Disease in plants
by H. Marshall Ward · English · Gutenberg #39011 · 468,678 chars · public domain (US)
The Project Gutenberg eBook of Disease in plants
This eBook is for the use of anyone anywhere in the United States and
most other parts of the world at no cost and with almost no restrictions
whatsoever. You may copy it, give it away or re-use it under the terms
of the Project Gutenberg License included with this eBook or online
at www.gutenberg.org. If you are not located in the United States,
you will have to check the laws of the country where you are located
before using this eBook.
Title: Disease in plants
Author: H. Marshall Ward
Release date: February 29, 2012 [eBook #39011]
Most recently updated: January 8, 2021
Language: English
Other information and formats: www.gutenberg.org/ebooks/39011
Credits: Produced by Chris Curnow, Lisa Reigel, and the Online
Distributed Proofreading Team at https://www.pgdp.net (This
file was produced from images generously made available
by The Internet Archive)
*** START OF THE PROJECT GUTENBERG EBOOK DISEASE IN PLANTS ***
Produced by Chris Curnow, Lisa Reigel, and the Online
Distributed Proofreading Team at https://www.pgdp.net (This
file was produced from images generously made available
by The Internet Archive)
Transcriber's Notes: Variations in spelling and hyphenation have been
left as in the original. Words in italics in the original are surrounded
by _underscores_. Words in bold in the original are surrounded by =equal
signs=. Ellipses match the original.
A few typographical errors have been corrected. A complete list as well
as other notes follows the text.
DISEASE IN PLANTS
DISEASE IN PLANTS
BY
H. MARSHALL WARD, Sc.D., F.R.S.
FELLOW OF SIDNEY SUSSEX COLLEGE, HONORARY FELLOW OF CHRIST'S COLLEGE
AND PROFESSOR OF BOTANY IN THE UNIVERSITY OF CAMBRIDGE
PRESIDENT OF THE BRITISH MYCOLOGICAL SOCIETY, AND FELLOW OF THE
LINNEAN AND ROYAL HORTICULTURAL SOCIETIES; HONORARY FELLOW
OF THE MANCHESTER LITERARY AND PHILOSOPHICAL SOCIETY
AND OF THE BOTANICAL SOCIETY OF EDINBURGH
London
MACMILLAN AND CO., LIMITED
NEW YORK: THE MACMILLAN COMPANY
1901
_All rights Reserved_
GLASGOW: PRINTED AT THE UNIVERSITY PRESS
BY ROBERT MACLEHOSE AND CO.
PREFACE.
It has often been represented to me that the cultivators of plants,
among whom are to be included planters and foresters, as well as
agriculturists and gardeners of every kind, are more particularly
concerned with, and interested in, the maladies themselves of the plants
they grow, than in the life-history of the fungi, insects or other
organisms to which they are due, or in the physiological processes which
are involved; and although it is impossible to really understand any
disease unless we also understand the processes by which it is brought
about, there is room for sympathy with the point of view of the
cultivator. He says, in effect, "I do not want to know all about the
biology of the fungus of wheat-rust, or of the _phylloxera_, nor do I
want to learn what experts can tell me about the action of bacteria in
soil, or the process of starch-formation in the leaves: I have neither
the time nor the means to master these details. What I want is guidance
as to what is wrong with my tomatoes, apple trees, chrysanthemums, fir
trees, turnips, etc., and what I am to do to set things right." Just
so. With the latter part of this cry one must sympathize, much as a
doctor does with the wail of the parent who calls him in to cure his
sick child--we need not stop to classify or compare the motives of the
parent and the cultivator, and perhaps I had done better to select a
breeder of sheep with his flock and a veterinary doctor in the
illustration, but we will let it pass; and as regards the former part of
the cry, I do not know that the plant-doctor can expect the cultivator
to be initiated in the aetiology of the disease any more than the
physician expects the parent to understand the biology of the typhoid
bacillus. That both the cultivator and the parent would be the better
for a real knowledge of the disease in either case must be admitted--nay
insisted on, provided the knowledge _is_ real--but we have to deal with
facts, and it is a fact that the clients of both doctors are impatient
of the details of the case.
Now, of course, I am aware that no short cut or "royal road" to science
exists, and if a man is going to train up trees or other plants, he
ought to know all about them in health and in sickness, in youth and in
old age, and he ought to learn everything about the soil they grow in,
the air that surrounds them, the enemies that beset them, and all the
multifarious relations of these one to another; but when I look at my
boy and reflect how much his nurse, his schoolmaster, his tutor, his
doctor, and his parents _ought_ to know successively and simultaneously
about him in sickness and in health, and about his surroundings, etc., I
begin to wonder whether there is not after all something to be said for
the cultivator's point of view.
Moreover, the cultivator knows a good deal about his plants which I do
not know, and although I should much like to know it, his plea of want
of time rings in my ears and the conviction strikes home that one ought
to try and meet his views, and tell him something about disease as
manifested in plants without insisting on his becoming a professional
mycologist, entomologist, agricultural chemist, and philosopher.
Of course, beyond a certain point, it is his lookout how much the
information is worth, and its educational value--a very different
matter--is sure to suffer from any restrictions imposed on the treatment
of the subject; but if the theme of disease in plants, treated from a
general point of view--I was about to write "treated in a popular
manner," but that is impossible until physiology and mycology are more
widely taught--enables him to understand better the questions he puts to
himself, and, still more, if it stimulates him to enquire further into
the inexhaustible field of science glimpsed at, something may come of
it.
The purpose of these essays is to treat the subject of disease in plants
with special reference to the patient itself, and to describe the
symptoms it exhibits and the course of the malady, with only such
references to the agents which induce or cause disease as are necessary
to an intelligent understanding of the subject, and of the kind of
treatment called for. Consequently I have avoided any unnecessary
classification or elaborate descriptions of parasitic fungi or insects,
histological details of the tissues of plants, chemical and physical
details regarding the soil, and even matters purely physiological as far
as possible. Several admirable works on these subjects are already
available, and must be referred to for further details.
It is, however, quite out of the question to avoid technicalities,
though I have chosen the simpler course wherever it was found feasible,
and have tried to so employ the examples selected that the student who
wishes to go further into the matters dealt with may turn to special
treatises for further information. For one eminently technical section I
ought perhaps to apologise, but the temptation to try and set forth, in
concrete form and suitable for the purposes of this book, some account
of what is known of the most essential and profound factors concerned in
the difficult question of the nature of life and death, health and
disease, was great. Probably my apology should go further, and apply to
what after all must be failure to explore this mystery to the bottom: my
only excuse must be that it may stimulate others to go further.
It was an afterthought to add, in Part I., the considerations on the
factors which influence the plant regarded as a living machine, so to
speak, in order that the student may the better apprehend the point of
view taken of the bearings of the matters discussed in Part II.
With regard to references, it seemed a better plan to give, in the form
of notes after each chapter, the titles of the principal books and
papers on which a student may base a further course of reading, than to
overweight the pages of what is, after all, merely an introductory
sketch to a huge subject, with detailed quotations from the numerous
sources of information made use of. I have freely expressed my own
opinions, but the sources for others are, I hope, as freely given. It
will, however, be understood that I have not aimed at a complete
bibliography, and, particularly, I have only given foreign references
where it seemed that adequate treatment of the subject could not be
found in English.
My sincere thanks are due to Mr. F. Darwin, F.R.S., who has kindly
looked through many of the proofs, and given me the benefit of several
suggestions: and to my wife for the very material aid she has afforded
me in the preparation of the index.
H. MARSHALL WARD.
CAMBRIDGE,
_November, 1900_.
CONTENTS.
_PART I.--SOME FACTORS._
CHAPTER I.
PAGE
THE PLANT AND ITS SURROUNDINGS, 1
CHAPTER II.
THE PLANT AND ITS FOOD, 7
CHAPTER III.
THE PLANT A LIVING MACHINE, 15
CHAPTER IV.
METABOLISM, 23
CHAPTER V.
ROOTS AND ROOT-HAIRS, 35
CHAPTER VI.
THE FUNCTIONS OF ROOT-HAIRS, 45
CHAPTER VII.
THE BIOLOGY OF SOIL, 56
CHAPTER VIII.
HYBRIDISATION AND SELECTION, 69
_PART II.--DISEASE IN PLANTS._
CHAPTER IX.
PHYTOPATHOLOGY. DERIVATION AND MEANING, 85
CHAPTER X.
HEALTH AND DISEASE, 91
CHAPTER XI.
CAUSES OF DISEASE, 99
CHAPTER XII.
CAUSES OF DISEASE. THE LIVING ENVIRONMENT, 108
CHAPTER XIII.
NATURE OF DISEASE, 119
CHAPTER XIV.
NATURE OF DISEASE (_Continued_), 130
CHAPTER XV.
SPREADING OF DISEASE AND EPIDEMICS, 142
CHAPTER XVI.
