface of the secretion. At first I thought—but, as
I now know, erroneously—that particles of such low specific gravity as
those of cork, thread, and paper, would never come into contact with
the surfaces of the glands. The particles cannot act simply by their
weight being added to that of the secretion, for small drops of water,
many times heavier than the particles, were repeatedly added, and never
produced any effect. Nor does the disturbance of the secretion produce
any effect, for long threads were drawn out by a needle, and affixed to
some adjoining object, and thus left for hours; but the tentacles
remained motionless.
I also carefully removed the secretion from four glands with a sharply
pointed piece of blotting-paper, so that they were exposed for a time
naked to the air, but this caused no movement; yet these glands were
[page 29] in an efficient state, for after 24 hrs. had elapsed, they
were tried with bits of meat, and all became quickly inflected. It then
occurred to me that particles floating on the secretion would cast
shadows on the glands, which might be sensitive to the interception of
the light. Although this seemed highly improbable, as minute and thin
splinters of colourless glass acted powerfully, nevertheless, after it
was dark, I put on, by the aid of a single tallow candle, as quickly as
possible, particles of cork and glass on the glands of a dozen
tentacles, as well as some of meat on other glands, and covered them up
so that not a ray of light could enter; but by the next morning, after
an interval of 13 hrs., all the particles were carried to the centres
of the leaves.
These negative results led me to try many more experiments, by placing
particles on the surface of the drops of secretion, observing, as
carefully as I could, whether they penetrated it and touched the
surface of the glands. The secretion, from its weight, generally forms
a thicker layer on the under than on the upper sides of the glands,
whatever may be the position of the tentacles. Minute bits of dry cork,
thread, blotting paper, and coal cinders were tried, such as those
previously employed; and I now observed that they absorbed much more of
the secretion, in the course of a few minutes, than I should have
thought possible; and as they had been laid on the upper surface of the
secretion, where it is thinnest, they were often drawn down, after a
time, into contact with at least some one point of the gland. With
respect to the minute splinters of glass and particles of hair, I
observed that the secretion slowly spread itself a little over their
surfaces, by which means they were likewise drawn downwards or
sideways, and thus one end, or some minute [page 30] prominence, often
came to touch, sooner or later, the gland.
In the foregoing and following cases, it is probable that the
vibrations, to which the furniture in every room is continually liable,
aids in bringing the particles into contact with the glands. But as it
was sometimes difficult, owing to the refraction of the secretion, to
feel sure whether the particles were in contact, I tried the following
experiment. Unusually minute particles of glass, hair, and cork, were
gently placed on the drops round several glands, and very few of the
tentacles moved. Those which were not affected were left for about half
an hour, and the particles were then disturbed or tilted up several
times with a fine needle under the microscope, the glands not being
touched. And now in the course of a few minutes almost all the hitherto
motionless tentacles began to move; and this, no doubt, was caused by
one end or some prominence of the particles having come into contact
with the surface of the glands. But as the particles were unusually
minute, the movement was small.
Lastly, some dark blue glass pounded into fine splinters was used, in
order that the points of the particles might be better distinguished
when immersed in the secretion; and thirteen such particles were placed
in contact with the depending and therefore thicker part of the drops
round so many glands. Five of the tentacles began moving after an
interval of a few minutes, and in these cases I clearly saw that the
particles touched the lower surface of the gland. A sixth tentacle
moved after 1 hr. 45 m., and the particle was now in contact with the
gland, which was not the case at first. So it was with the seventh
tentacle, but its movement did not begin until 3 hrs. 45 m. had [page
31] elapsed. The remaining six tentacles never moved as long as they
were observed; and the particles apparently never came into contact
with the surfaces of the glands.
From these experiments we learn that particles not containing soluble
matter, when placed on glands, often cause the tentacles to begin
bending in the course of from one to five minutes; and that in such
cases the particles have been from the first in contact with the
surfaces of the glands. When the tentacles do not begin moving for a
much longer time, namely, from half an hour to three or four hours, the
particles have been slowly brought into contact with the glands, either
by the secretion being absorbed by the particles or by its gradual
spreading over them, together with its consequent quicker evaporation.
When the tentacles do not move at all, the particles have never come
into contact with the glands, or in some cases the tentacles may not
have been in an active condition. In order to excite movement, it is
indispensable that the particles should actually rest on the glands;
for a touch once, twice, or even thrice repeated by any hard body is
not sufficient to excite movement.
Another experiment, showing that extremely minute particles act on the
glands when immersed in water, may here be given. A grain of sulphate
of quinine was added to an ounce of water, which was not afterwards
filtered; and on placing three leaves in ninety minims of this fluid, I
was much surprised to find that all three leaves were greatly inflected
in 15 m.; for I knew from previous trials that the solution does not
act so quickly as this. It immediately occurred to me that the
particles of the undissolved salt, which were so light as to float
about, might have come [page 32] into contact with the glands, and
caused this rapid movement. Accordingly I added to some distilled water
a pinch of a quite innocent substance, namely, precipitated carbonate
of lime, which consists of an impalpable powder; I shook the mixture,
and thus got a fluid like thin milk. Two leaves were immersed in it,
and in 6 m. almost every tentacle was much inflected. I placed one of
these leaves under the microscope, and saw innumerable atoms of lime
adhering to the external surface of the secretion. Some, however, had
penetrated it, and were lying on the surfaces of the glands; and no
doubt it was these particles which caused the tentacles to bend. When a
leaf is immersed in water, the secretion instantly swells much; and I
presume that it is ruptured here and there, so that little eddies of
water rush in. If so, we can understand how the atoms of chalk, which
rested on the surfaces of the glands, had penetrated the secretion.
Anyone who has rubbed precipitated chalk between his fingers will have
perceived how excessively fine the powder is. No doubt there must be a
limit, beyond which a particle would be too small to act on a gland;
but what this limit is, I know not. I have often seen fibres and dust,
which had fallen from the air, on the glands of plants kept in my room,
and these never induced any movement; but then such particles lay on
the surface of the secretion and never reached the gland itself.
Finally, it is an extraordinary fact that a little bit of soft thread,
1/50 of an inch in length and weighing 1/8197 of a grain, or of a human
hair, 8/1000 of an inch in length and weighing only 1/78740 of a grain
(.000822 milligramme), or particles of precipitated chalk, after
resting for a short time on a gland, should induce some change in its
cells, exciting them [page 33] to transmit a motor impulse throughout
the whole length of the pedicel, consisting of about twenty cells, to
near its base, causing this part to bend, and the tentacle to sweep
through an angle of above 180o. That the contents of the cells of the
glands, and afterwards those of the pedicels, are affected in a plainly
visible manner by the pressure of minute particles, we shall have
abundant evidence when we treat of the aggregation of protoplasm. But
the case is much more remarkable than as yet stated; for the particles
are supported by the viscid and dense secretion; nevertheless, even
smaller ones than those of which the measurements have been given, when
brought by an insensibly slow movement, through the means above
specified, into contact with the surface of a gland, act on it, and the
tentacle bends. The pressure exerted by the particle of hair, weighing
only 1/78740 of a grain and supported by a dense fluid, must have been
inconceivably slight. We may conjecture that it could hardly have
equalled the millionth of a grain; and we shall hereafter see that far
less than the millionth of a grain of phosphate of ammonia in solution,
when absorbed by a gland, acts on it and induces movement. A bit of
hair, 1/50 of an inch in length, and therefore much larger than those
used in the above experiments, was not perceived when placed on my
tongue; and it is extremely doubtful whether any nerve in the human
body, even if in an inflamed condition, would be in any way affected by
such a particle supported in a dense fluid, and slowly brought into
contact with the nerve. Yet the cells of the glands of Drosera are thus
excited to transmit a motor impulse to a distant point, inducing
movement. It appears to me that hardly any more remarkable fact than
this has been observed in the vegetable kingdom. [page 34]
The Inflection of the Exterior Tentacles, when their Glands are excited
by Repeated Touches.
We have already seen that, if the central glands are excited by being
gently brushed, they transmit a motor impulse to the exterior
tentacles, causing them to bend; and we have now to consider the
effects which follow from the glands of the exterior tentacles being
themselves touched. On several occasions, a large number of glands were
touched only once with a needle or fine brush, hard enough to bend the
whole flexible tentacle; and though this must have caused a
thousand-fold greater pressure than the weight of the above described
particles, not a tentacle moved. On another occasion forty-five glands
on eleven leaves were touched once, twice, or even thrice, with a
needle or stiff bristle. This was done as quickly as possible, but with
force sufficient to bend the tentacles; yet only six of them became
inflected,—three plainly, and three in a slight degree. In order to
ascertain whether these tentacles which were not affected were in an
efficient state, bits of meat were placed on ten of them, and they all
soon became greatly incurved. On the other hand, when a large number of
glands were struck four, five, or six times with the same force as
before, a needle or sharp splinter of glass being used, a much larger
proportion of tentacles became inflected; but the result was so
uncertain as to seem capricious. For instance, I struck in the above
manner three glands, which happened to be extremely sensitive, and all
three were inflected almost as quickly, as if bits of meat had been
placed on them. On another occasion I gave a single for- [page 35]
cible touch to a considerable number of glands, and not one moved; but
these same glands, after an interval of some hours, being touched four
or five times with a needle, several of the tentacles soon became
inflected.
The fact of a single touch or even of two or three touches not causing
inflection must be of some service to the plant; as during stormy
weather, the glands cannot fail to be occasionally touched by the tall
blades of grass, or by other plants growing near; and it would be a
great evil if the tentacles were thus brought into action, for the act
of re-expansion takes a considerable time, and until the tentacles are
re-expanded they cannot catch prey. On the other hand, extreme
sensitiveness to slight pressure is of the highest service to the
plant; for, as we have seen, if the delicate feet of a minute
struggling insect press ever so lightly on the surfaces of two or three
glands, the tentacles bearing these glands soon curl inwards and carry
the insect with them to the centre, causing, after a time, all the
circumferential tentacles to embrace it. Nevertheless, the movements of
the plant are not perfectly adapted to its requirements; for if a bit
of dry moss, peat, or other rubbish, is blown on to the disc, as often
happens, the tentacles clasp it in a useless manner. They soon,
however, discover their mistake and release such innutritious objects.
It is also a remarkable fact, that drops of water falling from a
height, whether under the form of natural or artificial rain, do not
cause the tentacles to move; yet the drops must strike the glands with
considerable force, more especially after the secretion has been all
washed away by heavy rain; and this often occurs, [page 36] though the
secretion is so viscid that it can be removed with difficulty merely by
waving the leaves in water. If the falling drops of water are small,
they adhere to the secretion, the weight of which must be increased in
a much greater degree, as before remarked, than by the addition of
minute particles of solid matter; yet the drops never cause the
tentacles to become inflected. It would obviously have been a great
evil to the plant (as in the case of occasional touches) if the
tentacles were excited to bend by every shower of rain; but this evil
has been avoided by the glands either having become through habit
insensible to the blows and prolonged pressure of drops of water, or to
their having been originally rendered sensitive solely to the contact
of solid bodies. We shall hereafter see that the filaments on the
leaves of Dionaea are likewise insensible to the impact of fluids,
though exquisitely sensitive to momentary touches from any solid body.
When the pedicel of a tentacle is cut off by a sharp pair of scissors
quite close beneath the gland, the tentacle generally becomes
inflected. I tried this experiment repeatedly, as I was much surprised
at the fact, for all other parts of the pedicels are insensible to any
stimulus. These headless tentacles after a time re-expand; but I shall
return to this subject. On the other hand, I occasionally succeeded in
crushing a gland between a pair of pincers, but this caused no
inflection. In this latter case the tentacles seem paralysed, as
likewise follows from the action of too strong solutions of certain
salts, and by too great heat, whilst weaker solutions of the same salts
and a more gentle heat cause movement. We shall also see in future
chapters that various other fluids, some [page 37] vapours, and oxygen
(after the plant has been for some time excluded from its action), all
induce inflection, and this likewise results from an induced galvanic
current.*
* My son Francis, guided by the observations of Dr. Burdon Sanderson on
Dionaea, finds that if two needles are inserted into the blade of a
leaf of Drosera, the tentacles do not move; but that if similar needles
in connection with the secondary coil of a Du Bois inductive apparatus
are inserted, the tentacles curve inwards in the course of a few
minutes. My son hopes soon to publish an account of his observations.
[page 38]
CHAPTER III.
AGGREGATION OF THE PROTOPLASM WITHIN THE CELLS OF THE TENTACLES.
Nature of the contents of the cells before aggregation—Various causes
which excite aggregation—The process commences within the glands and
travels down the tentacles— Description of the aggregated masses and of
their spontaneous movements—Currents of protoplasm along the walls of
the cells—Action of carbonate of ammonia—The granules in the protoplasm
which flows along the walls coalesce with the central masses—Minuteness
of the quantity of carbonate of ammonia causing aggregation—Action of
other salts of ammonia—Of other substances, organic fluids, &c.—Of
water—Of heat—Redissolution of the aggregated masses—Proximate causes
of the aggregation of the protoplasm—Summary and concluding
remarks—Supplementary observations on aggregation in the roots of
plants.
I will here interrupt my account of the movements of the leaves, and
describe the phenomenon of aggregation, to which subject I have already
alluded. If the tentacles of a young, yet fully matured leaf, that has
never been excited or become inflected, be examined, the cells forming
the pedicels are seen to be filled with homogeneous, purple fluid. The
walls are lined by a layer of colourless, circulating protoplasm; but
this can be seen with much greater distinctness after the process of
aggregation has been partly effected than before. The purple fluid
which exudes from a crushed tentacle is somewhat coherent, and does not
mingle with the surrounding water; it contains much flocculent or
granular matter. But this matter may have been generated by the cells
having been crushed; some degree of aggregation having been thus almost
instantly caused. [page 39]
If a tentacle is examined some hours after the gland has been excited
by repeated touches, or by an inorganic or organic particle placed on
it, or by the absorption of certain fluids, it presents a wholly
changed appearance. The cells, instead of being filled with homogeneous
purple fluid, now contain variously shaped masses of purple matter,
suspended in a colourless or almost colourless fluid. The change is so
conspicuous that it is visible through a weak lens, and even sometimes
by the naked eye; the tentacles now have a mottled appearance, so that
one thus affected can be picked out with ease from all the others. The
same result follows if the glands on the disc are irritated in any
manner, so that the exterior tentacles become inflected; for their
contents will then be found in an aggregated condition, although their
glands have not as yet touched any object. But aggregation may occur
independently of inflection, as we shall presently see. By whatever
cause the process may have been excited, it commences within the
glands, and then travels down the tentacles. It can be observed much
more distinctly in the upper cells of the pedicels than within the
glands, as these are somewhat opaque. Shortly after the tentacles have
re-expanded, the little masses of protoplasm are all redissolved, and
the purple fluid within the cells becomes as homogeneous and
transparent as it was at first. The process of redissolution travels
upwards from the bases of the tentacles to the glands, and therefore in
a reversed direction to that of aggregation. Tentacles in an aggregated
condition were shown to Prof. Huxley, Dr. Hooker, and Dr. Burdon
Sanderson, who observed the changes under the microscope, and were much
struck with the whole phenomenon. [page 40]
The little masses of aggregated matter are of the most diversified
shapes, often spherical or oval, sometimes much elongated, or quite
irregular with thread- or necklace-like or club-formed projections.