THE FACTORS OF AN EPIDEMIC, 149
CHAPTER XVII.
REMEDIAL MEASURES, 159
CHAPTER XVIII.
VARIATION AND DISEASE, 168
CHAPTER XIX.
SYMPTOMS OF DISEASE, 179
CHAPTER XX.
SYMPTOMS OF DISEASE (_Continued_), 186
CHAPTER XXI.
ARTIFICIAL WOUNDS, 194
CHAPTER XXII.
NATURAL WOUNDS, 204
CHAPTER XXIII.
EXCRESCENCES, 212
CHAPTER XXIV.
EXCRESCENCES (_Continued_), 222
CHAPTER XXV.
EXUDATIONS AND ROTTING, 227
CHAPTER XXVI.
NECROTIC DISEASES, 240
CHAPTER XXVII.
MONSTROSITIES AND MALFORMATIONS, 246
CHAPTER XXVIII.
PROLIFERATIONS, 257
CHAPTER XXIX.
GRAFTS, 262
CHAPTER XXX.
LIFE AND DEATH, 271
INDEX, 293
_PART I._
SOME FACTORS.
CHAPTER I.
THE PLANT AND ITS SURROUNDINGS.
_The plant the central object of study--soil, climate,
atmosphere, etc., are factors of its environment. Agricultural
chemistry. The plant a machine. Physiology._
If I were asked to sum up the most important result of the numerous
advances made during the past decade in agriculture and forestry, I
should reply--the clearer and wider recognition of the fact that the
plant itself is the centre of the subject, and not the soil, climate,
season, or other factors of its environment. Until comparatively recent
times it was the habit of farmers, foresters, planters, and gardeners,
all the world over, to look upon the plant as a mere item or as a
mysterious if important one in their calculations, and to regard the
soil as the chief factor in their studies.
Now all is changing, and the world is gradually awakening more and more
to the recognition of the truth that the soil and the clouds and the
atmosphere are merely reservoirs of more or less inert materials, from
which the living plant draws its supplies, and works them up, by means
of energy focussed from the sun, into new plant substance.
In other words, the more far-seeing pioneers of scientific agriculture
and forestry, etc., are recognising that agricultural chemistry is not
the be-all and end-all of agricultural science; but that, in place of
the study of the chemical analyses of dead soil, water, air, and
plant-remains, which has so long held sway, largely owing, I think, to
the influence of Liebig, the student should have his attention more
concentrated on the living plant itself and on the physiological actions
which make up its life. He must regard the living plant as a sort of
working machine--infinitely more complex than any machine made by man,
but a machine nevertheless--the purpose of which is to store up energy
from the sun, and so to add to our wealth on this planet, at the expense
of the extra-terrestrial universe.
It is not, be it noted, that the new study proposes to ignore or abandon
the old studies: modern physiology owes too much to the physics and
chemistry on which it is partly based, and to the labours of De
Saussure, Ingenhousz, Priestley, and others, for that. But it is that
the new study recognises that the central point, to which all views must
be focussed, is not the one that it was formerly supposed to be. The
student is still taught that the chemistry of soils yields valuable
information, and that lessons of importance are derived from comparisons
of the analyses of the ashes, etc., of plants; but he is no longer able
to cherish the hope, however remotely, that such studies solve his most
important problems.
The scene--or rather the point to which attention is now directed--is
the living, working, energy-accumulating plant itself, and not the dead
store of materials in the soil. The reason for the change is not far to
seek: it is due to the enormous strides made in the study of the
physiology of plants during the last quarter of a century, and the
subject abounds in examples illustrating the marvellous advances that
have been made, and at the same time showing how, in the progress of
researches, made for their own sake--_i.e._ in pursuit of satisfaction
for the intense curiosity of the scientific man--all kinds of side
issues turn up which prove to be of value in practice, and suggestive of
further thinking.
At the beginning of the nineteenth century--_i.e._ about 1820--the best
thinkers were giving up the old ideas that the environment supplied
food, as such, to plants, and had recognised that the plant takes up
substances from without and rearranges these in its own body.
The next twenty years or so form a very dark interval in plant
physiology, chiefly owing to the influence of the assumption of a
special "vital force," an assumption which was not allowed merely to
serve as a hypothesis put forward to stimulate research and suggest
better ideas, but which gained a hold over men's powers of reasoning to
an extent which now appears monstrous and phenomenal.
Many errors crept in during this reign of terror, one of the most fatal
of which was De Candolle's revival of the idea of "spongioles"; and
another, equally disastrous in many of its effects, was the conception
of a sort of vegetable food-extract, humus, existing in the soil in a
form peculiarly suitable for direct use by plants. It was during this
period that the confusion between the processes of respiration and
carbon-dioxide assimilation arose, and exerted its effects for evil into
our own day.
The now astounding statement that oxygen-respiration in plants did not
occur, laid the foundation of many subsequent difficulties, and so did
the positive and authoritative views on the uses of minerals to the
plant. Liebig, in fact, stood in the invidious position of being a high
authority on purely chemical questions, who was impelled to give
opinions on matters which can only be solved by physiological
experiments: his great service was to clear up mistakes as regards the
chemistry of soils and of plants--his great mistakes were due to his
pronouncing on physiological matters; and it may be doubted whether his
great services to the purely chemical side of subjects connected with
agricultural matters are the more to be admired, or the disastrous
influence of his statements on subjects which do not belong to the
domain of chemistry should be the more deplored. Be that as it may, he
handed on to succeeding generations some weighty errors as regards
plant-life, and taught the agriculturist to regard chemical analyses of
soils and plant ashes with a reverence which obstructed progress for
some time. As a set-off to this we must place his contributions to the
destruction of the bugbear vitalism, which was simply preventing
enquiry, and his services in bringing together and sifting with power
and originality all that had been then acquired as regards the chemistry
of the plant, the soil, and the atmosphere.
That Liebig was indispensable in 1840-1850 is one thing; but that his
influence should extend to the present day is quite another, and his
inevitable mistakes were almost as powerful for future evil, as his
clear exposition of the chemistry of his day was productive of immediate
good.
Boussingault, working at the same time, 1837-1855, but experimentally
with the living plant, taught us more about these matters than
any investigator of the time, though it is very probable that the
stimulus of Liebig's speculations, good and bad, had its effect in
impelling Boussingault to devote his splendid methods to problems of
plant-nutrition. Boussingault's contributions to our knowledge of the
composition of the dead plant cannot be over-estimated; but he did more
than this, because he so clearly apprehended the necessity for asking
his questions directly of the living plant, instead of deducing from
chemical principles what might be supposed to occur in it; and although
future researches showed that even so careful an investigator solved a
problem of first importance--viz. the question of the fixation of free
nitrogen--the wrong way, it will be found that so far as he did go his
conclusions were sound, and well calculated to inspire the confidence
with which the world received them. As we are here concerned more
especially with the botany of agriculture, however, it is unnecessary to
dwell longer on these matters, or on the similar and even more extensive
experiments, of world-wide reputation, carried on for so many years, and
still being carried on under the liberal auspices of Sir John Lawes, at
Rothamsted. Moreover it may be necessary to return to some of these
points later on.
NOTES TO CHAPTER I.
The reader will find a further general account of these
matters in Sachs' _Lectures on the Physiology of Plants_,
especially Lectures I. and XII., Engl. ed., Oxford, 1887. He
may then proceed to Pfeffer's _Physiology of Plants_, Engl.
ed., 1899, chapter I., and to the account of the history of
the subject in Sachs' _History of Botany_, Oxford, 1890,
especially pp. 359-375 and 445-524. References to more special
literature will be found in Pfeffer.
CHAPTER II.
THE PLANT AND ITS FOOD.
_The food of plants--"Vital force"--Other errors--Liebig and
Boussingault--The botany of agriculture. The synthesis of
carbohydrates--The physiology of plant-nutrition. The
persistence of misconceptions._
The year 1860 may be regarded as a landmark of importance in the history
of plant physiology, for it was in that year that Sachs discovered that
the bringing together of water and carbon-dioxide, in the green
chlorophyll-corpuscles of the plant exposed to sunlight, results in the
formation of the grains of starch found in these corpuscles.
Previous to this date Dutrochet (1826-37) had introduced the then crude
idea of osmosis into physiology; vegetable anatomy had improved, and the
modern conceptions of the living cell, protoplasm, nucleus, etc., were
slowly looming; sieve-tubes had been discovered, and the proteids and
starch in various parts of the plant examined; and the suggestion was
abroad, replacing Liebig's idea that plant acids were the first
products of carbon-assimilation, that some substance, of a slimy nature,
was manufactured in the cells of the leaves and thence distributed as
the formative material from which the plant constructed its parts. Davy
and Boussingault had even surmised that a carbohydrate might be the
first-formed product in assimilation.
There can be little doubt that Sachs' classical proof, by direct
physiological observation and experiment, first brought forward the
truth of organic synthesis in the plant in a concrete and convincing
form.
But it did more than that. It laid the foundation of the modern
physiology of plant-nutrition on ground already prepared by De Saussure
and the earlier workers; for, in addition to emphasising the truth of
organic synthesis--a truth which had been gradually impressing itself on
the world for some years--Sachs' discovery showed clearly the real
meaning of carbon-assimilation as a process for obtaining combustible
food, which the plant then proceeds to make use of.
Many points were rapidly cleared up at once, or if not explained were at
least put into a strong light for further enquiry, and plant-nutrition
soon ceased to be the mysterious subject for all kinds of wild
conjectures that it had hitherto been.
The meaning of thin leaves, with numerous stomata and finely ramified or
divided vascular bundles, became more apparent, as also did the
significance of the ascending transpiration current; the storage of
starch-grains in tubers, medullary rays, roots, seeds, etc., obtained
meanings not understood before; the spread of roots in the soil, and the
gradually discovered properties of the finer rootlets and of the
root-hairs, fitted naturally into their places; and, in short, a
thousand facts, otherwise isolated, became collated into an intelligible
system, full of suggestions for new work, such as has since gone on and
is now being pursued with an activity and success never before realised
in the history of science.
As time went on, while the general truth of Sachs' views was confirmed,
a number of detailed discoveries were made which seemed to contradict
them in certain points. It was found that not all leaves form starch,
for some contain sugar or oil; but Holle and Godlewski proved
experimentally that this oil may be replaced by starch if the conditions
of assimilation are slightly modified. More recently Hebert discovered
that the stalks and leaves of grasses contain a peculiar form of gum,
which was formerly confounded with starch, a substance not abundant in
them. Then came Schimper's discovery of starch-forming corpuscles,
which, if supplied with sugar, are able to form starch-grains in the
dark, as in tubers, etc., underground; and as subsequent researches have
proved that the chlorophyll-corpuscles--which are morphologically the
same as the starch-forming corpuscles and can be replaced by them--are
also able to form starch-grains from sugar, as proved by the experiments
of Boehm, Acton, Meyer, Laurent, Bokorny, Saposchnikoff, and others, it
soon became evident that nothing essential needed altering in Sachs'
view that starch is the first visible product of carbon-dioxide
assimilation, only it became clearer that the starch-grains are built up
by the protoplasm from glucose or some similar body, and represent so
many packets of reserve materials put by for the present because not
required for the immediate needs of the cell.
Boussingault showed, about thirty years ago, that assimilation soon
stops in green leaves if cut off from the plant, not because the leaves
die, but owing to some "maximum capacity" being attained. Sachs had
shown that the starch passes down to other parts of the plant in
solution as glucose.
Neither time nor space will permit me to go into the enormous field of
research and results opened up by these and similar observations made
between 1860-70. It must suffice to say that they led to the discovery
and study of the diastatic and other enzymes in the leaves and other
green parts of plants, and to a clearer understanding of what was
already known of them in seeds, and this knowledge reacted at once on
our insight into the processes of transport of reserve materials and
constructive materials from one part of the plant to another, matters
which will be referred to later on.
It remains to explain Boussingault's difficulty as regards the cessation
of assimilation. Recent researches confirm the view that at least three
causes are at work to bring about the inhibition of the
carbon-assimilation: first, the chlorophyll-corpuscles become filled to
excess with starch, which cannot get away because all the passages are
full and the products are inhibiting the further action of the enzymes
which should dissolve the solid granules; secondly, the leaf being
detached from the plant explains why the soluble products cannot get
away, for this makes a great difference in the rate of exhaustion of the
leaf; and, thirdly, the same fact involves that the leaf can obtain no
further supply of salts of potassium, etc., without which elements the
processes in question cannot go on.
These and numerous other deeper insights into the process of
assimilation, obviously strengthen the force of Sachs' discovery; though
it by no means necessarily follows that starch-grains are always the
resting form of the products of assimilation, and we now know that such
is often not the case: we now have much deeper glimpses into the initial
products of carbon-assimilation than Sachs had in 1860, but this
enhances rather than detracts from the importance of his splendidly
worked-out discovery. Put more generally, we may now say that the
process of carbon-dioxide assimilation in green leaves under the
influence of light is a process of synthesis--photo-synthesis--resulting
in the building up of a carbohydrate such as sugar, inulin or starch
from the elements carbon, hydrogen and oxygen.
But it must not be supposed that the importance of Sachs' discovery, and
the rapid consequent extensions of our knowledge, did their work
forthwith in disabusing men's minds of old and erroneous notions. To say
nothing of numerous smaller misconceptions which still held their ground
owing to the stupendous ignorance of plant-physiology which prevailed,
we find incompetent teachers and text-books were still propagating ideas
worthy of ancient times. The confusion between oxygen-respiration and
the gas interchanges in carbon-assimilation was by no means eliminated
even recently, though it can no longer withstand the deliberate
onslaughts now made on it. That the roots take up food as such from the
soil, and that that food is directly employed by the plant for its
nutrition is even yet implied in daily conversation around us; and
although matters have advanced so far that everyone now knows that the
substances at the roots must be in solution, ere they can be received
into the plant, it sometimes leads to astonishing replies, if we press
the question very far as to how the absorption takes place, in an
elementary examination of agricultural students. That manures are foods
to the plant, that sap circulates, that transpiration is of use to keep
the plant cool, and wood is a "porous body," etc., are only a few of the
misconceptions still current, in a decade that has found publishers for
a work advocating that roots are congealed sap, and that the leaves of
plants absorb the moisture and dust of the air, and so provide the plant
with food, and for a paper explaining the action of root-hairs as tubes
with open pores at their tips. But the gravest misapprehensions current
among us are due to the crude ideas as to what a plant really is: this,
I take it, is owing to the difficulty of grasping what physiologists
mean by organised structure, and leads to regarding the living being
either as a mere aggregation of chemical compounds, built up by the
ordinary play of chemical forces, as we know them, acting on dead
matter, or, as in the days before organic chemistry, as a mysterious
entity endowed with "vital force," and with properties not amenable to
scientific investigation. The mistaken notions as to the powers of roots
to "select" those substances which the plant requires, and to reject
useless ones was merely an expression of this belief.
The rock on which all are liable to come to grief--the chemist or
physicist who requires all his facts in terms of analyses and
proportions by weight, and therefore takes too mechanical a view of the
subject, or the man who is not scientifically trained at all, and
therefore is more liable to go to the other extreme and regard the plant
as a mysterious something which grows and has poetical associations and
traditions--is the great fact of organised structure, and it is the
recognition of this fact and some of its consequences which has altered
the whole position of the subject, and brought the study of the plant
into the domain of physiology. The living plant, its structure and
organisation, the functions of its mechanism, and its relations to the
environment, thus forms a subject apart from that which concerns the
chemical composition of the plant and its environment, and this
distinction designates, in a word, as it were, the change which has been
brought about by modern biology.
A point to be emphasised to the utmost where agricultural students are
concerned is that the essential process of feeding is the same in a
green plant, a fungus, and an animal; the greatest confusion still
exists with regard to this matter, owing to misconceptions as to the
real meaning of the functions of the chlorophyll-corpuscles when
supplied with carbon-dioxide and water and the energy of the sun's rays.
The plant does not feed on carbon-dioxide, any more than it feeds on
oxygen--it feeds on the organic material after it has been constructed,
and the chlorophyll-function is merely one mode of obtaining supplies of
such organic substance.
NOTES TO CHAPTER II.
In addition to the references in the last chapter, the student
should consult Sachs' _Lectures_, XVII.-XIX., and Pfeffer's
_Physiology_, pp. 287-329, for the further development of this
subject. An excellent resume, with new facts and points of
view, will be found in Dr. Horace Brown's "Address to the
Chemical Section," _British Association Reports_, Dover, 1899;
and "Chemistry and Physiology of Foliage Leaves" in _Trans.
Chem. Soc._, 1893, p. 604. See also Blackman, "Experimental
Researches on Vegetable Assimilation and Respiration," _Phil.
Trans._, 1895; and Parkin, "Formation, etc., of Carbohydrates
in Monocotyledons," _Phil. Trans._, 1899.
CHAPTER III.
THE PLANT A LIVING MACHINE.
_The plant a machine into which energy and material are
taken--Carbon assimilation--Feeding--Accumulation and
transformations in the plant. The action of light--The
chlorophyll-function._
The relations of the plant to the environment can only be understood by
taking into account the results of modern physiological discoveries.
These teach us that the living plant is a highly complex machine, the
details of its organisation and structure being much more numerous and
much more closely correlated at numerous points, than the parts of any
other machine known to us.
They also teach us that it is supplied with energy from without, as any
other machine; and that when so supplied, and properly working, the
living structure or machinery does work, also as other machines. But
modern physiology goes further, in that it renders some account of the
ways by which the external energy is taken into the plant, and there
applied to do work, or stored up for a time in order that it may be used
to do work at some future time.