They consist of thick, apparently viscid matter, which in the exterior
tentacles is of a purplish, and in the short distal tentacles of a
greenish, colour. These little masses incessantly change their forms
and positions, being never at rest. A single mass will often separate
into two, which afterwards reunite. Their movements are rather slow,
and resemble those of Amoebae or of the white corpuscles of the blood.
We
FIG. 7. (Drosera rotundifolia.) Diagram of the same cell of a tentacle,
showing the various forms successively assumed by the aggregated masses
of protoplasm.
may, therefore, conclude that they consist of protoplasm. If their
shapes are sketched at intervals of a few minutes, they are invariably
seen to have undergone great changes of form; and the same cell has
been observed for several hours. Eight rude, though accurate sketches
of the same cell, made at intervals of between 2 m. or 3 m., are here
given (fig. 7), and illustrate some of the simpler and commonest
changes. The cell A, when first sketched, included two oval masses of
purple protoplasm touching each other. These became separate, as shown
at B, and then reunited, as at C. After the next interval a very common
appearance was presented— [page 41] D, namely, the formation of an
extremely minute sphere at one end of an elongated mass. This rapidly
increased in size, as shown in E, and was then re-absorbed, as at F, by
which time another sphere had been formed at the opposite end.
The cell above figured was from a tentacle of a dark red leaf, which
had caught a small moth, and was examined under water. As I at first
thought that the movements of the masses might be due to the absorption
of water, I placed a fly on a leaf, and when after 18 hrs. all the
tentacles were well inflected, these were examined without being
immersed in water. The cell
FIG. 8. (Drosera rotundifolia.) Diagram of the same cell of a tentacle,
showing the various forms successively assumed by the aggregated masses
of protoplasm.
here represented (fig. 8) was from this leaf, being sketched eight
times in the course of 15 m. These sketches exhibit some of the more
remarkable changes which the protoplasm undergoes. At first, there was
at the base of the cell 1, a little mass on a short footstalk, and a
larger mass near the upper end, and these seemed quite separate.
Nevertheless, they may have been connected by a fine and invisible
thread of protoplasm, for on two other occasions, whilst one mass was
rapidly increasing, and another in the same cell rapidly decreasing, I
was able by varying the light and using a high power, to detect a
connecting thread of extreme tenuity, which evidently served as [page
42] the channel of communication between the two. On the other hand,
such connecting threads are sometimes seen to break, and their
extremities then quickly become club-headed. The other sketches in fig.
8 show the forms successively assumed.
Shortly after the purple fluid within the cells has become aggregated,
the little masses float about in a colourless or almost colourless
fluid; and the layer of white granular protoplasm which flows along the
walls can now be seen much more distinctly. The stream flows at an
irregular rate, up one wall and down the opposite one, generally at a
slower rate across the narrow ends of the elongated cells, and so round
and round. But the current sometimes ceases. The movement is often in
waves, and their crests sometimes stretch almost across the whole width
of the cell, and then sink down again. Small spheres of protoplasm,
apparently quite free, are often driven by the current round the cells;
and filaments attached to the central masses are swayed to and fro, as
if struggling to escape. Altogether, one of these cells with the ever
changing central masses, and with the layer of protoplasm flowing round
the walls, presents a wonderful scene of vital activity.
[Many observations were made on the contents of the cells whilst
undergoing the process of aggregation, but I shall detail only a few
cases under different heads. A small portion of a leaf was cut off,
placed under a high power, and the glands very gently pressed under a
compressor. In 15 m. I distinctly saw extremely minute spheres of
protoplasm aggregating themselves in the purple fluid; these rapidly
increased in size, both within the cells of the glands and of the upper
ends of the pedicels. Particles of glass, cork, and cinders were also
placed on the glands of many tentacles; in 1 hr. several of them were
inflected, but after 1 hr. 35 m. there was no aggregation. Other
tentacles with these particles were examined after 8 hrs., and [page
43] now all their cells had undergone aggregation; so had the cells of
the exterior tentacles which had become inflected through the
irritation transmitted from the glands of the disc, on which the
transported particles rested. This was likewise the case with the short
tentacles round the margins of the disc, which had not as yet become
inflected. This latter fact shows that the process of aggregation is
independent of the inflection of the tentacles, of which indeed we have
other and abundant evidence. Again, the exterior tentacles on three
leaves were carefully examined, and found to contain only homogeneous
purple fluid; little bits of thread were then placed on the glands of
three of them, and after 22 hrs. the purple fluid in their cells almost
down to their bases was aggregated into innumerable, spherical,
elongated, or filamentous masses of protoplasm. The bits of thread had
been carried some time previously to the central disc, and this had
caused all the other tentacles to become somewhat inflected; and their
cells had likewise undergone aggregation, which however, it should be
observed, had not as yet extended down to their bases, but was confined
to the cells close beneath the glands.
Not only do repeated touches on the glands* and the contact of minute
particles cause aggregation, but if glands, without being themselves
injured, are cut off from the summits of the pedicels, this induces a
moderate amount of aggregation in the headless tentacles, after they
have become inflected. On the other hand, if glands are suddenly
crushed between pincers, as was tried in six cases, the tentacles seem
paralysed by so great a shock, for they neither become inflected nor
exhibit any signs of aggregation.
Carbonate of Ammonia.—Of all the causes inducing aggregation, that
which, as far as I have seen, acts the quickest, and is the most
powerful, is a solution of carbonate of ammonia. Whatever its strength
may be, the glands are always affected first, and soon become quite
opaque, so as to appear black. For instance, I placed a leaf in a few
drops of a strong solution, namely, of one part to 146 of water (or 3
grs. to 1 oz.), and observed it under a high power. All the glands
began to
* Judging from an account of M. Heckel’s observations, which I have
only just seen quoted in the ‘Gardeners’ Chronicle’ (Oct. 10, 1874), he
appears to have observed a similar phenomenon in the stamens of
Berberis, after they have been excited by a touch and have moved; for
he says, “the contents of each individual cell are collected together
in the centre of the cavity.” [page 44]
darken in 10 s. (seconds); and in 13 s. were conspicuously darker. In 1
m. extremely small spherical masses of protoplasm could be seen arising
in the cells of the pedicels close beneath the glands, as well as in
the cushions on which the long-headed marginal glands rest. In several
cases the process travelled down the pedicels for a length twice or
thrice as great as that of the glands, in about 10 m. It was
interesting to observe the process momentarily arrested at each
transverse partition between two cells, and then to see the transparent
contents of the cell next below almost flashing into a cloudy mass. In
the lower part of the pedicels, the action proceeded slower, so that it
took about 20 m. before the cells halfway down the long marginal and
submarginal tentacles became aggregated.
We may infer that the carbonate of ammonia is absorbed by the glands,
not only from its action being so rapid, but from its effect being
somewhat different from that of other salts. As the glands, when
excited, secrete an acid belonging to the acetic series, the carbonate
is probably at once converted into a salt of this series; and we shall
presently see that the acetate of ammonia causes aggregation almost or
quite as energetically as does the carbonate. If a few drops of a
solution of one part of the carbonate to 437 of water (or 1 gr. to 1
oz.) be added to the purple fluid which exudes from crushed tentacles,
or to paper stained by being rubbed with them, the fluid and the paper
are changed into a pale dirty green. Nevertheless, some purple colour
could still be detected after 1 hr. 30 m. within the glands of a leaf
left in a solution of twice the above strength (viz. 2 grs. to 1 oz.);
and after 24 hrs. the cells of the pedicels close beneath the glands
still contained spheres of protoplasm of a fine purple tint. These
facts show that the ammonia had not entered as a carbonate, for
otherwise the colour would have been discharged. I have, however,
sometimes observed, especially with the long-headed tentacles on the
margins of very pale leaves immersed in a solution, that the glands as
well as the upper cells of the pedicels were discoloured; and in these
cases I presume that the unchanged carbonate had been absorbed. The
appearance above described, of the aggregating process being arrested
for a short time at each transverse partition, impresses the mind with
the idea of matter passing downwards from cell to cell. But as the
cells one beneath the other undergo aggregation when inorganic and
insoluble particles are placed on the glands, the process must be, at
least in these cases, one of molecular change, transmitted from the
glands, [page 45] independently of the absorption of any matter. So it
may possibly be in the case of the carbonate of ammonia. As, however,
the aggregation caused by this salt travels down the tentacles at a
quicker rate than when insoluble particles are placed on the glands, it
is probable that ammonia in some form is absorbed not only by the
glands, but passes down the tentacles.
Having examined a leaf in water, and found the contents of the cells
homogeneous, I placed it in a few drops of a solution of one part of
the carbonate to 437 of water, and attended to the cells immediately
beneath the glands, but did not use a very high power. No aggregation
was visible in 3 m.; but after 15 m. small spheres of protoplasm were
formed, more especially beneath the long-headed marginal glands; the
process, however, in this case took place with unusual slowness. In 25
m. conspicuous spherical masses were present in the cells of the
pedicels for a length about equal to that of the glands; and in 3 hrs.
to that of a third or half of the whole tentacle.
If tentacles with cells containing only very pale pink fluid, and
apparently but little protoplasm, are placed in a few drops of a weak
solution of one part of the carbonate to 4375 of water (1 gr. to 10
oz.), and the highly transparent cells beneath the glands are carefully
observed under a high power, these may be seen first to become slightly
cloudy from the formation of numberless, only just perceptible,
granules, which rapidly grow larger either from coalescence or from
attracting more protoplasm from the surrounding fluid. On one occasion
I chose a singularly pale leaf, and gave it, whilst under the
microscope, a single drop of a stronger solution of one part to 437 of
water; in this case the contents of the cells did not become cloudy,
but after 10 m. minute irregular granules of protoplasm could be
detected, which soon increased into irregular masses and globules of a
greenish or very pale purple tint; but these never formed perfect
spheres, though incessantly changing their shapes and positions.
With moderately red leaves the first effect of a solution of the
carbonate generally is the formation of two or three, or of several,
extremely minute purple spheres which rapidly increase in size. To give
an idea of the rate at which such spheres increase in size, I may
mention that a rather pale purple leaf placed under a slip of glass was
given a drop of a solution of one part to 292 of water, and in 13 m. a
few minute spheres of protoplasm were formed; one of these, after 2
hrs. 30 m., was about two-thirds of the diameter of the cell. After 4
hrs. 25 m. [page 46] it nearly equalled the cell in diameter; and a
second sphere about half as large as the first, together with a few
other minute ones, were formed. After 6 hrs. the fluid in which these
spheres floated was almost colourless. After 8 hrs. 35 m. (always
reckoning from the time when the solution was first added) four new
minute spheres had appeared. Next morning, after 22 hrs., there were,
besides the two large spheres, seven smaller ones, floating in
absolutely colourless fluid, in which some flocculent greenish matter
was suspended.
At the commencement of the process of aggregation, more especially in
dark red leaves, the contents of the cells often present a different
appearance, as if the layer of protoplasm (primordial utricle) which
lines the cells had separated itself and shrunk from the walls; an
irregularly shaped purple bag being thus formed. Other fluids, besides
a solution of the carbonate, for instance an infusion of raw meat,
produce this same effect. But the appearance of the primordial utricle
shrinking from the walls is certainly false;* for before giving the
solution, I saw on several occasions that the walls were lined with
colourless flowing protoplasm, and after the bag-like masses were
formed, the protoplasm was still flowing along the walls in a
conspicuous manner, even more so than before. It appeared indeed as if
the stream of protoplasm was strengthened by the action of the
carbonate, but it was impossible to ascertain whether this was really
the case. The bag-like masses, when once formed, soon begin to glide
slowly round the cells, sometimes sending out projections which
separate into little spheres; other spheres appear in the fluid
surrounding the bags, and these travel much more quickly. That the
small spheres are separate is often shown by sometimes one and then
another travelling in advance, and sometimes they revolve round each
other. I have occasionally seen spheres of this kind proceeding up and
down the same side of a cell, instead of round it. The bag-like masses
after a time generally divide into two rounded or oval masses, and
these undergo the changes shown in figs. 7 and 8. At other times
spheres appear within the bags; and these coalesce and separate in an
endless cycle of change.
After leaves have been left for several hours in a solution of the
carbonate, and complete aggregation has been effected, the
* With other plants I have often seen what appears to be a true
shrinking of the primordial utricle from the walls of the cells, caused
by a solution of carbonate of ammonia, as likewise follows from
mechanical injuries. [page 47]
stream of protoplasm on the walls of the cells ceases to be visible; I
observed this fact repeatedly, but will give only one instance. A pale
purple leaf was placed in a few drops of a solution of one part to 292
of water, and in 2 hrs. some fine purple spheres were formed in the
upper cells of the pedicels, the stream of protoplasm round their walls
being still quite distinct; but after an additional 4 hrs., during
which time many more spheres were formed, the stream was no longer
distinguishable on the most careful examination; and this no doubt was
due to the contained granules having become united with the spheres, so
that nothing was left by which the movement of the limpid protoplasm
could be perceived. But minute free spheres still travelled up and down
the cells, showing that there was still a current. So it was next
morning, after 22 hrs., by which time some new minute spheres had been
formed; these oscillated from side to side and changed their positions,
proving that the current had not ceased, though no stream of protoplasm
was visible. On another occasion, however, a stream was seen flowing
round the cell-walls of a vigorous, dark-coloured leaf, after it had
been left for 24 hrs. in a rather stronger solution, namely, of one
part of the carbonate to 218 of water. This leaf, therefore, was not
much or at all injured by an immersion for this length of time in the
above solution of two grains to the ounce; and on being afterwards left
for 24 hrs. in water, the aggregated masses in many of the cells were
re-dissolved, in the same manner as occurs with leaves in a state of
nature when they re-expand after having caught insects.