The accumulation of energy thus ensured is associated with corresponding
changes of material substance, and the principal means for bringing this
about is recognised in the assimilation of carbon-dioxide--photo-synthesis.
In this process energy enters the chlorophyll-corpuscle in the form of
the radiant energy of the sun, it is there directed in the mechanism of
the protoplasm, so as to do work on the molecules of water and
carbon-dioxide which have also been brought into the machinery; this it
does, breaking asunder their stable structure into unstable bodies,
which then re-combine in different ways to form a carbohydrate, such as
starch, and this starch is temporarily stored as grains, while oxygen
escapes.
Each starch-grain, therefore, is to be regarded as a packet of matter
and of potential energy, as it were, capable of yielding up the latter
at any future time, when put under such circumstances that it must do
so. Such stores of energy-yielding substance, if I may use the
much-abused phrase, form the principal food of the plant--or of an
animal, if it steps in and takes them--and we now see that the process
of carbon-dioxide assimilation, as it has perhaps unfortunately been
called, is not the same thing as the process of feeding, for the
_feeding_--_i.e._ the nutrition proper--of the plant does not begin
until the _food_ has been thus obtained.
We now see what the real position of the plant is, to its environment,
whether the latter be living or dead. From our point of view, the plant
serves as a centre for bringing together the substances obtainable from
the soil, and those derived from the atmosphere, and so focussing and
directing the radiant energy of the sun upon these substances, that they
are broken up, and some of their constituents synthesised, with
absorption of energy, into a body, such as starch, containing more
energy than did the original substances taken together or separate. It
matters little whether the actual carbohydrate thus synthesised is
starch, or sugar or inulin: the point is that energy has been gained
from outside and bound up with the acquired material for further use.
But modern physiology has carried matters much further than this, and
especially in the three following directions.
In the first place, it has shown that much of the energy thus stored
from without in the plant is again liberated in the process of oxygen
respiration, and expended partly as appreciable heat and partly as
driving force for stimulating the machinery of the living plant to
further activities.
In the second place, part of it is rearranged with the rearrangement of
the molecules with which the energy is bound up, as it were, so that
work of various kinds is done _in_ the machinery of the plant: I refer
to various metabolic and surface-actions resulting from the peculiar
mode of presentment of the resulting substances, for instance the
production of osmotic pressures in the cell.
And, thirdly, part of the synthesised substance is worked up into higher
bodies, by processes which obviously entail the further doing of work on
the constituents.
The further pursuit of this theme would evidently carry us beyond the
more immediate subject of this book; but I want to make clear that
recent researches render it more and more certain that the living plant
is a complex piece of co-ordinated machinery which brings together
matter and energy from the external universe, and then gets work out of
these.
This proposition is the more important because the whole question of the
enrichment of our planet with new food, new building materials, and new
fuel, to compensate the daily losses, depends on it, and is of course to
be referred fundamentally to the acquirement of new supplies of energy
from the sun. Enormous activity has been displayed by physiologists,
since 1860, in attempting to solve the question, which of the many
different rays known to proceed from the sun are absorbed by the
chlorophyll-corpuscle, and directed to the performance of the work above
referred to.
The names of Draper, Sachs and Pfeffer stand forth prominently as
pioneers in this; while those of Lommel, Engelmann, Timiriazeff and
Langley have been among the most active in making important
contributions to the subject, and in attempting to answer the further
questions connected with the mode in which the chlorophyll is concerned
in utilising the energy of the solar radiations. The point is one of
supreme importance, because it goes on all fours with modern questions
as to the rays of light absorbed or dispersed in our atmosphere at
different seasons of the year, or in special climatic conditions, to say
nothing of its other scientific aspects. Unfortunately, however, we have
no satisfactory explanation of the actual role played by the chlorophyll
substance itself, in spite of much industrious work which has been done
in the subject in this country and elsewhere. As regards the rays
employed, it was first proved that the most effective belong to the red
end of the visible spectrum, and that the effect as measured by the
amounts of oxygen given off, and of starch formed in given periods of
time, is more or less proportionable to the intensity of the solar
light. Then it was established that no monochromatic light is so
powerful as the white light from which it was obtained, though the
relative numbers expressing the activity in the red and yellow regions
may stand to those in the blue as something like 12:1. The latest
results place the maximum assimilation in the red-orange, and this
coincides with the maximum absorption in the chlorophyll. If we may
accept the current views as to the distribution of energy in the
spectrum of solar light, which depends on the complete absorption of all
the rays by a black body, where they are estimated as heat, we have the
interesting result that the agricultural or forest plant is adapted to
catch and retain, broadly speaking, just those particular rays which
possess most energy.
The probability is increasing that the protoplasmic machinery is the
really effective mechanism in the process, and we may figure this
machinery as so holding or presenting the molecules of carbon-dioxide
and water to the impact of the light-vibrations, that the latter are
enabled to undo the molecular structure; the atomic combinations thereby
liberated may then be supposed to form a body like formic-aldehyde,
which by polymerisation becomes a carbohydrate of the nature of a sugar
such as glucose, which the protoplasm then builds up into its substance
and subsequently deposits as starch, and stores temporarily in the form
of grains or as amorphous material.
This is partly hypothetical, and is largely due to the careful
deductions of the chemists, but there are very many facts now to hand
which bear out its probability, especially the recent advances in our
knowledge of the sugars, and the experimental feeding of leaves and
plants deprived of starch with such substances as dextrose, levulose,
maltose, and other sugars, as well as glycerine and other bodies which
should be convertible into, or yield them, if the theory is true. In
this last connection, the careful and extensive experiments of Acton, A.
Meyer, Boehm, and Laurent should be mentioned. It would be interesting
to enlarge upon Engelmann's beautiful physiological experiments in
connection with this subject of absorption of solar energy, where the
maximum accumulation of oxygen-loving bacteria at those parts of a green
alga which lie in the red-orange of the spectrum, are used as indicators
of the maximum oxygen evolution (and therefore of the maximum
carbon-dioxide assimilation), but space will not admit of this. For a
similar reason I must also pass over the same observer's experiments
with plants which assimilate in protoplasm behind a red instead of a
green substance, and which absorb chiefly other rays between the yellow
and blue, with the remark that they also seem to imply that it is the
protoplasmic machinery which turns the energy on to the carbon-dioxide
molecule, the coloured screen being secondary in the matter. Recent
experiments which show that green plants will not assimilate
carbon-dioxide in a light which has passed through a solution of
chlorophyll--and therefore left its red rays behind; nor behind a screen
of iodine dissolved in carbon-dioxide--which lets no visible rays
between the red and blue pass--should be noticed, as showing the
importance of the chlorophyll and the special rays referred to, however;
and I ought at least to mention Timiriazeff's beautiful proof, published
in 1890, that if, on the leaf of a plant left in the dark long enough to
render it free of starch, a bright solar spectrum is steadily projected
for 3-6 hours, the chlorophyll then removed by alcohol and the
decolorised leaf placed in iodine, the image of the spectrum is
reproduced by the different intensities of the starch bands, blue with
iodine, in the different parts. Here, again, the maximum coloration
coincides with the maximum absorption in and near the red.
Microscopic observations and photo-chemical experiments alike convince
us that the chlorophyll-corpuscle is itself a complex piece of
protoplasmic machinery, working for and with the rest of the plant, and
there can be little question as to the greater accuracy of our reasoning
on the whole question I am discussing, since Meyer, Schimper,
Pringsheim, and others have established the importance of its structural
peculiarities.
I must now pass on to consider another aspect of the question of
carbon-assimilation.
NOTES TO CHAPTER III.
In addition to the references in the last chapter, the reader
may be referred to Sachs' _Lectures_, XXV., and Pfeffer's
_Physiology_, pp. 329-356, where the voluminous literature is
given.
CHAPTER IV.
METABOLISM.
_Quantities of starch formed, and their significance for the
plant. The absorption of energy--the conversion of energy in
the plant. The plant is a complex machine for concentrating
and storing energy and material from without._
Sachs measured the increase in dry weight (due to the carbohydrates
formed in the chlorophyll-corpuscles) per square meter of leaf-surface,
exposed for a definite period, by drying rapidly at 100 deg. C. equal areas
of the leaves concerned, and comparing the weights.
Of course the results are not to be pushed too far, in view of the fact
that some of the starch is continually passing away to be utilised, and
of the difficulties of comparing the weather, the intensity of light,
currents of air, hygroscopic conditions of atmosphere, and other
variable factors which influence the matter. For instance, the stomata
open and close to different extents according to the conditions of
light and moisture, and this affects the whole mechanism of
transpiration especially, and therefore the supplies of water and
mineral salts. Nevertheless, some interesting and valuable results have
been obtained in connection with this important subject.