In a leaf which had been left for 22 hrs. in a solution of one part of
the carbonate to 292 of water, some spheres of protoplasm (formed by
the self-division of a bag-like mass) were gently pressed beneath a
covering glass, and then examined under a high power. They were now
distinctly divided by well-defined radiating fissures, or were broken
up into separate fragments with sharp edges; and they were solid to the
centre. In the larger broken spheres the central part was more opaque,
darker-coloured, and less brittle than the exterior; the latter alone
being in some cases penetrated by the fissures. In many of the spheres
the line of separation between the outer and inner parts was tolerably
well defined. The outer parts were of exactly the same very pale purple
tint, as that of the last formed smaller spheres; and these latter did
not include any darker central core.
From these several facts we may conclude that when vigorous
dark-coloured leaves are subjected to the action of carbonate of [page
48] ammonia, the fluid within the cells of the tentacles often
aggregates exteriorly into coherent viscid matter, forming a kind of
bag. Small spheres sometimes appear within this bag, and the whole
generally soon divides into two or more spheres, which repeatedly
coalesce and redivide. After a longer or shorter time the granules in
the colourless layer of protoplasm, which flows round the walls, are
drawn to and unite with the larger spheres, or form small independent
spheres; these latter being of a much paler colour, and more brittle
than the first aggregated masses. After the granules of protoplasm have
been thus attracted, the layer of flowing protoplasm can no longer be
distinguished, though a current of limpid fluid still flows round the
walls.
If a leaf is immersed in a very strong, almost concentrated, solution
of carbonate of ammonia, the glands are instantly blackened, and they
secrete copiously; but no movement of the tentacles ensues. Two leaves
thus treated became after 1 hr. flaccid, and seemed killed; all the
cells in their tentacles contained spheres of protoplasm, but these
were small and discoloured. Two other leaves were placed in a solution
not quite so strong, and there was well-marked aggregation in 30 m.
After 24 hrs. the spherical or more commonly oblong masses of
protoplasm became opaque and granular, instead of being as usual
translucent; and in the lower cells there were only innumerable minute
spherical granules. It was evident that the strength of the solution
had interfered with the completion of the process, as we shall see
likewise follows from too great heat.
All the foregoing observations relate to the exterior tentacles, which
are of a purple colour; but the green pedicels of the short central
tentacles are acted on by the carbonate, and by an infusion of raw
meat, in exactly the same manner, with the sole difference that the
aggregated masses are of a greenish colour; so that the process is in
no way dependent on the colour of the fluid within the cells.
Finally, the most remarkable fact with respect to this salt is the
extraordinary small amount which suffices to cause aggregation. Full
details will be given in the seventh chapter, and here it will be
enough to say that with a sensitive leaf the absorption by a gland of
1/134400 of a grain (.000482 mgr.) is enough to cause in the course of
one hour well-marked aggregation in the cells immediately beneath the
gland.
The Effects of certain other Salts and Fluids.—Two leaves were placed
in a solution of one part of acetate of ammonia to about [page 49] 146
of water, and were acted on quite as energetically, but perhaps not
quite so quickly, as by the carbonate. After 10 m. the glands were
black, and in the cells beneath them there were traces of aggregation,
which after 15 m. was well marked, extending down the tentacles for a
length equal to that of the glands. After 2 hrs. the contents of almost
all the cells in all the tentacles were broken up into masses of
protoplasm. A leaf was immersed in a solution of one part of oxalate of
ammonia to 146 of water; and after 24 m. some, but not a conspicuous,
change could be seen within the cells beneath the glands. After 47 m.
plenty of spherical masses of protoplasm were formed, and these
extended down the tentacles for about the length of the glands. This
salt, therefore, does not act so quickly as the carbonate. With respect
to the citrate of ammonia, a leaf was placed in a little solution of
the above strength, and there was not even a trace of aggregation in
the cells beneath the glands, until 56 m. had elapsed; but it was well
marked after 2 hrs. 20 m. On another occasion a leaf was placed in a
stronger solution, of one part of the citrate to 109 of water (4 grs.
to 1 oz.), and at the same time another leaf in a solution of the
carbonate of the same strength. The glands of the latter were blackened
in less than 2 m., and after 1 hr. 45 m. the aggregated masses, which
were spherical and very dark-coloured, extended down all the tentacles,
for between half and two-thirds of their lengths; whereas in the leaf
immersed in the citrate the glands, after 30 m., were of a dark red,
and the aggregated masses in the cells beneath them pink and elongated.
After 1 hr. 45 m. these masses extended down for only about one-fifth
or one-fourth of the length of the tentacles.
Two leaves were placed, each in ten minims of a solution of one part of
nitrate of ammonia to 5250 of water (1 gr. to 12 oz.), so that each
leaf received 1/576 of a grain (.1124 mgr.). This quantity caused all
the tentacles to be inflected, but after 24 hrs. there was only a trace
of aggregation. One of these same leaves was then placed in a weak
solution of the carbonate, and after 1 hr. 45 m. the tentacles for half
their lengths showed an astonishing degree of aggregation. Two other
leaves were then placed in a much stronger solution of one part of the
nitrate to 146 of water (3 grs. to 1 oz.); in one of these there was no
marked change after 3 hrs.; but in the other there was a trace of
aggregation after 52 m., and this was plainly marked after 1 hr. 22 m.,
but even after 2 hrs. 12 m. there was certainly not more aggregation
than would have fol- [page 50] lowed from an immersion of from 5 m. to
10 m. in an equally strong solution of the carbonate.
Lastly, a leaf was placed in thirty minims of a solution of one part of
phosphate of ammonia to 43,750 of water (1 gr. to 100 oz.), so that it
received 1/1600 of a grain (.04079 mgr.); this soon caused the
tentacles to be strongly inflected; and after 24 hrs. the contents of
the cells were aggregated into oval and irregularly globular masses,
with a conspicuous current of protoplasm flowing round the walls. But
after so long an interval aggregation would have ensued, whatever had
caused inflection.
Only a few other salts, besides those of ammonia, were tried in
relation to the process of aggregation. A leaf was placed in a solution
of one part of chloride of sodium to 218 of water, and after 1 hr. the
contents of the cells were aggregated into small, irregularly globular,
brownish masses; these after 2 hrs. were almost disintegrated and
pulpy. It was evident that the protoplasm had been injuriously
affected; and soon afterwards some of the cells appeared quite empty.
These effects differ altogether from those produced by the several
salts of ammonia, as well as by various organic fluids, and by
inorganic particles placed on the glands. A solution of the same
strength of carbonate of soda and carbonate of potash acted in nearly
the same manner as the chloride; and here again, after 2 hrs. 30 m.,
the outer cells of some of the glands had emptied themselves of their
brown pulpy contents. We shall see in the eighth chapter that solutions
of several salts of soda of half the above strength cause inflection,
but do not injure the leaves. Weak solutions of sulphate of quinine, of
nicotine, camphor, poison of the cobra, &c., soon induce well-marked
aggregation; whereas certain other substances (for instance, a solution
of curare) have no such tendency.
Many acids, though much diluted, are poisonous; and though, as will be
shown in the eighth chapter, they cause the tentacles to bend, they do
not excite true aggregation. Thus leaves were placed in a solution of
one part of benzoic acid to 437 of water; and in 15 m. the purple fluid
within the cells had shrunk a little from the walls, yet when carefully
examined after 1 hr. 20 m., there was no true aggregation; and after 24
hrs. the leaf was evidently dead. Other leaves in iodic acid, diluted
to the same degree, showed after 2 hrs. 15 m. the same shrunken
appearance of the purple fluid within the cells; and these, after 6
hrs. 15 m., were seen under a high power to be filled with excessively
minute spheres of dull reddish protoplasm, [page 51] which by the next
morning, after 24 hrs., had almost disappeared, the leaf being
evidently dead. Nor was there any true aggregation in leaves immersed
in propionic acid of the same strength; but in this case the protoplasm
was collected in irregular masses towards the bases of the lower cells
of the tentacles.
A filtered infusion of raw meat induces strong aggregation, but not
very quickly. In one leaf thus immersed there was a little aggregation
after 1 hr. 20 m., and in another after 1 hr. 50 m. With other leaves a
considerably longer time was required: for instance, one immersed for 5
hrs. showed no aggregation, but was plainly acted on in 5 m.; when
placed in a few drops of a solution of one part of carbonate of ammonia
to 146 of water. Some leaves were left in the infusion for 24 hrs., and
these became aggregated to a wonderful degree, so that the inflected
tentacles presented to the naked eye a plainly mottled appearance. The
little masses of purple protoplasm were generally oval or beaded, and
not nearly so often spherical as in the case of leaves subjected to
carbonate of ammonia. They underwent incessant changes of form; and the
current of colourless protoplasm round the walls was conspicuously
plain after an immersion of 25 hrs. Raw meat is too powerful a
stimulant, and even small bits generally injure, and sometimes kill,
the leaves to which they are given: the aggregated masses of protoplasm
become dingy or almost colourless, and present an unusual granular
appearance, as is likewise the case with leaves which have been
immersed in a very strong solution of carbonate of ammonia. A leaf
placed in milk had the contents of its cells somewhat aggregated in 1
hr. Two other leaves, one immersed in human saliva for 2 hrs. 30 m.,
and another in unboiled white of egg for 1 hr. 30 m., were not action
on in this manner; though they undoubtedly would have been so, had more
time been allowed. These same two leaves, on being afterwards placed in
a solution of carbonate of ammonia (3 grs. to 1 oz.), had their cells
aggregated, the one in 10 m. and the other in 5 m.
Several leaves were left for 4 hrs. 30 m. in a solution of one part of
white sugar to 146 of water, and no aggregation ensued; on being placed
in a solution of this same strength of carbonate of ammonia, they were
acted on in 5 m.; as was likewise a leaf which had been left for 1 hr.
45 m. in a moderately thick solution of gum arabic. Several other
leaves were immersed for some hours in denser solutions of sugar, gum,
and starch, and they had the contents of their cells greatly
aggregated. This [page 52] effect may be attributed to exosmose; for
the leaves in the syrup became quite flaccid, and those in the gum and
starch somewhat flaccid, with their tentacles twisted about in the most
irregular manner, the longer ones like corkscrews. We shall hereafter
see that solutions of these substances, when placed on the discs of
leaves, do not incite inflection. Particles of soft sugar were added to
the secretion round several glands and were soon dissolved, causing a
great increase of the secretion, no doubt by exosmose; and after 24
hrs. the cells showed a certain amount of aggregation, though the
tentacles were not inflected. Glycerine causes in a few minutes
well-pronounced aggregation, commencing as usual within the glands and
then travelling down the tentacles; and this I presume may be
attributed to the strong attraction of this substance for water.
Immersion for several hours in water causes some degree of aggregation.
Twenty leaves were first carefully examined, and re-examined after
having been left immersed in distilled water for various periods, with
the following results. It is rare to find even a trace of aggregation
until 4 or 5 and generally not until several more hours have elapsed.
When however a leaf becomes quickly inflected in water, as sometimes
happens, especially during very warm weather, aggregation may occur in
little over 1 hr. In all cases leaves left in water for more than 24
hrs. have their glands blackened, which shows that their contents are
aggregated; and in the specimens which were carefully examined, there
was fairly well-marked aggregation in the upper cells of the pedicels.
These trials were made with cut off-leaves, and it occurred to me that
this circumstance might influence the result, as the footstalks would
not perhaps absorb water quickly enough to supply the glands as they
continued to secrete. But this view was proved erroneous, for a plant
with uninjured roots, bearing four leaves, was submerged in distilled
water for 47 hrs., and the glands were blackened, though the tentacles
were very little inflected. In one of these leaves there was only a
slight degree of aggregation in the tentacles; in the second rather
more, the purple contents of the cells being a little separated from
the walls; in the third and fourth, which were pale leaves, the
aggregation in the upper parts of the pedicels was well marked. In
these leaves the little masses of protoplasm, many of which were oval,
slowly changed their forms and positions; so that a submergence for 47
hrs. had not killed the protoplasm. In a previous trial with a
submerged plant, the tentacles were not in the least inflected. [page
53]
Heat induces aggregation. A leaf, with the cells of the tentacles
containing only homogeneous fluid, was waved about for 1 m. in water at
130° Fahr. (54°.4 Cent.) and was then examined under the microscope as
quickly as possible, that is in 2 m. or 3 m.; and by this time the
contents of the cells had undergone some degree of aggregation. A
second leaf was waved for 2 m. in water at 125° (51°.6 Cent.) and
quickly examined as before; the tentacles were well inflected; the
purple fluid in all the cells had shrunk a little from the walls, and
contained many oval and elongated masses of protoplasm, with a few
minute spheres. A third leaf was left in water at 125°, until it
cooled, and when examined after 1 hr. 45 m., the inflected tentacles
showed some aggregation, which became after 3 hrs. more strongly
marked, but did not subsequently increase. Lastly, a leaf was waved for
1 m. in water at 120° (48°.8 Cent.) and then left for 1 hr. 26 m. in
cold water; the tentacles were but little inflected, and there was only
here and there a trace of aggregation. In all these and other trials
with warm water the protoplasm showed much less tendency to aggregate
into spherical masses than when excited by carbonate of ammonia.
Redissolution of the Aggregated Masses of Protoplasm.—As soon as
tentacles which have clasped an insect or any inorganic object, or have
been in any way excited, have fully re-expanded, the aggregated masses
of protoplasm are redissolved and disappear; the cells being now
refilled with homogeneous purple fluid as they were before the
tentacles were inflected. The process of redissolution in all cases
commences at the bases of the tentacles, and proceeds up them towards
the glands. In old leaves, however, especially in those which have been
several times in action, the protoplasm in the uppermost cells of the
pedicels remains in a permanently more or less aggregated condition. In
order to observe the process of redissolution, the following
observations were made: a leaf was left for 24 hrs. in a little
solution of one part of carbonate of ammonia to 218 of water, and the
protoplasm was as usual aggregated into numberless purple spheres,
which were incessantly changing their forms. The leaf was then washed
and placed in distilled water, and after 3 hrs. 15 m. some few of the
spheres began to show by their less clearly defined edges signs of
redissolution. After 9 hrs. many of them had become elongated, and the
surrounding fluid in the cells was slightly more coloured, showing
plainly that redissolution had commenced. After 24 hrs., though many
cells still contained spheres, here and there one [page 54] could be
seen filled with purple fluid, without a vestige of aggregated
protoplasm; the whole having been redissolved. A leaf with aggregated
masses, caused by its having been waved for 2 m. in water at the
temperature of 125° Fahr., was left in cold water, and after 11 hrs.
the protoplasm showed traces of incipient redissolution. When again
examined three days after its immersion in the warm water, there was a
conspicuous difference, though the protoplasm was still somewhat
aggregated. Another leaf, with the contents of all the cells strongly
aggregated from the action of a weak solution of phosphate of ammonia,
was left for between three and four days in a mixture (known to be
innocuous) of one drachm of alcohol to eight drachms of water, and when
re-examined every trace of aggregation had disappeared, the cells being
now filled with homogeneous fluid.