It was found, for instance, that the foliage of a sun-flower or of a
vegetable-marrow may be forming starch at a rate of considerably over a
gram per hour in every square meter of leaf-surface exposed on a fine
day; while in particularly clear and warm sunny weather Sachs obtained
as much as 24 to 25 grams per square meter per diem.
When one reflects that 200 square meters is not an extravagant estimate
for the area of leaf-surface exposed on a tree, for a period which even
in our latitudes may be considerably over 100 days of, say, ten hours'
light, we need no longer wonder at the rapidity with which wood is
produced in the stems, and similar estimates (which I have purposely
kept lower than the estimates for continental and tropical climates) may
suffice to show how quickly potatoes or the ears of corn, etc., may fill
up with the starch or other carbohydrates which render them valuable as
crops. We want more measurements in these connections, moreover, for
there are several ways in which they are of scientific value and
practical importance.
It is evident from what has been said that every grain of starch formed
represents so much energy, packed away for the moment in the
storehouses of the plant; and we know that--quite apart, however, from
intermediate transformations of the energy thus stored--this energy
reappears in the kinetic state eventually, when the starch is burned
off, in presence of oxygen, and transformed into carbon-dioxide and
water. It matters not how quickly or how gradually this combustion
occurs, or whether it is accomplished by burning in a fire, or by slow
and complex stages in respiration or metabolism: the point is that the
unit of weight of starch yields so many units of heat when its structure
tumbles down to the original components, carbon-dioxide and water.
Clearly, if we know how many units of heat are yielded by the combustion
of one gram of starch, we can obtain an estimate of the amount of
energy, measured in terms of heat, which the foliage gains and stores
up--an estimate which will approach the truth in proportion as our
estimate of the total assimilative activity is correct.
A word of warning is necessary here, however, for those best acquainted
with physiology recognise that however useful such calculations as the
above may be, and undoubtedly are, to give a general idea of the fact
that the energy represented is large, it would be a mistake to suppose
that such estimates give even an approximate measure of the energy of
potential which may be got from the carbohydrate, and still less of the
amount of work that may be got from its employment, according to the way
it is employed or presented in the plant. To take a single instance
only. If the carbohydrate is rapidly burned off to carbon-dioxide and
water, very little is got out of it in the way of work--most, if not
all, of the energy set free escapes as heat: whereas if the carbohydrate
is slowly and gradually oxydised, passing through various stages and
giving rise to powerfully osmotic bodies in the process, or if it is
built up into protoplasm, or into the structure of a cell-wall,
relatively enormous quantities of work may be got out of its
surface-energy, and heat may be absorbed. Whence it follows that we
cannot measure the power for physiological work of a body by merely
obtaining its heat of combustion, any more than we can infer its
significance in metabolism from its chemical properties.
The general conclusion that the plant stores large quantities of energy
may of course be arrived at by simply estimating the enormous quantities
of food-material which we obtain annually from agricultural plants.
Modern physiologists have attempted to proceed further than this,
however, in their essays to form an estimate of the relations between
the available energy in the solar rays and that used and stored in the
plant.
If we reflect on such phenomena as the cool shade of a tree, and the
deep gloom of a forest, and on experiments which show that an ordinary
leaf certainly lets very little of the radiant energy of the spectrum
pass through it, it becomes evident that many of the rays which fall on
the leaf are absorbed in some form, and it becomes very probable that
much of the solar energy, other than that we term light, is retained in
the leaf for other purposes than assimilation--or, at least, no other
conclusion seems possible in view of all the facts. Engelmann's
researches with purple bacteria are almost conclusive on this point, and
we may regard it as extremely probable that the plant makes other uses
of rays, perceived by us as heat-rays, as sources of energy. Researches
on the influences of temperature on assimilation and other functions
point to the same conclusion; and Pfeffer and Rodemann definitely state
that heat is converted into work in the osmotic cells. And the study of
the absorption bands in the spectrum of the living leaf becomes more
intelligible in the light of these conclusions. Moreover, the fact that
a plant still carries on processes of metabolism when active
transpiration has lowered its temperature below that of the surrounding
air--and the plant therefore receives heat from the environment--points
to similar conclusions.
The importance of the conclusion is immense, for even if the plant had
no other sources of energy than the darker heat rays of the solar
spectrum, it is clear that it ought to be able to do work.
The above may suffice for the general establishment of the conclusion
that the plant absorbs more radiant energy than it employs solely for
assimilation, and emphasises our deduction that it is a machine for
storing energy.
The question now arises, how is this relatively enormous gain in energy
employed by the plant? Our answer to the question is not complete, but
modern discoveries in various directions have supplied clues here and
there which enable us to sketch in some degree the kinds of changes that
must go on.
Not the least startling result is that, important as carbon-assimilation
is as the chief mode of supplying energy, it is not the only means that
the plant has of obtaining such from the environment, and it is even
possible--not to say probable--that energy from the external universe
may be conveyed into the body of the plant in forms quite different from
those perceptible to our eyes as light.
In the most recent survey of this domain, it is pointed out that we may
distinguish between radiant energy, as not necessarily or obviously
connected with ponderable matter, and mechanical energy, which is always
connected in some way with material substance. All mechanical
performances in the plants depend on transformation of some form of
these, evident either as actual energy doing mechanical work, or as
energy of potential ready to do work.
In so far as molecular movements are concerned, we have the special form
of chemical energy. The evolution of heat, light and electricity by
plants are instances of radiant energy, and so on.
Many transformations of energy in the plants are due to non-vital
processes--_e.g._ transpiration, warping actions, etc., but we cannot
always draw sharp lines between the various cases. Nor can we directly
measure the work done in the living machinery; but from the effects of
pressures and strains, the lifting of heavy weights, driving of
root-tips into soil, osmotic phenomena, etc., it is certain that the
values may be very high.
The following classes of processes in living protoplasm and cells may be
taken as indicators. First we have transformation of chemical energy,
without which continued life is impossible: in many cases--_e.g._ the
processes connected with oxygen respiration--these result in the
development of heat. Secondly, we have those remarkable manifestations
of energy known as osmotic processes, which depend on surface actions,
and with which may be associated other surface effects, such as
imbibition, secretion, etc., and in connection with which heat may be
evolved or absorbed. It is true the substances which exhibit the
properties here referred to may be produced, or placed in position, by
chemical energy, or they may be absorbed by roots, etc.; but the
proximate energy exhibited by them is not derived from chemical energy,
and may be out of all proportion to the chemical energy of the substance
or substances concerned. Moreover it is significant to note that a
highly oxydised body may develop much osmotic energy, as well as a
highly combustible one.
It is of the greatest importance to realise the truth that much work can
be, and is done in the living plant, by conversions of energy of
potential independent of and out of proportion to the chemical energy
available by decomposing the substances concerned; even the heat of
respiration may be superfluous here, for the plant may absorb heat from
without, and convert it into work.
Tensions often arise in the plant, and do work expressed as
movements--_e.g._ the springing of elastic Balsam fruits, stamens of
_Parietaria_, etc.
Osmotic energy not only results in enormous pressures and tensions, but
causes movements by diffusion and diosmosis, and any given osmotic
substance which carries this energy with it is not necessarily formed
always in the same way in the cell--_e.g._ glucose may arise from
starch, or from carbon-dioxide, or from oil.
Surface-energy is also expressed in the powerful attractions for water
exhibited in imbibition, swelling, capillarity, absorption, surface
tensions, etc.
Transpiration induces relatively enormous disturbances of equilibrium,
and does work in moving water quite independent of chemical energy.
Again, what may be termed excretion-energy, as expressed in the
separation of a solid body--_e.g._ a crystal--from a solution, may be
for our purposes regarded separately. Any change in the condition of
aggregation of a substance in the plant may result in movements and the
overcoming of resistances.
It will be evident from this short digression--and this is the point I
wish to emphasise--that in the interval between the securing of a grain
of starch, representing so much energy won from the external universe,
and the reconversion of this grain into its equivalent carbon-dioxide
and water, by respiration, resulting in the loss of the above energy as
heat, the starch referred to may have undergone numerous transformations
in the living machinery of the plant, and have played at various times a
role in connection with the most various evolutions of energy.
If we try to picture a possible case, we may take the following. A given
starch-granule, after being built up in the chlorophyll-corpuscle, is
decomposed, and yields part of itself as glucose, which passes down into
other parts of the plant in solution. Part of it is merely re-converted
into starch, and temporarily stored: another part passes into the arena
of oxydation-processes, the sum of which constitute respiration, and may
serve for a time in the molecules of an organic acid: yet another part
may be converted into a constituent of the cellulose cell-walls; while
part may be brought into play in the reconstruction of protoplasm.
In this last connection a discovery made by Schulze about 1878, and
followed up later by Pfeffer, Palladin, and others is of importance.
Seedlings growing in the dark, or in an atmosphere devoid of
carbon-dioxide in the light, become surcharged with nitrogenous bodies
known as amides, formed during the breaking down of the proteids in the
destructive process preceding and accompanying respiration: if the
seedlings are allowed free access to light and carbon-dioxide, however,
the amides disappear. The explanation is that they are combined with
some of the materials of the carbohydrates, and again built up into the
material of the living protoplasm.