We have seen that leaves immersed for some hours in dense solutions of
sugar, gum, and starch, have the contents of their cells greatly
aggregated, and are rendered more or less flaccid, with the tentacles
irregularly contorted. These leaves, after being left for four days in
distilled water, became less flaccid, with their tentacles partially
re-expanded, and the aggregated masses of protoplasm were partially
redissolved. A leaf with its tentacles closely clasped over a fly, and
with the contents of the cells strongly aggregated, was placed in a
little sherry wine; after 2 hrs. several of the tentacles had
re-expanded, and the others could by a mere touch be pushed back into
their properly expanded positions, and now all traces of aggregation
had disappeared, the cells being filled with perfectly homogeneous pink
fluid. The redissolution in these cases may, I presume, be attributed
to endosmose.]
_On the Proximate Causes of the Process of Aggregation._
As most of the stimulants which cause the inflection of the tentacles
likewise induce aggregation in the contents of their cells, this latter
process might be thought to be the direct result of inflection; but
this is not the case. If leaves are placed in rather strong solutions
of carbonate of ammonia, for instance of three or four, and even
sometimes of only two grains to the ounce of water (i.e. one part to
109, or 146, or [page 55] 218, of water), the tentacles are paralysed,
and do not become inflected, yet they soon exhibit strongly marked
aggregation. Moreover, the short central tentacles of a leaf which has
been immersed in a weak solution of any salt of ammonia, or in any
nitrogenous organic fluid, do not become in the least inflected;
nevertheless they exhibit all the phenomena of aggregation. On the
other hand, several acids cause strongly pronounced inflection, but no
aggregation.
It is an important fact that when an organic or inorganic object is
placed on the glands of the disc, and the exterior tentacles are thus
caused to bend inwards, not only is the secretion from the glands of
the latter increased in quantity and rendered acid, but the contents of
the cells of their pedicels become aggregated. The process always
commences in the glands, although these have not as yet touched any
object. Some force or influence must, therefore, be transmitted from
the central glands to the exterior tentacles, first to near their bases
causing this part to bend, and next to the glands causing them to
secrete more copiously. After a short time the glands, thus indirectly
excited, transmit or reflect some influence down their own pedicels,
inducing aggregation in cell beneath cell to their bases.
It seems at first sight a probable view that aggregation is due to the
glands being excited to secrete more copiously, so that sufficient
fluid is not left in their cells, and in the cells of the pedicels, to
hold the protoplasm in solution. In favour of this view is the fact
that aggregation follows the inflection of the tentacles, and during
the movement the glands generally, or, as I believe, always, secrete
more copiously than they did before. Again, during the re-expansion
[page 56] of the tentacles, the glands secrete less freely, or quite
cease to secrete, and the aggregated masses of protoplasm are then
redissolved. Moreover, when leaves are immersed in dense vegetable
solutions, or in glycerine, the fluid within the gland-cells passes
outwards, and there is aggregation; and when the leaves are afterwards
immersed in water, or in an innocuous fluid of less specific gravity
than water, the protoplasm is redissolved, and this, no doubt, is due
to endosmose.
Opposed to this view, that aggregation is caused by the outward passage
of fluid from the cells, are the following facts. There seems no close
relation between the degree of increased secretion and that of
aggregation. Thus a particle of sugar added to the secretion round a
gland causes a much greater increase of secretion, and much less
aggregation, than does a particle of carbonate of ammonia given in the
same manner. It does not appear probable that pure water would cause
much exosmose, and yet aggregation often follows from an immersion in
water of between 16 hrs. and 24 hrs., and always after from 24 hrs. to
48 hrs. Still less probable is it that water at a temperature of from
125° to 130° Fahr. (51°.6 to 54°.4 Cent.) should cause fluid to pass,
not only from the glands, but from all the cells of the tentacles down
to their bases, so quickly that aggregation is induced within 2 m. or 3
m. Another strong argument against this view is, that, after complete
aggregation, the spheres and oval masses of protoplasm float about in
an abundant supply of thin colourless fluid; so that at least the
latter stages of the process cannot be due to the want of fluid to hold
the protoplasm in solution. There is still stronger evidence that
aggregation is independent of secretion; for the papillae, described in
the first chapter, with which the [page 57] leaves are studded are not
glandular, and do not secrete, yet they rapidly absorb carbonate of
ammonia or an infusion of raw meat, and their contents then quickly
undergo aggregation, which afterwards spreads into the cells of the
surrounding tissues. We shall hereafter see that the purple fluid
within the sensitive filaments of Dionaea, which do not secrete,
likewise undergoes aggregation from the action of a weak solution of
carbonate of ammonia.
The process of aggregation is a vital one; by which I mean that the
contents of the cells must be alive and uninjured to be thus affected,
and they must be in an oxygenated condition for the transmission of the
process at the proper rate. Some tentacles in a drop of water were
strongly pressed beneath a slip of glass; many of the cells were
ruptured, and pulpy matter of a purple colour, with granules of all
sizes and shapes, exuded, but hardly any of the cells were completely
emptied. I then added a minute drop of a solution of one part of
carbonate of ammonia to 109 of water, and after 1 hr. examined the
specimens. Here and there a few cells, both in the glands and in the
pedicels, had escaped being ruptured, and their contents were well
aggregated into spheres which were constantly changing their forms and
positions, and a current could still be seen flowing along the walls;
so that the protoplasm was alive. On the other hand, the exuded matter,
which was now almost colourless instead of being purple, did not
exhibit a trace of aggregation. Nor was there a trace in the many cells
which were ruptured, but which had not been completely emptied of their
contents. Though I looked carefully, no signs of a current could be
seen within these ruptured cells. They had evidently been killed by the
pressure; and the matter which they [page 58] still contained did not
undergo aggregation any more than that which had exuded. In these
specimens, as I may add, the individuality of the life of each cell was
well illustrated.
A full account will be given in the next chapter of the effects of heat
on the leaves, and I need here only state that leaves immersed for a
short time in water at a temperature of 120° Fahr. (48°.8 Cent.),
which, as we have seen, does not immediately induce aggregation, were
then placed in a few drops of a strong solution of one part of
carbonate of ammonia to 109 of water, and became finely aggregated. On
the other hand, leaves, after an immersion in water at 150° (65°.5
Cent.), on being placed in the same strong solution, did not undergo
aggregation, the cells becoming filled with brownish, pulpy, or muddy
matter. With leaves subjected to temperatures between these two
extremes of 120° and 150° Fahr. (48°.8 and 65°.5 Cent.), there were
gradations in the completeness of the process; the former temperature
not preventing aggregation from the subsequent action of carbonate of
ammonia, the latter quite stopping it. Thus, leaves immersed in water,
heated to 130° (54°.4 Cent.), and then in the solution, formed
perfectly defined spheres, but these were decidedly smaller than in
ordinary cases. With other leaves heated to 140° (60° Cent.), the
spheres were extremely small, yet well defined, but many of the cells
contained, in addition, some brownish pulpy matter. In two cases of
leaves heated to 145° (62°.7 Cent.), a few tentacles could be found
with some of their cells containing a few minute spheres; whilst the
other cells and other whole tentacles included only the brownish,
disintegrated or pulpy matter.
The fluid within the cells of the tentacles must be in an oxygenated
condition, in order that the force or [page 59] influence which induces
aggregation should be transmitted at the proper rate from cell to cell.
A plant, with its roots in water, was left for 45 m. in a vessel
containing 122 oz. of carbonic acid. A leaf from this plant, and, for
comparison, one from a fresh plant, were both immersed for 1 hr. in a
rather strong solution of carbonate of ammonia. They were then
compared, and certainly there was much less aggregation in the leaf
which had been subjected to the carbonic acid than in the other.
Another plant was exposed in the same vessel for 2 hrs. to carbonic
acid, and one of its leaves was then placed in a solution of one part
of the carbonate to 437 of water; the glands were instantly blackened,
showing that they had absorbed, and that their contents were
aggregated; but in the cells close beneath the glands there was no
aggregation even after an interval of 3 hrs. After 4 hrs. 15 m. a few
minute spheres of protoplasm were formed in these cells, but even after
5 hrs. 30 m. the aggregation did not extend down the pedicels for a
length equal to that of the glands. After numberless trials with fresh
leaves immersed in a solution of this strength, I have never seen the
aggregating action transmitted at nearly so slow a rate. Another plant
was left for 2 hrs. in carbonic acid, but was then exposed for 20 m. to
the open air, during which time the leaves, being of a red colour,
would have absorbed some oxygen. One of them, as well as a fresh leaf
for comparison, were now immersed in the same solution as before. The
former were looked at repeatedly, and after an interval of 65 m. a few
spheres of protoplasm were first observed in the cells close beneath
the glands, but only in two or three of the longer tentacles. After 3
hrs. the aggregation had travelled down the pedicels of a few of the
tentacles [page 60] for a length equal to that of the glands. On the
other hand, in the fresh leaf similarly treated, aggregation was plain
in many of the tentacles after 15 m.; after 65 m. it had extended down
the pedicels for four, five, or more times the lengths of the glands;
and after 3 hrs. the cells of all the tentacles were affected for
one-third or one-half of their entire lengths. Hence there can be no
doubt that the exposure of leaves to carbonic acid either stops for a
time the process of aggregation, or checks the transmission of the
proper influence when the glands are subsequently excited by carbonate
of ammonia; and this substance acts more promptly and energetically
than any other. It is known that the protoplasm of plants exhibits its
spontaneous movements only as long as it is in an oxygenated condition;
and so it is with the white corpuscles of the blood, only as long as
they receive oxygen from the red corpuscles;* but the cases above given
are somewhat different, as they relate to the delay in the generation
or aggregation of the masses of protoplasm by the exclusion of oxygen.
A Summary and Concluding Remarks.—The process of aggregation is
independent of the inflection of the tentacles and of increased
secretion from the glands. It commences within the glands, whether
these have been directly excited, or indirectly by a stimulus received
from other glands. In both cases the process is transmitted from cell
to cell down the whole length of the tentacles, being arrested for a
short time at each transverse partition. With pale-coloured leaves the
first change which is perceptible, but only
* With respect to plants, Sachs, ‘Traité de Bot.’ 3rd edit., 1874, p.
864. On blood corpuscles, see ‘Quarterly Journal of Microscopical
Science,’ April 1874, p. 185.’ [page 61]
under a high power, is the appearance of the finest granules in the
fluid within the cells, making it slightly cloudy. These granules soon
aggregate into small globular masses. I have seen a cloud of this kind
appear in 10 s. after a drop of a solution of carbonate of ammonia had
been given to a gland. With dark red leaves the first visible change
often is the conversion of the outer layer of the fluid within the
cells into bag-like masses. The aggregated masses, however they may
have been developed, incessantly change their forms and positions. They
are not filled with fluid, but are solid to their centres. Ultimately
the colourless granules in the protoplasm which flows round the walls
coalesce with the central spheres or masses; but there is still a
current of limpid fluid flowing within the cells. As soon as the
tentacles fully re-expand, the aggregated masses are redissolved, and
the cells become filled with homogeneous purple fluid, as they were at
first. The process of redissolution commences at the bases of the
tentacles, thence proceeding upwards to the glands; and, therefore, in
a reversed direction to that of aggregation.
Aggregation is excited by the most diversified causes,—by the glands
being several times touched,—by the pressure of particles of any kind,
and as these are supported by the dense secretion, they can hardly
press on the glands with the weight of a millionth of a grain,*—by the
tentacles being cut off close beneath
* According to Hofmeister (as quoted by Sachs, ‘Traité de Bot.’ 1874,
p. 958), very slight pressure on the cell-membrane arrests immediately
the movements of the protoplasm, and even determines its separation
from the walls. But the process of aggregation is a different
phenomenon, as it relates to the contents of the cells, and only
secondarily to the layer of protoplasm which flows along the walls;
though no doubt the effects of pressure or of a touch on the outside
must be transmitted through this layer. [page 62]
the glands,—by the glands absorbing various fluids or matter dissolved
out of certain bodies,—by exosmose,—and by a certain degree of heat. On
the other hand, a temperature of about 150° Fahr. (65°.5 Cent.) does
not excite aggregation; nor does the sudden crushing of a gland. If a
cell is ruptured, neither the exuded matter nor that which still
remains within the cell undergoes aggregation when carbonate of ammonia
is added. A very strong solution of this salt and rather large bits of
raw meat prevent the aggregated masses being well developed. From these
facts we may conclude that the protoplasmic fluid within a cell does
not become aggregated unless it be in a living state, and only
imperfectly if the cell has been injured. We have also seen that the
fluid must be in an oxygenated state, in order that the process of
aggregation should travel from cell to cell at the proper rate.
Various nitrogenous organic fluids and salts of ammonia induce
aggregation, but in different degrees and at very different rates.
Carbonate of ammonia is the most powerful of all known substances; the
absorption of 1/134400 of a grain (.000482 mg.) by a gland suffices to
cause all the cells of the same tentacle to become aggregated. The
first effect of the carbonate and of certain other salts of ammonia, as
well as of some other fluids, is the darkening or blackening of the
glands. This follows even from long immersion in cold distilled water.
It apparently depends in chief part on the strong aggregation of their
cell-contents, which thus become opaque, and do not reflect light. Some
other fluids render the glands of a brighter red; whilst certain acids,
though much diluted, the poison of the cobra-snake, &c., make the
glands perfectly white and opaque; and this seems to depend on the
coagulation of their contents without [page 63] any aggregation.
Nevertheless, before being thus affected, they are able, at least in
some cases, to excite aggregation in their own tentacles.
That the central glands, if irritated, send centrifugally some
influence to the exterior glands, causing them to send back a
centripetal influence inducing aggregation, is perhaps the most
interesting fact given in this chapter. But the whole process of
aggregation is in itself a striking phenomenon. Whenever the peripheral
extremity of a nerve is touched or pressed, and a sensation is felt, it
is believed that an invisible molecular change is sent from one end of
the nerve to the other; but when a gland of Drosera is repeatedly
touched or gently pressed, we can actually see a molecular change
proceeding from the gland down the tentacle; though this change is
probably of a very different nature from that in a nerve. Finally, as
so many and such widely different causes excite aggregation, it would
appear that the living matter within the gland-cells is in so unstable
a condition that almost any disturbance suffices to change its
molecular nature, as in the case of certain chemical compounds. And
this change in the glands, whether excited directly, or indirectly by a
stimulus received from other glands, is transmitted from cell to cell,
causing granules of protoplasm either to be actually generated in the
previously limpid fluid or to coalesce and thus to become visible.
Supplementary Observations on the Process of Aggregation in the Roots
of Plants.
It will hereafter be seen that a weak solution of the carbonate of
ammonia induces aggregation in the cells of the roots of Drosera; and
this led me to make a few trials on the roots of other plants. I dug up
in the latter part of October the first weed which I met with, viz.
Euphorbia peplus, being care- [page 64] ful not to injure the roots;
these were washed and placed in a little solution of one part of
carbonate of ammonia to 146 of water. In less than one minute I saw a
cloud travelling from cell to cell up the roots, with wonderful
rapidity. After from 8 m. to 9 m. the fine granules, which caused this
cloudy appearance, became aggregated towards the extremities of the
roots into quadrangular masses of brown matter; and some of these soon
changed their forms and became spherical. Some of the cells, however,
remained unaffected. I repeated the experiment with another plant of
the same species, but before I could get the specimen into focus under
the microscope, clouds of granules and quadrangular masses of reddish
and brown matter were formed, and had run far up all the roots. A fresh
root was now left for 18 hrs. in a drachm of a solution of one part of
the carbonate to 437 of water, so that it received 1/8 of a grain, or
2.024 mg. When examined, the cells of all the roots throughout their
whole length contained aggregated masses of reddish and brown matter.
Before making these experiments, several roots were closely examined,
and not a trace of the cloudy appearance or of the granular masses
could be seen in any of them. Roots were also immersed for 35 m. in a
solution of one part of carbonate of potash to 218 of water; but this
salt produced no effect.
I may here add that thin slices of the stem of the Euphorbia were
placed in the same solution, and the cells which were green instantly
became cloudy, whilst others which were before colourless were clouded
with brown, owing to the formation of numerous granules of this tint. I
have also seen with various kinds of leaves, left for some time in a
solution of carbonate of ammonia, that the grains of chlorophyll ran
together and partially coalesced; and this seems to be a form of
aggregation.
Plants of duck-weed (Lemna) were left for between 30 m. and 45 m. in a
solution of one part of this same salt to 146 of water, and three of
their roots were then examined. In two of them, all the cells which had
previously contained only limpid fluid now included little green
spheres. After from 1 1/2 hr. to 2 hrs. similar spheres appeared in the
cells on the borders of the leaves; but whether the ammonia had
travelled up the roots or had been directly absorbed by the leaves, I
cannot say. As one species, Lemna arrhiza, produces no roots, the
latter alternative is perhaps the most probable. After about 2 1/2 hrs.
some of the little green spheres in the roots were broken up into small
granules which exhibited Brownian movements. Some duck-weed was also
left for 1 hr. 30 m. in a solution of one part of [page 65] carbonate
of potash to 218 of water, and no decided change could be perceived in
the cells of the roots; but when these same roots were placed for 25 m.
in a solution of carbonate of ammonia of the same strength, little
green spheres were formed.
A green marine alga was left for some time in this same solution, but
was very doubtfully affected. On the other hand, a red marine alga,
with finely pinnated fronds, was strongly affected. The contents of the
cells aggregated themselves into broken rings, still of a red colour,
which very slowly and slightly changed their shapes, and the central
spaces within these rings became cloudy with red granular matter. The
facts here given (whether they are new, I know not) indicate that
interesting results would perhaps be gained by observing the action of
various saline solutions and other fluids on the roots of plants. [page
66]
CHAPTER IV.
THE EFFECTS OF HEAT ON THE LEAVES.
Nature of the experiments—Effects of boiling water—Warm water causes
rapid inflection—Water at a higher temperature does not cause immediate
inflection, but does not kill the leaves, as shown by their subsequent
re-expansion and by the aggregation of the protoplasm—A still higher
temperature kills the leaves and coagulates the albuminous contents of
the glands.
In my observations on Drosera rotundifolia, the leaves seemed to be
more quickly inflected over animal substances, and to remain inflected
for a longer period during very warm than during cold weather. I
wished, therefore, to ascertain whether heat alone would induce
inflection, and what temperature was the most efficient. Another
interesting point presented itself, namely, at what degree life was
extinguished; for Drosera offers unusual facilities in this respect,
not in the loss of the power of inflection, but in that of subsequent
re-expansion, and more especially in the failure of the protoplasm to
become aggregated, when the leaves after being heated are immersed in a
solution of carbonate of ammonia.*
* When my experiments on the effects of heat were made, I was not aware
that the subject had been carefully investigated by several observers.
For instance, Sachs is convinced (‘Traité de Botanique,’ 1874, pp. 772,
854) that the most different kinds of plants all perish if kept for 10
m. in water at 45° to 46° Cent., or 113° to 115° Fahr.; and he
concludes that the protoplasm within their cells always coagulates, if
in a damp condition, at a temperature of between 50° and 60° Cent., or
122° to 140° Fahr. Max Schultze and Kühne (as quoted by Dr. Bastian in
‘Contemp. Review,’ 1874, p. 528) “found that the protoplasm of
plant-cells, with which they experimented, was always killed and [[page
67]] altered by a very brief exposure to a temperature of 118 1/2°
Fahr. as a maximum.” As my results are deduced from special phenomena,
namely, the subsequent aggregation of the protoplasm and the
re-expansion of the tentacles, they seem to me worth giving. We shall
find that Drosera resists heat somewhat better than most other plants.
That there should be considerable differences in this respect is not
surprising, considering that some low vegetable organisms grow in hot
springs—cases of which have been collected by Prof. Wyman (‘American
Journal of Science,’ vol. xliv. 1867). Thus, Dr. Hooker found Confervae
in water at 168° Fahr.; Humboldt, at 185° Fahr.; and Descloizeaux, at
208° Fahr.) [page 67]
[My experiments were tried in the following manner. Leaves were cut
off, and this does not in the least interfere with their powers; for
instance, three cut off leaves, with bits of meat placed on them, were
kept in a damp atmosphere, and after 23 hrs. closely embraced the meat
both with their tentacles and blades; and the protoplasm within their
cells was well aggregated. Three ounces of doubly distilled water was
heated in a porcelain vessel, with a delicate thermometer having a long
bulb obliquely suspended in it. The water was gradually raised to the
required temperature by a spirit-lamp moved about under the vessel; and
in all cases the leaves were continually waved for some minutes close
to the bulb. They were then placed in cold water, or in a solution of
carbonate of ammonia. In other cases they were left in the water, which
had been raised to a certain temperature, until it cooled. Again in
other cases the leaves were suddenly plunged into water of a certain
temperature, and kept there for a specified time. Considering that the
tentacles are extremely delicate, and that their coats are very thin,
it seems scarcely possible that the fluid contents of their cells
should not have been heated to within a degree or two of the
temperature of the surrounding water. Any further precautions would, I
think, have been superfluous, as the leaves from age or constitutional
causes differ slightly in their sensitiveness to heat.
It will be convenient first briefly to describe the effects of
immersion for thirty seconds in boiling water. The leaves are rendered
flaccid, with their tentacles bowed backwards, which, as we shall see
in a future chapter, is probably due to their outer surfaces retaining
their elasticity for a longer period than their inner surfaces retain
the power of contraction. The purple fluid within the cells of the
pedicels is rendered finely granular, but there is no true aggregation;
nor does this follow [page 68] when the leaves are subsequently placed
in a solution of carbonate of ammonia. But the most remarkable change
is that the glands become opaque and uniformly white; and this may be
attributed to the coagulation of their albuminous contents.
My first and preliminary experiment consisted in putting seven leaves
in the same vessel of water, and warming it slowly up to the
temperature of 110° Fahr. (43°.3 Cent.); a leaf being taken out as soon
as the temperature rose to 80° (26°.6 Cent.), another at 85°, another
at 90°, and so on. Each leaf, when taken out, was placed in water at
the temperature of my room, and the tentacles of all soon became
slightly, though irregularly, inflected. They were now removed from the
cold water and kept in damp air, with bits of meat placed on their
discs. The leaf which had been exposed to the temperature of 110o
became in 15 m. greatly inflected; and in 2 hrs. every single tentacle
closely embraced the meat. So it was, but after rather longer
intervals, with the six other leaves. It appears, therefore, that the
warm bath had increased their sensitiveness when excited by meat.
I next observed the degree of inflection which leaves underwent within
stated periods, whilst still immersed in warm water, kept as nearly as
possible at the same temperature; but I will here and elsewhere give
only a few of the many trials made. A leaf was left for 10 m. in water
at 100° (37°.7 Cent.), but no inflection occurred. A second leaf,
however, treated in the same manner, had a few of its exterior
tentacles very slightly inflected in 6 m., and several irregularly but
not closely inflected in 10 m. A third leaf, kept in water at 105° to
106° (40°.5 to 41°.1 Cent.), was very moderately inflected in 6 m. A
fourth leaf, in water at 110° (43°.3 Cent.), was somewhat inflected in
4 m., and considerably so in from 6 to 7 m.
Three leaves were placed in water which was heated rather quickly, and
by the time the temperature rose to 115°-116° (46°.1 to 46°.06 Cent.),
all three were inflected. I then removed the lamp, and in a few minutes
every single tentacle was closely inflected. The protoplasm within the
cells was not killed, for it was seen to be in distinct movement; and
the leaves, having been left in cold water for 20 hrs., re-expanded.
Another leaf was immersed in water at 100o (37.7° Cent.), which was
raised to 120° (48°.8 Cent.); and all the tentacles, except the extreme
marginal ones, soon became closely inflected. The leaf was now placed
in cold water, and in 7 hrs. 30 m. it had partly, and in 10 hrs. fully,
re-expanded. On the following morning it was immersed in a weak
solution of carbonate of [page 69] ammonia, and the glands quickly
became black, with strongly marked aggregation in the tentacles,
showing that the protoplasm was alive, and that the glands had not lost
their power of absorption. Another leaf was placed in water at 110o
(43°.3 Cent.) which was raised to 120° (48°.8 Cent.); and every
tentacle, excepting one, was quickly and closely inflected. This leaf
was now immersed in a few drops of a strong solution of carbonate of
ammonia (one part to 109 of water); in 10 m. all the glands became
intensely black, and in 2 hrs. the protoplasm in the cells of the
pedicels was well aggregated. Another leaf was suddenly plunged, and as
usual waved about, in water at 120°, and the tentacles became inflected
in from 2 m. to 3 m., but only so as to stand at right angles to the
disc. The leaf was now placed in the same solution (viz. one part of
carbonate of ammonia to 109 of water, or 4 grs. to 1 oz., which I will
for the future designate as the strong solution), and when I looked at
it again after the interval of an hour, the glands were blackened, and
there was well-marked aggregation. After an additional interval of 4
hrs. the tentacles had become much more inflected. It deserves notice
that a solution as strong as this never causes inflection in ordinary
cases. Lastly a leaf was suddenly placed in water at 125° (51°.6
Cent.), and was left in it until the water cooled; the tentacles were
rendered of a bright red and soon became inflected. The contents of the
cells underwent some degree of aggregation, which in the course of
three hours increased; but the masses of protoplasm did not become
spherical, as almost always occurs with leaves immersed in a solution
of carbonate of ammonia.]
We learn from these cases that a temperature of from 120° to 125°
(48°.8 to 51°.6 Cent.) excites the tentacles into quick movement, but
does not kill the leaves, as shown either by their subsequent
re-expansion or by the aggregation of the protoplasm. We shall now see
that a temperature of 130° (54°.4 Cent.) is too high to cause immediate
inflection, yet does not kill the leaves.
[Experiment 1.—A leaf was plunged, and as in all cases waved about for
a few minutes, in water at 130° (54°.4 Cent.), but there was no trace
of inflection; it was then placed in cold water, and after an interval
of 15 m. very slow movement was [page 70] distinctly seen in a small
mass of protoplasm in one of the cells of a tentacle.* After a few
hours all the tentacles and the blade became inflected.
Experiment 2.—Another leaf was plunged into water at 130o to 131o, and
as before there was no inflection. After being kept in cold water for
an hour, it was placed in the strong solution of ammonia, and in the
course of 55 m. the tentacles were considerably inflected. The glands,
which before had been rendered of a brighter red, were now blackened.
The protoplasm in the cells of the tentacles was distinctly aggregated;
but the spheres were much smaller than those generated in unheated
leaves when subjected to carbonate of ammonia. After an additional 2
hrs. all the tentacles, excepting six or seven, were closely inflected.
Experiment 3.—A similar experiment to the last, with exactly the same
results.
Experiment 4.—A fine leaf was placed in water at 100° (37°.7 Cent.),
which was then raised to 145° (62°.7 Cent.). Soon after immersion,
there was, as might have been expected, strong inflection. The leaf was
now removed and left in cold water; but from having been exposed to so
high a temperature, it never re-expanded.
Experiment 5.—Leaf immersed at 130° (54°.4 Cent.), and the water raised
to 145° (62°.7 Cent.), there was no immediate inflection; it was then
placed in cold water, and after 1 hr. 20 m. some of the tentacles on
one side became inflected. This leaf was now placed in the strong
solution, and in 40 m. all the submarginal tentacles were well
inflected, and the glands blackened. After an additional interval of 2
hrs. 45 m. all the tentacles, except eight or ten, were closely
inflected, with their cells exhibiting a slight degree of aggregation;
but the spheres of protoplasm were very small, and the cells of the
exterior tentacles contained some pulpy or disintegrated brownish
matter.
Experiments 6 and 7.—Two leaves were plunged in water at 135° (57°.2
Cent.) which was raised to 145° (62°.7 Cent.); neither became
inflected. One of these, however, after having been left for 31 m. in
cold water, exhibited some slight inflection, which increased after an
additional interval of 1 hr. 45 m., until
* Sachs states (‘Traité de Botanique,’ 1874, p. 855) that the movements
of the protoplasm in the hairs of a Cucurbita ceased after they were
exposed for 1 m. in water to a temperature of 47° to 48° Cent., or 117°
to 119° Fahr. [page 71]
all the tentacles, except sixteen or seventeen, were more or less
inflected; but the leaf was so much injured that it never re-expanded.
The other leaf, after having been left for half an hour in cold water,
was put into the strong solution, but no inflection ensued; the glands,
however, were blackened, and in some cells there was a little
aggregation, the spheres of protoplasm being extremely small; in other
cells, especially in the exterior tentacles, there was much
greenish-brown pulpy matter.
Experiment 8.—A leaf was plunged and waved about for a few minutes in
water at 140° (60° Cent.), and was then left for half an hour in cold
water, but there was no inflection. It was now placed in the strong
solution, and after 2 hrs. 30 m. the inner submarginal tentacles were
well inflected, with their glands blackened, and some imperfect
aggregation in the cells of the pedicels. Three or four of the glands
were spotted with the white porcelain-like structure, like that
produced by boiling water. I have seen this result in no other instance
after an immersion of only a few minutes in water at so low a
temperature as 140°, and in only one leaf out of four, after a similar
immersion at a temperature of 145° Fahr. On the other hand, with two
leaves, one placed in water at 145° (62°.7 Cent.), and the other in
water at 140° (60° Cent.), both being left therein until the water
cooled, the glands of both became white and porcelain-like. So that the
duration of the immersion is an important element in the result.