Returning to our hypothetical starch-grain--or, rather, its parts--we
have some of it retained as starch, in excess, simply because it is not
needed at the moment: another portion gives up its energy in
respiration, and this does work on the spot, or is lost as heat; or in
the body of an organic acid, or its salt, the part in question may do
lifting or pressing work by osmosis, or cause diffusion-currents from
one cell to another. In the constitution of the cell-wall we may have
part of our starch-grain aiding in imbibition or in the establishment of
elastic tensions in turgidity: and, finally, parts may be built up into
the living protoplasmic machinery of the plant.
What is true for the starch-grain is also true for any particle of salt,
or water, or gas which enters into the metabolism of the living plant,
regard being paid to the particular case, and circumstances in each
case.
Enough has been said to show that the plant cannot be properly studied
merely as the subject of chemical analysis or of physical investigation;
you might as well expect to understand a watch by assays of the gold,
silver, steel and diamonds of which its parts are made up, or to learn
what can be got out of the proper working of a lace machine by
analysing the silk put into it, and the fabric which comes out, and by
taking the specific gravity of its parts and testing the physical
properties of its wheels and levers.
This is not the same thing as denying the value of such knowledge, in
the case of either the dead machine or the living plant: it is merely
emphasising the supreme importance of the study of the structure and
working of the active machinery in both cases.
Nor is it pertinent to remark on the apparent hopelessness of physiology
being at present able to explain the seemingly infinite complexity of
the living machinery of protoplasm and its activities. The modern
locomotive is also a complex affair in its way, but it is profitable to
investigate it and to know all one can of its working and possibilities,
for obvious reasons: a little reflection will convince us that it is
also worth while to investigate that complex machine, the plant--the
working organism which alone can really enrich a country. Moreover, we
ought to be encouraged by the satisfactory progress now being made, and
the splendid practical results which are accruing, rather than dismayed
by the prospect of unflagging labour which will be required in the
future.
Enough has perhaps been said to establish the general truth that the
plant is a complex machine for storing energy and material from outside,
and we have seen that modern research has at least gone a long way
towards determining how the living machine works.
It is hardly necessary to point out that important practical
consequences may result from these phenomena of the accumulation of
surplus starch or other carbohydrates in the leaves during the day, and
of their disappearance during the night into the lower parts of the
plant. For instance, foliage cut for fodder in the morning is far poorer
in starch than if cut in the evening, and it would be very instructive
to have experiments made on a large scale to test the result of feeding
caterpillars or rabbits, for instance, with mulberry, vine, or other
leaves in the two conditions.
Again, we now see what complications may arise if a parasitic organism
gains access to the stores of carbohydrates in process of accumulation,
or attacks and injures the machinery which is building up such
materials, etc.
NOTES TO CHAPTER IV.
The student who desires to pursue this subject further should
read Sachs' _Lectures_, XX. and XXV., and Pfeffer's
_Physiology_, pp. 442-566, but he will hardly arrive at the
best that has been done without consulting Pfeffer's "Studien
zur Energetik der Pflanzen" in the _Abhandl. der Math.-Phys.
Classe der Kgl. Sachss. Gesellsch. der Wiss._ (Leipzig, 1892),
p. 151; and Kassowitz, _Allgemeine Biologie_ (Vienna, 1899),
Bk. I., pp. 1-127.
CHAPTER V.
ROOTS AND ROOT-HAIRS.
_Older views as to root-hairs--Root-hairs and their
development--Surface--Variations--Conditions for maximum
formation--Minute structure--Adhesion to particles of
soil--Functions._
On the roots of most plants are to be found delicate, silky-looking,
tubular prolongations of some of the superficial cells, known as
root-hairs. Malpighi (1687) seems to have been the first to observe
them, and he took them for capillary tubes. Grew (1682) seems to have
been responsible for the view that the roots act like sponges in taking
up water.
Simon (1768) was probably the originator of the idea that these
root-hairs were excretory tubules, a view that became very popular at
the beginning of this century.
Meyer (1838) was perhaps the first to give a comparative account of
them, and he supposed them to be delicate prolongations of the
root-surface to facilitate the absorption of water.
The real importance of these organs, however, has only become apparent
since Sachs, in 1859, recognised their relations to the particles of
soil between which they extend and to which they cling.
In 1883 Schwarz made a very thorough study of their biological
character, and in 1887 Molisch gave us new facts as to their physiology.
Our knowledge of them has been rendered very much more intimate by the
researches of Pfeffer and De Vries on osmotic and plasmolytic phenomena,
and they serve as an excellent study of some of the best results of
modern physiology.
In the normal case, such as is exemplified by a seedling wheat or bean,
the root-hairs arise some distance behind the growing tip of the root,
an obvious adaptation which prevents their being rubbed off by the soil,
as they would be if developed on parts still actively lengthening. As
those behind die off, new ones replace them in front, and so we find a
wave of succession of functionally active root-hairs some little
distance behind the tip of the root: the same order of events holds for
each new rootlet as it emerges from the parent root, and so successive
borings in the soil, made by the diverging root-tips, are thoroughly
explored by these root-hairs.
Measurements have shown that in various plants the surface of root on 1
mm. of length is increased by the root-hairs in proportions given in
the following table:
------------+---------------------+-----------------+--------------
PLANT. | Area of surface | Area of | No. of times
| without root-hairs. | root and hairs. | greater.
------------+---------------------+-----------------+--------------
Maize, | 4.52 sq. mm. | 25.13 sq. mm. | 5.5
Pea, | 4.71 sq. mm. | 58.33 sq. mm. | 12.4
Scindapsus, | 14.02 sq. mm. | 261.9 sq. mm. | 18.7
------------+---------------------+-----------------+--------------
--which sufficiently establishes the general proposition that the area
of the root-surface is enormously increased by these hairs.
But this does not give us any definite idea of the length of the
cylinders of soil explored by these surfaces, until we find that plants
such as an ordinary sunflower, hemp, or vegetable-marrow may have roots
penetrating into a cubic meter of soil, in all directions, and so
closely that probably no volume so large as a cubic centimeter is left
unexplored. Clark found by actual measurement that the roots of a large
gourd, if put end to end, extended over 25 kilometers, and Nobbe gives
520 meters for the roots of a wheat. Vetches may go nine feet deep, and
oats more than three feet. The Sal, a tree of the forests of India, has
roots which penetrate to a depth of 50 to 60 feet.
Some rough notion of the lengths, superficies and penetrating capacities
of the roots of a large tree may be gathered from the above, but it is
doubtful whether we can form any adequate ideas as to the millions of
root-hairs which must be developed along the course of these
subterranean boring organs.
One of the most striking results of modern enquiry into these matters,
is the discovery that the number and superficial area of these
root-hairs, on one and the same plant, may vary to a large extent
according to the structure, as it were, of the soil, and the degree of
moisture it is capable of retaining; or, with the same soil, according
to the amount of water which it receives and holds. Correlations have
also been observed between the development in length and surface of the
rootlets themselves.
The following illustrations will suffice to show this:
Six young wheat-plants in soil kept constantly wet, developed roots the
total length of which measured 365 mm. each, on the average, and almost
devoid of root-hairs.
Six similar plants in soil only moderately moist, averaged 668 mm., and
were well furnished (though not densely covered) with root-hairs.
Six similar plants in soil which would be termed dry, averaged 371 mm.,
but were densely covered with rich crops of root-hairs.
Further researches have shown that the conditions which rule the
development of the root-system and root-hairs in the soil are very
complex, and not always easy to trace. The most general statements we
can make are the following:
There is an optimum degree of moisture in the soil which promotes the
maximum development of root-hairs. If the soil is too wet they are not
developed.
These facts are of importance as correlated with the ease or difficulty
experienced by the roots in obtaining water, and plants such as our
ordinary agricultural plants show this very distinctly.
Although, as shown in the experiments with wheat, the short roots in dry
soil were more densely covered with root-hairs than the much longer
roots in moderately moist soil, subsequent closer investigation shows
that the total quantity and area of root-hairs is less in the former
case than in the latter.
The greatest number of root-hairs are developed on roots which are
growing at their best: too much moisture may prevent the formation of
root-hairs: too little may induce dense growths of root-hairs locally,
but the total number is reduced.
Another set of events which exerts influence on the development of
root-hairs is the composition of the dilute solution--water containing
dissolved salts--which surrounds them in the soil.
Thus, Schwarz found that when similar oat and wheat plants were grown
with their roots in solutions of various salts, the results differed as
follows:
Oats in a 15 per cent. solution of calcium chloride developed no
root-hairs, though they formed in a 5 per cent. solution, and were very
numerous in a 0.5 per cent. solution, or in water alone. In a 10 per
cent. nutritive solution the plants developed no root-hairs, though
they were abundant in a 1 per cent. solution.