Experiment 9.—A leaf was placed in water at 140° (60° Cent.), which was
raised to 150° (65°.5 Cent.); there was no inflection; on the contrary,
the outer tentacles were somewhat bowed backwards. The glands became
like porcelain, but some of them were a little mottled with purple. The
bases of the glands were often more affected than their summits. This
leaf having been left in the strong solution did not undergo any
inflection or aggregation.
Experiment 10.—A leaf was plunged in water at 150° to 150 1/2° (65°.5
Cent.); it became somewhat flaccid, with the outer tentacles slightly
reflexed, and the inner ones a little bent inwards, but only towards
their tips; and this latter fact shows that the movement was not one of
true inflection, as the basal part alone normally bends. The tentacles
were as usual rendered of a very bright red, with the glands almost
white like porcelain, yet tinged with pink. The leaf having been placed
in the strong solution, the cell-contents of the tentacles became of a
muddy-brown, with no trace of aggregation. [page 72]
Experiment 11.—A leaf was immersed in water at 145° (62°.7 Cent.),
which was raised to 156° (68°.8 Cent.). The tentacles became bright red
and somewhat reflexed, with almost all the glands like porcelain; those
on the disc being still pinkish, those near the margin quite white. The
leaf being placed as usual first in cold water and then in the strong
solution, the cells in the tentacles became of a muddy greenish brown,
with the protoplasm not aggregated. Nevertheless, four of the glands
escaped being rendered like porcelain, and the pedicels of these glands
were spirally curled, like a French horn, towards their upper ends; but
this can by no means be considered as a case of true inflection. The
protoplasm within the cells of the twisted portions was aggregated into
distinct though excessively minute purple spheres. This case shows
clearly that the protoplasm, after having been exposed to a high
temperature for a few minutes, is capable of aggregation when
afterwards subjected to the action of carbonate of ammonia, unless the
heat has been sufficient to cause coagulation.]
Concluding Remarks.—As the hair-like tentacles are extremely thin and
have delicate walls, and as the leaves were waved about for some
minutes close to the bulb of the thermometer, it seems scarcely
possible that they should not have been raised very nearly to the
temperature which the instrument indicated. From the eleven last
observations we see that a temperature of 130° (54°.4 Cent.) never
causes the immediate inflection of the tentacles, though a temperature
from 120° to 125° (48°.8 to 51°.6 Cent.) quickly produces this effect.
But the leaves are paralysed only for a time by a temperature of 130°,
as afterwards, whether left in simple water or in a solution of
carbonate of ammonia, they become inflected and their protoplasm
undergoes aggregation. This great difference in the effects of a higher
and lower temperature may be compared with that from immersion in
strong and weak solutions of the salts of ammonia; for the former do
not excite movement, whereas the latter act energetically. A temporary
suspension of the [page 73] power of movement due to heat is called by
Sachs* heat-rigidity; and this in the case of the sensitive-plant
(Mimosa) is induced by its exposure for a few minutes to humid air,
raised to 120°-122° Fahr., or 49° to 50° Cent. It deserves notice that
the leaves of Drosera, after being immersed in water at 130° Fahr., are
excited into movement by a solution of the carbonate so strong that it
would paralyse ordinary leaves and cause no inflection.
The exposure of the leaves for a few minutes even to a temperature of
145° Fahr. (62°.7 Cent.) does not always kill them; as when afterwards
left in cold water, or in a strong solution of carbonate of ammonia,
they generally, though not always, become inflected; and the protoplasm
within their cells undergoes aggregation, though the spheres thus
formed are extremely small, with many of the cells partly filled with
brownish muddy matter. In two instances, when leaves were immersed in
water, at a lower temperature than 130° (54°.4 Cent.), which was then
raised to 145° (62°.7 Cent.), they became during the earlier period of
immersion inflected, but on being afterwards left in cold water were
incapable of re-expansion. Exposure for a few minutes to a temperature
of 145o sometimes causes some few of the more sensitive glands to be
speckled with the porcelain-like appearance; and on one occasion this
occurred at a temperature of 140° (60° Cent.). On another occasion,
when a leaf was placed in water at this temperature of only 140o, and
left therein till the water cooled, every gland became like porcelain.
Exposure for a few minutes to a temperature of 150° (65°.5 Cent.)
generally produces this effect, yet many glands retain a
* ‘Traité de Bot.’ 1874, p. 1034. [page 74]
pinkish colour, and many present a speckled appearance. This high
temperature never causes true inflection; on the contrary, the
tentacles commonly become reflexed, though to a less degree than when
immersed in boiling water; and this apparently is due to their passive
power of elasticity. After exposure to a temperature of 150° Fahr., the
protoplasm, if subsequently subjected to carbonate of ammonia, instead
of undergoing aggregation, is converted into disintegrated or pulpy
discoloured matter. In short, the leaves are generally killed by this
degree of heat; but owing to differences of age or constitution, they
vary somewhat in this respect. In one anomalous case, four out of the
many glands on a leaf, which had been immersed in water raised to 156°
(68°.8 Cent.), escaped being rendered porcellanous;* and the protoplasm
in the cells close beneath these glands underwent some slight, though
imperfect, degree of aggregation.
Finally, it is a remarkable fact that the leaves of Drosera
rotundifolia, which flourishes on bleak upland moors throughout Great
Britain, and exists (Hooker) within the Arctic Circle, should be able
to withstand for even a short time immersion in water heated to a
temperature of 145°.**
It may be worth adding that immersion in cold
* As the opacity and porcelain-like appearance of the glands is
probably due to the coagulation of the albumen, I may add, on the
authority of Dr. Burdon Sanderson, that albumen coagulates at about
155o, but, in presence of acids, the temperature of coagulation is
lower. The leaves of Drosera contain an acid, and perhaps a difference
in the amount contained may account for the slight differences in the
results above recorded.
** It appears that cold-blooded animals are, as might have been
expected, far more sensitive to an increase of temperature than is
Drosera. Thus, as I hear from Dr. Burdon Sanderson, a frog begins to be
distressed in water at a temperature of only 85° Fahr. At 95° the
muscles become rigid, and the animal dies in a stiffened condition.
[page 75]
water does not cause any inflection: I suddenly placed four leaves,
taken from plants which had been kept for several days at a high
temperature, generally about 75° Fahr. (23°.8 Cent.), in water at 45°
(7°.2 Cent.), but they were hardly at all affected; not so much as some
other leaves from the same plants, which were at the same time immersed
in water at 75°; for these became in a slight degree inflected. [page
76]
CHAPTER V.
THE EFFECTS OF NON-NITROGENOUS AND NITROGENOUS ORGANIC FLUIDS ON THE
LEAVES.
Non-nitrogenous fluids—Solutions of gum arabic—Sugar—Starch—Diluted
alcohol—Olive oil— Infusion and decoction of tea—Nitrogenous
fluids—Milk—Urine—Liquid albumen—Infusion of raw meat—Impure
mucus—Saliva—Solution of isinglass—Difference in the action of these
two sets of fluids—Decoction of green peas—Decoction and infusion of
cabbage—Decoction of grass leaves.
When, in 1860, I first observed Drosera, and was led to believe that
the leaves absorbed nutritious matter from the insects which they
captured, it seemed to me a good plan to make some preliminary trials
with a few common fluids, containing and not containing nitrogenous
matter; and the results are worth giving.
In all the following cases a drop was allowed to fall from the same
pointed instrument on the centre of the leaf; and by repeated trials
one of these drops was ascertained to be on an average very nearly half
a minim, or 1/960 of a fluid ounce, or .0295 ml. But these measurements
obviously do not pretend to any strict accuracy; moreover, the drops of
the viscid fluids were plainly larger than those of water. Only one
leaf on the same plant was tried, and the plants were collected from
two distant localities. The experiments were made during August and
September. In judging of the effects, one caution is necessary: if a
drop of any adhesive fluid is placed on an old or feeble leaf, the
glands of which have ceased to secrete copiously, the drop sometimes
dries up, especially if the plant [page 77] is kept in a room, and some
of the central and submarginal tentacles are thus drawn together,
giving to them the false appearance of having become inflected. This
sometimes occurs with water, as it is rendered adhesive by mingling
with the viscid secretion. Hence the only safe criterion, and to this
alone I have trusted, is the bending inwards of the exterior tentacles,
which have not been touched by the fluid, or at most only at their
bases. In this case the movement is wholly due to the central glands
having been stimulated by the fluid, and transmitting a motor impulse
to the exterior tentacles. The blade of the leaf likewise often curves
inwards, in the same manner as when an insect or bit of meat is placed
on the disc. This latter movement is never caused, as far as I have
seen, by the mere drying up of an adhesive fluid and the consequent
drawing together of the tentacles.
First for the non-nitrogenous fluids. As a preliminary trial, drops of
distilled water were placed on between thirty and forty leaves, and no
effect whatever was produced; nevertheless, in some other and rare
cases, a few tentacles became for a short time inflected; but this may
have been caused by the glands having been accidentally touched in
getting the leaves into a proper position. That water should produce no
effect might have been anticipated, as otherwise the leaves would have
been excited into movement by every shower of rain.
[Gum arabic.—Solutions of four degrees of strength were made; one of
six grains to the ounce of water (one part to 73); a second rather
stronger, yet very thin; a third moderately thick, and a fourth so
thick that it would only just drop from a pointed instrument. These
were tried on fourteen leaves; the drops being left on the discs from
24 hrs. to 44 hrs.; generally about [page 78] 30 hrs. Inflection was
never thus caused. It is necessary to try pure gum arabic, for a friend
tried a solution bought ready prepared, and this caused the tentacles
to bend; but he afterwards ascertained that it contained much animal
matter, probably glue.
Sugar.—Drops of a solution of white sugar of three strengths (the
weakest containing one part of sugar to 73 of water) were left on
fourteen leaves from 32 hrs. to 48 hrs.; but no effect was produced.
Starch.—A mixture about as thick as cream was dropped on six leaves and
left on them for 30 hrs., no effect being produced. I am surprised at
this fact, as I believe that the starch of commerce generally contains
a trace of gluten, and this nitrogenous substance causes inflection, as
we shall see in the next chapter.
Alcohol, Diluted.—One part of alcohol was added to seven of water, and
the usual drops were placed on the discs of three leaves. No inflection
ensued in the course of 48 hrs. To ascertain whether these leaves had
been at all injured, bits of meat were placed on them, and after 24
hrs. they were closely inflected. I also put drops of sherry-wine on
three other leaves; no inflection was caused, though two of them seemed
somewhat injured. We shall hereafter see that cut off leaves immersed
in diluted alcohol of the above strength do not become inflected.
Olive Oil.—drops were placed on the discs of eleven leaves, and no
effect was produced in from 24 hrs. to 48 hrs. Four of these leaves
were then tested by bits of meat on their discs, and three of them were
found after 24 hrs. with all their tentacles and blades closely
inflected, whilst the fourth had only a few tentacles inflected. It
will, however, be shown in a future place, that cut off leaves immersed
in olive oil are powerfully affected.
Infusion and Decoction of Tea.—Drops of a strong infusion and
decoction, as well as of a rather weak decoction, of tea were placed on
ten leaves, none of which became inflected. I afterwards tested three
of them by adding bits of meat to the drops which still remained on
their discs, and when I examined them after 24 hrs. they were closely
inflected. The chemical principle of tea, namely theine, was
subsequently tried and produced no effect. The albuminous matter which
the leaves must originally have contained, no doubt, had been rendered
insoluble by their having been completely dried.]
We thus see that, excluding the experiments with water, sixty-one
leaves were tried with drops of the [page 79] above-named
non-nitrogenous fluids; and the tentacles were not in a single case
inflected.
[With respect to nitrogenous fluids, the first which came to hand were
tried. The experiments were made at the same time and in exactly the
same manner as the foregoing. As it was immediately evident that these
fluids produced a great effect, I neglected in most cases to record how
soon the tentacles became inflected. But this always occurred in less
than 24 hrs.; whilst the drops of non-nitrogenous fluids which produced
no effect were observed in every case during a considerably longer
period.
Milk.—Drops were placed on sixteen leaves, and the tentacles of all, as
well as the blades of several, soon became greatly inflected. The
periods were recorded in only three cases, namely, with leaves on which
unusually small drops had been placed. Their tentacles were somewhat
inflected in 45 m.; and after 7 hrs. 45 m. the blades of two were so
much curved inwards that they formed little cups enclosing the drops.
These leaves re-expanded on the third day. On another occasion the
blade of a leaf was much inflected in 5 hrs. after a drop of milk had
been placed on it.
Human Urine.—Drops were placed on twelve leaves, and the tentacles of
all, with a single exception, became greatly inflected. Owing, I
presume, to differences in the chemical nature of the urine on
different occasions, the time required for the movements of the
tentacles varied much, but was always effected in under 24 hrs. In two
instances I recorded that all the exterior tentacles were completely
inflected in 17 hrs., but not the blade of the leaf. In another case
the edges of a leaf, after 25 hrs. 30 m., became so strongly inflected
that it was converted into a cup. The power of urine does not lie in
the urea, which, as we shall hereafter see, is inoperative.
Albumen (fresh from a hen’s egg), placed on seven leaves, caused the
tentacles of six of them to be well inflected. In one case the edge of
the leaf itself became much curled in after 20 hrs. The one leaf which
was unaffected remained so for 26 hrs., and was then treated with a
drop of milk, and this caused the tentacles to bend inwards in 12 hrs.
Cold Filtered Infusion of Raw Meat.—This was tried only on a single
leaf, which had most of its outer tentacles and the blade inflected in
19 hrs. During subsequent years, I repeatedly used this infusion to
test leaves which had been experimented [page 80] on with other
substances, and it was found to act most energetically, but as no exact
account of these trials was kept, they are not here introduced.
Mucus.—Thick and thin mucus from the bronchial tubes, placed on three
leaves, caused inflection. A leaf with thin mucus had its marginal
tentacles and blade somewhat curved inward in 5 hrs. 30 m., and greatly
so in 20 hrs. The action of this fluid no doubt is due either to the
saliva or to some albuminous matter* mingled with it, and not, as we
shall see in the next chapter, to mucin or the chemical principle of
mucus.
Saliva.—Human saliva, when evaporated, yields** from 1.14 to 1.19 per
cent. of residue; and this yields 0.25 per cent. of ashes, so that the
proportion of nitrogenous matter which saliva contains must be small.