Wheat plants with their roots in a 15 per cent. solution of potassium
nitrate bore no root-hairs, but they were numerous in a 2 per cent.
solution of the same salt.
These are extreme cases, for, although the roots were not killed, they
were strongly inhibited in their growth by the more concentrated
solutions. However, experiments of this kind at least bring vividly
before us what variations are possible, and suggest that similar events
on a smaller scale may occur in a soil which yields large quantities of
soluble substances, _e.g._ when freshly manured. Obviously these facts
have a practical significance as regards kind of soil, drainage, season
(_e.g._ drought or wet), etc.
But there are other factors which rule the development of root-hairs,
and some experiments by Lesage show that the correlations between the
development of root-hairs and roots are probably much more complex than
had been suspected; for he finds that if the lateral rootlets of a Bean,
in a water culture, are suppressed, the main rootlet develops numerous
and very long hairs to compensate the loss in surface, a matter of
obvious importance in the discussion of cases where roots have been
injured in the soil.
Before proceeding further it is necessary to look a little more closely
into the structure of a single hair.
It is a tubular prolongation of a single cell of the external covering
of the young root, usually about 1 to 3 mm. in length, and 0.01 to 0.10
mm. in diameter. In special cases the root-hairs of some water plants
may reach 5 to 18 mm. in length, but of course I am referring to the
ordinary land plants of agriculture and forestry. This tubular
prolongation is closed and rounded off at the distal free end, and opens
at the proximal end into the cell of which it is a protrusion.
The whole structure is bounded by an extremely delicate and elastic wall
of cellulose, which Frank says is of special composition, almost too
thin to measure in many cases, but often somewhere near 0.005 to 0.001
mm. in thickness. This thin membrane is remarkably permeable by water,
or dilute solutions, as is shown by the rapidity with which a root-hair
collapses if exposed to evaporation, or with which dense solutions
abstract water from it, or with which solutions may be seen to penetrate
it under the microscope.
Overlying the thin cell-wall proper, on the outside, is a thin
gelatinous layer, a product of alteration of the outermost lamellae of
the former.
Closely lining the proper cell-wall on the inside, is an extremely thin
layer of living protoplasm, and somewhere in this protoplasm is a
distinct cell-nucleus.
The interior of the tube is filled with cell-sap, and it is the osmotic
pressure of this cell-sap which keeps the whole living instrument tense
and rigid, and the thin protoplasmic film close pressed against the
cellulose cell-wall.
Nothing whatever can pass into the cell-sap, or out from it, without
traversing both the lining of living protoplasm and the cell-wall.
If we gently pull a living root, of wheat, pea, mustard, etc., from a
normal soil, we find particles of soil so closely adherent to the
root-hairs that they cannot all be washed off without tearing the hairs:
the root-hairs establish relations of contact with these particles, so
close that they are cemented to the solid surfaces by means of the
gelatinous layer already referred to. This peculiarity has the following
consequences. In the first place, the enormous holdfast, ensured by the
millions of points of adherence, enables the plant to withstand even
powerful lever actions from above, and provides fixed points against
which the root-tips can work as they drive deeper into the soil. In the
second place, the intimate contact of the root-hairs and particles of
soil, ensures that the films of water held by surface-action on the
soil-particles and root-hairs shall be in continuity with the water
saturating the cell-walls of the latter, and therefore with the
protoplasm and cell-sap in their interior. The importance of this at
periods when the soil is "dry" will be obvious, when we reflect that no
soil is ever naturally so dry that surface-films of water are absent
from the particles.
The fact that the root-hair contains living protoplasm, enables us to
understand to a certain extent the results of the following
experiments.
If we have a leafy and healthy plant, with roots, bearing numerous
root-hairs, properly established in suitably moist soil in the pot, the
roots cease to absorb water if the temperature of the soil falls below a
certain minimum, though they recommence to do so if the temperature is
raised again: this has nothing to do with the temperature of the upper
parts of the plant, or of the air, and the latter may be so high that
the plant rapidly droops from loss of water at the leaves, which is not
being compensated owing to the inactivity of the roots.
Similarly we may have the air so cold, at a time when the soil is warm
enough to keep the root-hairs actively at work, that the plant becomes
surcharged with water, which escapes from the leaves like drops of dew.
The temperatures necessary to cause these disturbances in the action of
the living root-hairs vary for different plants, and even for different
varieties of the same species.
Similar arrestation of the functions of the roots may be brought about
by removing the oxygen from the soil around the root-hairs, and
replacing it by carbon-dioxide, or the vapour of chloroform. If not kept
too long in such a condition, the plant recovers rapidly on admitting
atmospheric oxygen, which is always present in a normal well-drained
soil both as gas in the capillary interspaces, and dissolved in the
water on the surfaces of the particles. If the access of oxygen is
delayed, however, as often happens in rainy seasons and in wet soils,
the root-hairs are killed, and rot sets in. A good instance of this has
lately been given by Heinricher in the case of potatoes.
NOTES ON CHAPTER V.
For the further pursuit of this subject the reader should
consult Sachs' _Lectures_, II. and XV.; Sorauer, _A Popular
Treatise on the Physiology of Plants_, 1895, chapters II. and
IV., and Pfeffer's _Physiology_, pp. 149-163. The principal
paper on root-hairs referred to in the text is Schwarz, "Die
Wurzelhaare der Pflanzen," in _Unters. aus dem bot. Inst. zu
Wuerzburg_, I. Heft 2, 1883, p. 140, where a very exhaustive
account of these organs will be found.
CHAPTER VI.
THE FUNCTIONS OF ROOT-HAIRS.
_Excretions from root-hairs--Osmotic phenomena--Turgescence--
Plasmolysis--Control of the protoplasm in absorption, etc.
Selective absorption._
We see then that the root-hairs are the active living instruments in
absorbing the water (containing small quantities of dissolved
substances) of the soil.
If the living root-hairs are so numerous and so active, however, a
natural inference is that they must exert some influence on the
composition or arrangement of their environment. All the teachings of
modern physiology go to show that such a living cell as I have sketched
cannot carry on its life, brief though it be--the root-hairs are active
for about four or five days--without forming substances of the nature of
excreta, and we should expect some of these to pass out to the soil.
Sachs showed, in 1860, that roots growing in contact with polished
marble corrode the surface of the mineral, and Nobbe, in 1876, showed
that the roots of seedlings reduce potassium permanganate, a fact which
Molisch confirmed in 1887. The latter observer also proved that living
root-hairs secrete substances which colour a solution of guaiacum blue,
oxidise pyrogallic acid and other organic substances, and rendered it
probable that they excrete some substance which inverts cane-sugar, and
in some cases even small quantities of a diastatic enzyme.
Molisch also confirmed an old observation, that roots excrete
carbon-dioxide; and he and Czapek showed that the root-hairs excrete
acids more permanent in their nature than carbonic acid, and published a
method for showing this by means of a dilute solution, slightly
alkaline, of phenolphthalein.
Molisch declared that the substances secreted by root-hairs may even be
observed, dissolved in drops which ooze from the surfaces of the
root-hairs.
That these root-excretions, and particularly the acids, may be of
service in dissolving and rendering more available various constituents
of the soil is an obvious suggestion, and it is borne out by Sachs'
discovery of the corrosion of marble, and by Molisch's observation that
living roots slowly corrode ivory if continuously kept in contact with
it.
But a deeper insight into the physiology of these organs was only
possible when the meaning of the phenomena of osmosis had been rendered
clearer by the researches of Pfeffer and De Vries in 1877.
De Vries showed that the turgescence of the living cell can be
diminished, and even reduced to nothing, by placing the cell in contact
with solutions of substances which attract water from the cell-sap: as
the turgescence diminishes, the cell contracts, owing to the elasticity
of the cell-wall, which was previously distended; if the abstraction of
water continues, the living protoplasmic membrane lining the cell-wall
contracts away from the latter. He then proved that no injury need
accrue to the cell by this process of plasmolysis, since the turgescence
can be restored by washing out the salt with a more dilute solution, or
with pure water; and the cell may go on living and even growing as
before. These phenomena can only be produced in cells where the
protoplasmic lining is intact and alive.
Pfeffer showed that the whole matter depends on the properties of the
living protoplasmic membrane, which, so long as it is alive, has the
power of governing the entrance or exit of dissolved substances, but is
as a rule freely permeable for water. If, then, substances with a
powerful attraction for water are formed in the cell cavity, and of such
a nature that the protoplasm does not permit their free diffusion to the
exterior, they attract water, and hold it fast, and so set up the
condition of hydrostatic pressure known as turgescence, the limit of
which depends on the attainment of a state of equilibrium between the
elastic reaction of the cell-wall and the distending power of the
absorbed water. When this limit is reached, water begins to filter back
again through the cell-wall. Numerous researches during the last fifteen
years have shown that the sap of such a living cell as the root-hair is
charged with substances of various degrees of osmotic power; bodies like
sugars, amides, vegetable acids and their salts, being formed by the
metabolic activity of the protoplasm and accumulated there. Moreover, we
now know that the salts of the vegetable acids in particular are
effective, and the researches of Warburg and Palladin in 1886 have
placed it beyond reasonable doubt that these acids are continually being
developed and destroyed in the living cell during normal growth and
respiration, and that great variations as to quantity may be brought
about by alterations in the conditions of the environment--_e.g._
temperature, oxygen, etc.