Nevertheless, drops placed on the discs of eight leaves acted on them
all. In one case all the exterior tentacles, excepting nine, were
inflected in 19 hrs. 30 m.; in another case a few became so in 2 hrs.,
and after 7 hrs. 30 m. all those situated near where the drop lay, as
well as the blade, were acted on. Since making these trials, I have
many scores of times just touched glands with the handle of my scalpel
wetted with saliva, to ascertain whether a leaf was in an active
condition; for this was shown in the course of a few minutes by the
bending inwards of the tentacles. The edible nest of the Chinese
swallow is formed of matter secreted by the salivary glands; two grains
were added to one ounce of distilled water (one part to 218), which was
boiled for several minutes, but did not dissolve the whole. The
usual-sized drops were placed on three leaves, and these in 1 hr. 30 m.
were well, and in 2 hrs. 15 m. closely, inflected.
Isinglass.—Drops of a solution about as thick as milk, and of a still
thicker solution, were placed on eight leaves, and the tentacles of all
became inflected. In one case the exterior tentacles were well curved
in after 6 hrs. 30 m., and the blade of the leaf to a partial extent
after 24 hrs. As saliva acted so efficiently, and yet contains so small
a proportion of nitrogenous matter, I tried how small a quantity of
isinglass would act. One part was dissolved in 218 parts of distilled
water, and drops were placed on four leaves. After 5 hrs. two of these
were considerably and two moderately inflected; after 22 hrs. the
former were greatly and the latter much more inflected. In the course
of 48 hrs.
* Mucus from the air-passages is said in Marshall, ‘Outlines of
Physiology,’ vol. ii. 1867, p. 364, to contain some albumen.
** Müller’s ‘Elements of Physiology,’ Eng. Trans. vol. i., p. 514.
[page 81]
from the time when the drops were placed on the leaves, all four had
almost re-expanded. They were then given little bits of meat, and these
acted more powerfully than the solution. One part of isinglass was next
dissolved in 437 of water; the fluid thus formed was so thin that it
could not be distinguished from pure water. The usual-sized drops were
placed on seven leaves, each of which thus received 1/960 of a grain
(.0295 mg.). Three of them were observed for 41 hrs., but were in no
way affected; the fourth and fifth had two or three of their exterior
tentacles inflected after 18 hrs.; the sixth had a few more; and the
seventh had in addition the edge of the leaf just perceptibly curved
inwards. The tentacles of the four latter leaves began to re-expand
after an additional interval of only 8 hrs. Hence the 1/960 of a grain
of isinglass is sufficient to affect very slightly the more sensitive
or active leaves. On one of the leaves, which had not been acted on by
the weak solution, and on another, which had only two of its tentacles
inflected, drops of the solution as thick as milk were placed; and next
morning, after an interval of 16 hrs., both were found with all their
tentacles strongly inflected.]
Altogether I experimented on sixty-four leaves with the above
nitrogenous fluids, the five leaves tried only with the extremely weak
solution of isinglass not being included, nor the numerous trials
subsequently made, of which no exact account was kept. Of these
sixty-four leaves, sixty-three had their tentacles and often their
blades well inflected. The one which failed was probably too old and
torpid. But to obtain so large a proportion of successful cases, care
must be taken to select young and active leaves. Leaves in this
condition were chosen with equal care for the sixty-one trials with
non-nitrogenous fluids (water not included); and we have seen that not
one of these was in the least affected. We may therefore safely
conclude that in the sixty-four experiments with nitrogenous fluids the
inflection of the exterior tentacles was due to the absorption of [page
82] nitrogenous matter by the glands of the tentacles on the disc.
Some of the leaves which were not affected by the non-nitrogenous
fluids were, as above stated, immediately afterwards tested with bits
of meat, and were thus proved to be in an active condition. But in
addition to these trials, twenty-three of the leaves, with drops of
gum, syrup, or starch, still lying on their discs, which had produced
no effect in the course of between 24 hrs. and 48 hrs., were then
tested with drops of milk, urine, or albumen. Of the twenty-three
leaves thus treated, seventeen had their tentacles, and in some cases
their blades, well inflected; but their powers were somewhat impaired,
for the rate of movement was decidedly slower than when fresh leaves
were treated with these same nitrogenous fluids. This impairment, as
well as the insensibility of six of the leaves, may be attributed to
injury from exosmose, caused by the density of the fluids placed on
their discs.
[The results of a few other experiments with nitrogenous fluids may be
here conveniently given. Decoctions of some vegetables, known to be
rich in nitrogen, were made, and these acted like animal fluids. Thus,
a few green peas were boiled for some time in distilled water, and the
moderately thick decoction thus made was allowed to settle. Drops of
the superincumbent fluid were placed on four leaves, and when these
were looked at after 16 hrs., the tentacles and blades of all were
found strongly inflected. I infer from a remark by Gerhardt* that
legumin is present in peas “in combination with an alkali, forming an
incoagulable solution,” and this would mingle with boiling water. I may
mention, in relation to the above and following experiments, that
according to Schiff** certain forms of albumen
* Watts’ ‘Dictionary of Chemistry,’ vol. iii., p. 568.
** ‘Leçons sur la Phys. de la Digestion,’ tom. i, p. 379; tom. ii. pp.
154, 166, on legumin. [page 83]
exist which are not coagulated by boiling water, but are converted into
soluble peptones.
On three occasions chopped cabbage-leaves* were boiled in distilled
water for 1 hr. or for 1 1/4 hr.; and by decanting the decoction after
it had been allowed to rest, a pale dirty green fluid was obtained. The
usual-sized drops were placed on thirteen leaves. Their tentacles and
blades were inflected after 4 hrs. to a quite extraordinary degree.
Next day the protoplasm within the cells of the tentacles was found
aggregated in the most strongly marked manner. I also touched the
viscid secretion round the glands of several tentacles with minute
drops of the decoction on the head of a small pin, and they became well
inflected in a few minutes. The fluid proving so powerful, one part was
diluted with three of water, and drops were placed on the discs of five
leaves; and these next morning were so much acted on that their blades
were completely doubled over. We thus see that a decoction of
cabbage-leaves is nearly or quite as potent as an infusion of raw meat.
About the same quantity of chopped cabbage-leaves and of distilled
water, as in the last experiment, were kept in a vessel for 20 hrs. in
a hot closet, but not heated to near the boiling-point. Drops of this
infusion were placed on four leaves. One of these, after 23 hrs., was
much inflected; a second slightly; a third had only the submarginal
tentacles inflected; and the fourth was not at all affected. The power
of this infusion is therefore very much less than that of the
decoction; and it is clear that the immersion of cabbage-leaves for an
hour in water at the boiling temperature is much more efficient in
extracting matter which excites Drosera than immersion during many
hours in warm water. Perhaps the contents of the cells are protected
(as Schiff remarks with respect to legumin) by the walls being formed
of cellulose, and that until these are ruptured by boiling-water, but
little of the contained albuminous matter is dissolved. We know from
the strong odour of cooked cabbage-leaves that boiling water produces
some chemical change in them, and that they are thus rendered far more
digestible and nutritious to man. It is therefore an interesting
* The leaves of young plants, before the heart is formed, such as were
used by me, contain 2.1 per cent. of albuminous matter, and the outer
leaves of mature plants 1.6 per cent. Watts’ ‘Dictionary of Chemistry,’
vol. i. p. 653. [page 84]
fact that water at this temperature extracts matter from them which
excites Drosera to an extraordinary degree.
Grasses contain far less nitrogenous matter than do peas or cabbages.
The leaves and stalks of three common kinds were chopped and boiled for
some time in distilled water. Drops of this decoction (after having
stood for 24 hrs.) were placed on six leaves, and acted in a rather
peculiar manner, of which other instances will be given in the seventh
chapter on the salts of ammonia. After 2 hrs. 30 m. four of the leaves
had their blades greatly inflected, but not their exterior tentacles;
and so it was with all six leaves after 24 hrs. Two days afterwards the
blades, as well as the few submarginal tentacles which had been
inflected, all re-expanded; and much of the fluid on their discs was by
this time absorbed. It appears that the decoction strongly excites the
glands on the disc, causing the blade to be quickly and greatly
inflected; but that the stimulus, differently from what occurs in
ordinary cases, does not spread, or only in a feeble degree, to the
exterior tentacles.
I may here add that one part of the extract of belladonna (procured
from a druggist) was dissolved in 437 of water, and drops were placed
on six leaves. Next day all six were somewhat inflected, and after 48
hrs. were completely re-expanded. It was not the included atropine
which produced this effect, for I subsequently ascertained that it is
quite powerless. I also procured some extract of hyoscyamus from three
shops, and made infusions of the same strength as before. Of these
three infusions, only one acted on some of the leaves, which were
tried. Though druggists believe that all the albumen is precipitated in
the preparation of these drugs, I cannot doubt that some is
occasionally retained; and a trace would be sufficient to excite the
more sensitive leaves of Drosera. [page 85]
CHAPTER VI.
THE DIGESTIVE POWER OF THE SECRETION OF DROSERA.
The secretion rendered acid by the direct and indirect excitement of
the glands—Nature of the acid—Digestible substances—Albumen, its
digestion arrested by alkalies, recommences by the addition of an
acid—Meat—Fibrin—Syntonin—Areolar tissue—Cartilage—Fibro-cartilage—
Bone—Enamel and dentine—Phosphate of lime—Fibrous basis of
bone—Gelatine—Chondrin— Milk, casein and
cheese—Gluten—Legumin—Pollen—Globulin—Haematin—Indigestible
substances—Epidermic productions—Fibro-elastic
tissue—Mucin—Pepsin—Urea—Chitine— Cellulose—Gun-cotton—Chlorophyll—Fat
and oil—Starch—Action of the secretion on living seeds—Summary and
concluding remarks.
As we have seen that nitrogenous fluids act very differently on the
leaves of Drosera from non-nitrogenous fluids, and as the leaves remain
clasped for a much longer time over various organic bodies than over
inorganic bodies, such as bits of glass, cinder, wood, &c., it becomes
an interesting inquiry, whether they can only absorb matter already in
solution, or render it soluble,—that is, have the power of digestion.
We shall immediately see that they certainly have this power, and that
they act on albuminous compounds in exactly the same manner as does the
gastric juice of mammals; the digested matter being afterwards
absorbed. This fact, which will be clearly proved, is a wonderful one
in the physiology of plants. I must here state that I have been aided
throughout all my later experiments by many valuable suggestions and
assistance given me with the greatest kindness by Dr. Burdon Sanderson.
[page 86]
It may be well to premise for the sake of any reader who knows nothing
about the digestion of albuminous compounds by animals that this is
effected by means of a ferment, pepsin, together with weak hydrochloric
acid, though almost any acid will serve. Yet neither pepsin nor an acid
by itself has any such power.* We have seen that when the glands of the
disc are excited by the contact of any object, especially of one
containing nitrogenous matter, the outer tentacles and often the blade
become inflected; the leaf being thus converted into a temporary cup or
stomach. At the same time the discal glands secrete more copiously, and
the secretion becomes acid. Moreover, they transmit some influence to
the glands of the exterior tentacles, causing them to pour forth a more
copious secretion, which also becomes acid or more acid than it was
before.
As this result is an important one, I will give the evidence. The
secretion of many glands on thirty leaves, which had not been in any
way excited, was tested with litmus paper; and the secretion of
twenty-two of these leaves did not in the least affect the colour,
whereas that of eight caused an exceedingly feeble and sometimes
doubtful tinge of red. Two other old leaves, however, which appeared to
have been inflected several times, acted much more decidedly on the
paper. Particles of clean glass were then placed on five of the leaves,
cubes of albumen on six, and bits of raw meat on three, on none of
which was the secretion at this time in the least acid. After an
interval of 24 hrs., when almost all the tentacles on
* It appears, however, according to Schiff, and contrary to the opinion
of some physiologists, that weak hydrochloric dissolves, though slowly,
a very minute quantity of coagulated albumen. Schiff, ‘Phys. de la
Digestion,’ tom. ii. 1867, p. 25. [page 87]
these fourteen leaves had become more or less inflected, I again tested
the secretion, selecting glands which had not as yet reached the centre
or touched any object, and it was now plainly acid. The degree of
acidity of the secretion varied somewhat on the glands of the same
leaf. On some leaves, a few tentacles did not, from some unknown cause,
become inflected, as often happens; and in five instances their
secretion was found not to be in the least acid; whilst the secretion
of the adjoining and inflected tentacles on the same leaf was decidedly
acid. With leaves excited by particles of glass placed on the central
glands, the secretion which collects on the disc beneath them was much
more strongly acid than that poured forth from the exterior tentacles,
which were as yet only moderately inflected. When bits of albumen (and
this is naturally alkaline), or bits of meat were placed on the disc,
the secretion collected beneath them was likewise strongly acid. As raw
meat moistened with water is slightly acid, I compared its action on
litmus paper before it was placed on the leaves, and afterwards when
bathed in the secretion; and there could not be the least doubt that
the latter was very much more acid. I have indeed tried hundreds of
times the state of the secretion on the discs of leaves which were
inflected over various objects, and never failed to find it acid. We
may, therefore, conclude that the secretion from unexcited leaves,
though extremely viscid, is not acid or only slightly so, but that it
becomes acid, or much more strongly so, after the tentacles have begun
to bend over any inorganic or organic object; and still more strongly
acid after the tentacles have remained for some time closely clasped
over any object.
I may here remind the reader that the secretion [page 88] appears to be
to a certain extent antiseptic, as it checks the appearance of mould
and infusoria, thus preventing for a time the discoloration and decay
of such substances as the white of an egg, cheese, &c. It therefore
acts like the gastric juice of the higher animals, which is known to
arrest putrefaction by destroying the microzymes.
[As I was anxious to learn what acid the secretion contained, 445
leaves were washed in distilled water, given me by Prof. Frankland; but
the secretion is so viscid that it is scarcely possible to scrape or
wash off the whole. The conditions were also unfavourable, as it was
late in the year and the leaves were small. Prof. Frankland with great
kindness undertook to test the fluid thus collected. The leaves were
excited by clean particles of glass placed on them 24 hrs. previously.
No doubt much more acid would have been secreted had the leaves been
excited by animal matter, but this would have rendered the analysis
more difficult. Prof. Frankland informs me that the fluid contained no
trace of hydrochloric, sulphuric, tartaric, oxalic, or formic acids.
This having been ascertained, the remainder of the fluid was evaporated
nearly to dryness, and acidified with sulphuric acid; it then evolved
volatile acid vapour, which was condensed and digested with carbonate
of silver. “The weight of the silver salt thus produced was only .37
gr., much too small a quantity for the accurate determination of the
molecular weight of the acid. The number obtained, however,
corresponded nearly with that of propionic acid; and I believe that
this, or a mixture of acetic and butyric acids, were present in the
liquid. The acid doubtless belongs to the acetic or fatty series.”
Prof. Frankland, as well as his assistant, observed (and this is an
important fact) that the fluid, “when acidified with sulphuric acid,
emitted a powerful odour like that of pepsin.” The leaves from which
the secretion had been washed were also sent to Prof. Frankland; they
were macerated for some hours, then acidified with sulphuric acid and
distilled, but no acid passed over. Therefore the acid which fresh
leaves contain, as shown by their discolouring litmus paper when
crushed, must be of a different nature from that present in the
secretion. Nor was any odour of pepsin emitted by them. [page 89]
Although it has long been known that pepsin with acetic acid has the
power of digesting albuminous compounds, it appeared advisable to
ascertain whether acetic acid could be replaced, without the loss of
digestive power, by the allied acids which are believed to occur in the
secretion of Drosera, namely, propionic, butyric, or valerianic. Dr.
Burdon Sanderson was so kind as to make for me the following
experiments, the results of which are valuable, independently of the
present inquiry. Prof. Frankland supplied the acids.
“1. The purpose of the following experiments was to determine the
digestive activity of liquids containing pepsin, when acidulated with
certain volatile acids belonging to the acetic series, in comparison
with liquids acidulated with hydrochloric acid, in proportion similar
to that in which it exists in gastric juice.
“2. It has been determined empirically that the best results are
obtained in artificial digestion when a liquid containing two per
thousand of hydrochloric acid gas by weight is used. This corresponds
to about 6.25 cubic centimetres per litre of ordinary strong
hydrochloric acid. The quantities of propionic, butyric, and valerianic
acids respectively which are required to neutralise as much base as
6.25 cubic centimetres of HCl, are in grammes 4.04 of propionic acid,
4.82 of butyric acid, and 5.68 of valerianic acid. It was therefore
judged expedient, in comparing the digestive powers of these acids with
that of hydrochloric acid, to use them in these proportions.
“3. Five hundred cub. cent. of a liquid containing about 8 cub. cent.
of a glycerine extract of the mucous membrane of the stomach of a dog
killed during digestion having been prepared, 10 cub. cent. of it were
evaporated and dried at 110o. This quantity yielded 0.0031 of residue.
“4. Of this liquid four quantities were taken which were severally
acidulated with hydrochloric, propionic, butyric, and valerianic acids,
in the proportions above indicated. Each liquid was then placed in a
tube, which was allowed to float in a water bath, containing a
thermometer which indicated a temperature of 38° to 40° Cent. Into
each, a quantity of unboiled fibrin was introduced, and the whole
allowed to stand for four hours, the temperature being maintained
during the whole time, and care being taken that each contained
throughout an excess of fibrin. At the end of the period each liquid
was filtered. Of the filtrate, which of course contained as much of the
fibrin as had been digested during the four hours, [page 90] 10 cub.
cent. were measured out and evaporated, and dried at 110° as before.
The residues were respectively—
“In the liquid containing hydrochloric acid 0.4079 ” ” propionic acid
0.0601 ” ” butyric acid 0.1468 ” ” valerianic acid 0.1254
“Hence, deducting from each of these the above-mentioned residue, left
when the digestive liquid itself was evaporated, viz. 0.0031, we have,
“For propionic acid 0.0570 ” butyric acid 0.1437 ” valerianic acid
0.1223
as compared with 0.4048 for hydrochloric acid; these several numbers
expressing the quantities of fibrin by weight digested in presence of
equivalent quantities of the respective acids under identical
conditions.
“The results of the experiment may be stated thus:—If 100 represent the
digestive power of a liquid containing pepsin with the usual proportion
of hydrochloric acid, 14.0, 35.4, and 30.2, will represent respectively
the digestive powers of the three acids under investigation.
“5. In a second experiment in which the procedure was in every respect
the same, excepting that all the tubes were plunged into the same
water-bath, and the residues dried at 115o C., the results were as
follows:—
“Quantity of fibrin dissolved in four hours by 10 cub. cent. of the
liquid:—
“Propionic acid 0.0563 Butyric acid 0.0835 Valerianic acid 0.0615
“The quantity digested by a similar liquid containing hydrochloric acid
was 0.3376. Hence, taking this as 100, the following numbers represent
the relative quantities digested by the other acids:—
“Propionic acid 16.5 Butyric acid 24.7 Valerianic acid 16.1
“6. A third experiment of the same kind gave: [page 91]
“Quantity of fibrin digested in four hours by 10 cub. cent. of the
liquid:—
“Hydrochloric acid 0.2915 Propionic acid 0.1490 Butyric acid 0.1044
Valerianic acid 0.0520
“Comparing, as before, the three last numbers with the first taken as
100, the digestive power of propionic acid is represented by 16.8; that
of butyric acid by 35.8; and that of valerianic by 17.8.
“The mean of these three sets of observations (hydrochloric acid being
taken as 100) gives for
“Propionic acid 15.8 Butyric acid 32.0 Valerianic acid 21.4
“7. A further experiment was made to ascertain whether the digestive
activity of butyric acid (which was selected as being apparently the
most efficacious) was relatively greater at ordinary temperatures than
at the temperature of the body. It was found that whereas 10 cub. cent.
of a liquid containing the ordinary proportion of hydrochloric acid
digested 0.1311 gramme, a similar liquid prepared with butyric acid
digested 0.0455 gramme of fibrin.
“Hence, taking the quantities digested with hydrochloric acid at the
temperature of the body as 100, we have the digestive power of
hydrochloric acid at the temperature of 16° to 18° Cent. represented by
44.9; that of butyric acid at the same temperature being 15.6.”
We here see that at the lower of these two temperatures, hydrochloric
acid with pepsin digests, within the same time, rather less than half
the quantity of fibrin compared with what it digests at the higher
temperature; and the power of butyric acid is reduced in the same
proportion under similar conditions and temperatures. We have also seen
that butyric acid, which is much more efficacious than propionic or
valerianic acids, digests with pepsin at the higher temperature less
than a third of the fibrin which is digested at the same temperature by
hydrochloric acid.] [page 92]
I will now give in detail my experiments on the digestive power of the
secretion of Drosera, dividing the substances tried into two series,
namely those which are digested more or less completely, and those
which are not digested. We shall presently see that all these
substances are acted on by the gastric juice of the higher animals in
the same manner. I beg leave to call attention to the experiments under
the head albumen, showing that the secretion loses its power when
neutralised by an alkali, and recovers it when an acid is added.
Substances which are completely or partially digested by the Secretion
of Drosera.
Albumen.—After having tried various substances, Dr. Burdon Sanderson
suggested to me the use of cubes of coagulated albumen or hard-boiled
egg. I may premise that five cubes of the same size as those used in
the following experiments were placed for the sake of comparison at the
same time on wet moss close to the plants of Drosera. The weather was
hot, and after four days some of the cubes were discoloured and mouldy,
with their angles a little rounded; but they were not surrounded by a
zone of transparent fluid as in the case of those undergoing digestion.
Other cubes retained their angles and white colour. After eight days
all were somewhat reduced in size, discoloured, with their angles much
rounded. Nevertheless in four out of the five specimens, the central
parts were still white and opaque. So that their state differed widely,
as we shall see, from that of the cubes subjected to the action of the
secretion.
[Experiment 1.
Rather large cubes of albumen were first tried; the tentacles were well
inflected in 24 hrs.; after an [page 93] additional day the angles of
the cubes were dissolved and rounded;* but the cubes were too large, so
that the leaves were injured, and after seven days one died and the
others were dying. Albumen which has been kept for four or five days,
and which, it may be presumed, has begun to decay slightly, seems to
act more quickly than freshly boiled eggs. As the latter were generally
used, I often moistened them with a little saliva, to make the
tentacles close more quickly.
Experiment 2.—A cube of 1/10 of an inch (i.e. with each side 1/10 of an
inch, or 2.54 mm. in length) was placed on a leaf, and after 50 hrs. it
was converted into a sphere about 3/40 of an inch (1.905 mm.) in
diameter, surrounded by perfectly transparent fluid. After ten days the
leaf re-expanded, but there was still left on the disc a minute bit of
albumen now rendered transparent. More albumen had been given to this
leaf than could be dissolved or digested.
Experiment 3.—Two cubes of albumen of 1/20 of an inch (1.27 mm.) were
placed on two leaves. After 46 hrs. every atom of one was dissolved,
and most of the liquefied matter was absorbed, the fluid which remained
being in this, as in all other cases, very acid and viscid. The other
cube was acted on at a rather slower rate.
Experiment 4.—Two cubes of albumen of the same size as the last were
placed on two leaves, and were converted in 50 hrs. into two large
drops of transparent fluid; but when these were removed from beneath
the inflected tentacles, and viewed by reflected light under the
microscope, fine streaks of white opaque matter could be seen in the
one, and traces of similar streaks in the other. The drops were
replaced on the leaves, which re-expanded after 10 days; and now
nothing was left except a very little transparent acid fluid.
Experiment 5.—This experiment was slightly varied, so that the albumen
might be more quickly exposed to the action of the secretion. Two
cubes, each of about 1/40 of an inch (.635 mm.), were placed on the
same leaf, and two similar cubes on another
* In all my numerous experiments on the digestion of cubes of albumen,
the angles and edges were invariably first rounded. Now, Schiff states
(‘Leçons phys. de la Digestion,’ vol. ii. 1867, page 149) that this is
characteristic of the digestion of albumen by the gastric juice of
animals. On the other hand, he remarks “les dissolutions, en chimie,
ont lieu sur toute la surface des corps en contact avec l’agent
dissolvant.” [page 94]
leaf. These were examined after 21 hrs. 30 m., and all four were found
rounded. After 46 hrs. the two cubes on the one leaf were completely
liquefied, the fluid being perfectly transparent; on the other leaf
some opaque white streaks could still be seen in the midst of the
fluid. After 72 hrs. these streaks disappeared, but there was still a
little viscid fluid left on the disc; whereas it was almost all
absorbed on the first leaf. Both leaves were now beginning to
re-expand.]
The best and almost sole test of the presence of some ferment analogous
to pepsin in the secretion appeared to be to neutralise the acid of the
secretion with an alkali, and to observe whether the process of
digestion ceased; and then to add a little acid and observe whether the
process recommenced. This was done, and, as we shall see, with success,
but it was necessary first to try two control experiments; namely,
whether the addition of minute drops of water of the same size as those
of the dissolved alkalies to be used would stop the process of
digestion; and, secondly, whether minute drops of weak hydrochloric
acid, of the same strength and size as those to be used, would injure
the leaves. The two following experiments were therefore tried:—
Experiment 6.—Small cubes of albumen were put on three leaves, and
minute drops of distilled water on the head of a pin were added two or
three times daily. These did not in the least delay the process; for,
after 48 hrs., the cubes were completely dissolved on all three leaves.
On the third day the leaves began to re-expand, and on the fourth day
all the fluid was absorbed.
Experiment 7.—Small cubes of albumen were put on two leaves, and minute
drops of hydrochloric acid, of the strength of one part to 437 of
water, were added two or three times. This did not in the least delay,
but seemed rather to hasten, the process of digestion; for every trace
of the albumen disappeared in 24 hrs. 30 m. After three days the leaves
partially re-expanded, and by this time almost all the viscid fluid on
their discs was absorbed. It is almost superfluous to state that [page
95] cubes of albumen of the same size as those above used, left for
seven days in a little hydrochloric acid of the above strength,
retained all their angles as perfect as ever.
Experiment 8.—Cubes of albumen (of 1/20 of an inch, or 2.54 mm.) were
placed on five leaves, and minute drops of a solution of one part of
carbonate of soda to 437 of water were added at intervals to three of
them, and drops of carbonate of potash of the same strength to the
other two. The drops were given on the head of a rather large pin, and
I ascertained that each was equal to about 1/10 of a minim (.0059 ml.),
so that each contained only 1/4800 of a grain (.0135 mg.) of the
alkali. This was not sufficient, for after 46 hrs. all five cubes were
dissolved.
Experiment 9.—The last experiment was repeated on four leaves, with
this difference, that drops of the same solution of carbonate of soda
were added rather oftener, as often as the secretion became acid, so
that it was much more effectually neutralised. And now after 24 hrs.
the angles of three of the cubes were not in the least rounded, those
of the fourth being so in a very slight degree. Drops of extremely weak
hydrochloric acid (viz. one part to 847 of water) were then added, just
enough to neutralise the alkali which was still present; and now
digestion immediately recommenced, so that after 23 hrs. 30 m. three of
the cubes were completely dissolved, whilst the fourth was converted
into a minute sphere, surrounded by transparent fluid; and this sphere
next day disappeared.
Experiment 10.—Stronger solutions of carbonate of soda and of potash
were next used, viz. one part to 109 of water; and as the same-sized
drops were given as before, each drop contained 1/1200 of a grain
(.0539 mg.) of either salt. Two cubes of albumen (each about 1/40 of an
inch, or .635 mm.) were placed on the same leaf, and two on another.
Each leaf received, as soon as the secretion became slightly acid (and
this occurred four times within 24 hrs.), drops either of the soda or
potash, and the acid was thus effectually neutralised. The experiment
now succeeded perfectly, for after 22 hrs. the angles of the cubes were
as sharp as they were at first, and we know from experiment 5 that such
small cubes would have been completely rounded within this time by the
secretion in its natural state. Some of the fluid was now removed with
blotting-paper from the discs of the leaves, and minute drops of
hydrochloric acid of the strength of the one part to 200 of water was
added. Acid of this greater strength was used as the solutions of the
alkalies were stronger. The [page 96] process of digestion now
commenced, so that within 48 hrs. from the time when the acid was given
the four cubes were not only completely dissolved, but much of the
liquefied albumen was absorbed.
Experiment 11.—Two cubes of albumen (1/40 of an inch, or .635 mm.) were
placed on two leaves, and were treated with alkalies as in the last
experiment, and with the same result; for after 22 hrs. they had their
angles perfectly sharp, showing that the digestive process had been
completely arrested. I then wished to ascertain what would be the
effect of using stronger hydrochloric acid; so I added minute drops of
the strength of 1 per cent. This proved rather too strong, for after 48
hrs. from the time when the acid was added one cube was still almost
perfect, and the other only very slightly rounded, and both were
stained slightly pink. This latter fact shows that the leaves were
injured,* for during the normal process of digestion the albumen is not
thus coloured, and we can thus understand why the cubes were not
dissolved.]
From these experiments we clearly see that the secretion has the power
of dissolving albumen, and we further see that if an alkali is added,
the process of digestion is stopped, but immediately recommences as
soon as the alkali is neutralised by weak hydrochloric acid. Even i
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