If, now, we bring a solution of some salt, such as potassium nitrate,
which has a powerful attraction for water, on the outside of the living
root-hair, the question whether the water remains in the cell, or passes
out of it, merely depends on whether the substances inside or that
outside have the most powerful attraction on the water in the sap, since
the protoplasm allows water to pass freely.
But the protoplasmic lining may affect the whole matter in another way;
for it may allow the dissolved salt, or other substance, in the solution
outside or inside the cell to pass through it also, or it may take it
up and fix it, or break it up or otherwise alter it.
More recent researches, and especially those of Pfeffer, have shown that
these diosmotic properties of the living protoplasm are of the utmost
importance in the whole matter of absorption of substances from the
soil.
Let us suppose the following case. A root-hair, in full vigour, is
allowed to bathe freely in a dilute solution of various substances, such
as sugar, potassium nitrate, phosphates, sulphates and carbonates of
iron, soda, lime, magnesium and others known by experiment to be
harmless to its life.
Now it turns out to be by no means a foregone conclusion that all or any
of the substances, even though freely soluble in the water, can pass
through the protoplasm into the interior of the cell. Some may be
allowed easy access, others may only be permitted to pass in small
quantities, and others, again, may be absolutely refused access by the
delicate living filter, so long as it is vigorously alive. Nor, as
proved by numerous experimental cultures since De Saussure's time, is
the entrance of a salt, etc., ruled by its indispensability or otherwise
in the economy of the plant. And it is important to notice that only
experiment can prove the point and determine which substances are
absorbed and which refused by the root-hair.
If we now suppose the protoplasm to give rise to powerfully osmotic
substances which accumulate in the sap-vacuole, but which are not
permitted free egress through the protoplasm (and the formation of such
bodies will occur if the protoplasm is actively respiring), the
conditions for absorption of water, with or without any dissolved salts,
which the protoplasm allows to traverse it, are set up.
But the above supposed case is realised, as Pfeffer showed in 1886, when
he found by a series of beautiful experiments that certain aniline dyes
can accumulate in living root-hairs, and other living cells, whereas
others cannot pass the living protoplasm. After accumulating for some
time, the dye may either remain stored there, or may eventually diffuse
out.
Pfeffer made another discovery, of equal importance, namely, that under
the influence of dilute organic acids, such as citric acid, the
permeability of the living protoplasm may be altered, so that it allows
substances to pass which could not otherwise have traversed it. De Vries
had also shown that the condition of the protoplasm affects its power of
retaining the colouring matter in the sap of the Beet: so long as the
protoplasm is alive, the crimson sap is retained, even when the cell is
plasmolysed, but immediately it begins to die the colour escapes through
it. A similar case exists when the chlorophyll-corpuscles retain their
colour in living cells known to be charged with acids: so long as the
protoplasm is alive and normally active the green bodies are protected.
These, and numerous other experiments of the same kind, prove that the
healthy root-hair is a living instrument for taking up dilute solutions
out of the soil, and holding them in the sap-cavity for a time. If
killed, by frost for instance, it loses these powers.
The researches of the last ten years have also shown that a time comes
when the turgid cell, if an isolated one, and if sufficient supplies of
water are present, is so tightly distended that the surplus water begins
to diffuse out again under the pressure proper to the hydrostatic
conditions set up.
Now we arrive at a very critical point.
When the water, or dilute solution of various substances, begins to
exude under pressure from the living root-hair, what is to prevent its
escape into the soil? And if it thus diffuses out, where is the object
of absorption?
The questions are obviously pertinent, and they may seem the more so in
that the cells adjoining the root-hair on its inner side are also
turgid, and possess similar properties to those of the root-hairs. To
establish a condition of things which shall bring about the inward flow
of the absorbed water, one of the three following cases is conceivable.
(1) The cells, as we pass radially into the root, have different
properties on the wall of the two sides; or (2) they are more and more
greedy of water owing to some process of extraction of their water by
tissues in the centre of the root; or (3) these successive series of
cells possess osmotically more powerful contents at periods coincident
with the escape of the water from the now osmotically weaker root-hairs.
A little reflection will show that where we have a group of such cells
as the above, all capable of absorbing water and dilute solutions and of
becoming turgid, movements of the absorbed water must go on until all
the cells are in equilibrium, as regards their osmotic pressures.
Now the living rootlet is just such a system, the various cells of which
are in different conditions of osmotic pressure at any given time: some
of these cells are old, and their protoplasm is allowing sap to filter
out under pressure: others are in the height of their vigour, and their
protoplasm extremely impervious to the highly osmotic sap-constituents
which it itself is forming actively: others are too young to have
attained their full turgescence: while others again are in stages
intermediate between the above.
There is another point of importance, however, to explain some
peculiarities in the absorption of these dilute solutions of salts,
etc., by the root-hairs from the soil, and by cells lying deeper in the
plant from these root-hairs.
It is easy to understand that if a root-hair absorbs a given
substance--say calcium sulphate, for illustration--and hands it over to
other cells unchanged, a time must be supposed to arrive when, the sap
of all the cells being equally charged with calcium sulphate, no more
could be absorbed: the rate of absorption of this particular substance,
and the quantity absorbed, up to the hypothetical point of equilibrium
chosen, would then depend simply on the ease with which its molecules
traversed the living protoplasmic membrane, and the degree of their
solubility in the sap.
But now suppose the following new factor to come in. Suppose that
calcium sulphate undergoes decomposition in some one of the internal
cells of the system of absorbing cells, or that it is even merely
crystallised out in such a cell, or in any other way removed from
solution (_e.g._ by deposition in cell-walls). This alters the state of
affairs considerably. The separation of the molecules from the
sap-solution is itself a cause for the flow of more of the solution to
the cell concerned, and such causes of diffusion are very common in the
plant.
The importance of this principle consists in that it lies at the base of
the whole question of selective absorption, application of manures, and
the rotation of crops; and those who are acquainted with the excellent
analytical results of De Saussure, Boussingault, Wolff, Trinchinetti,
Goedechen, etc., and the water-culture experiments of Sachs, Nobbe, and
others, will understand what an illuminating effect on these points was
produced by the above generalisation, which we owe especially to
Pfeffer's splendid researches into the nature of osmotic phenomena.
It will now be clear, I hope, why we regard the living root-hairs as
instruments--as pieces of living machinery--for the active absorption of
water, with substances dissolved in it, from the soil; and it will also
be evident, I think, that no one can form a proper conception of this
matter of absorption, so important in all agricultural questions, unless
he pays attention to these biological phenomena. It was hopeless to
expect to understand these matters merely in the light of chemical
analyses of plants and soils, and one expression of this hopelessness
was the belief in the power of roots to select only the substances
useful to it. We now know that the expression "selective power of roots"
has a totally different meaning from that implied in the minds of the
last generation of agriculturalists, and it would be easy to devise
experiments, with solutions of different strength, where the plant
should be made to take up relatively large quantities of harmless, but
useless minerals, etc., and to starve in the midst of plenty of the
elements proper to its structure, simply because the former are offered
in a form in which they easily traverse the protoplasm of the
root-hairs, while the latter are presented in a form unsuitable for
absorption. That all these matters are of importance in regard to
manuring and choice of soils, etc., needs no emphasising.
These remarks, of course, do not detract from the value of good
comparative chemical analyses, when viewed in the light of physiological
knowledge, as I need hardly say; but they do, and emphatically so,
attack the position that such analyses alone can explain the problems of
agriculture.
On the other hand, we must not rest satisfied with the suggestions so
far put forward to account for the processes referred to, since it is
impossible to overlook the fact that in their present form they merely
afford proximate explanations, and are too crudely mechanical for
finality.
NOTES ON CHAPTER VI.
In addition to the works referred to in the last chapter, the
student should consult Pfeffer's _Physiology_, pp. 86-149, and
pp. 410-441. With reference to water cultures, Sachs'
_Lectures_, XVII., may also be consulted. The standard work on
ash constituents of plants is Wolff, _Aschen-analysen_, 1871
and 1880, an indispensable book of reference in this
connection, though there are others, quoted in Pfeffer, where
further literature may also be found.
CHAPTER VII.
THE BIOLOGY OF SOIL.
_Soil not a dead matrix--Organic materials--The living
organisms of the soil--Their activities--Their numbers and
importance. Abandonment of the notion that chemical analysis
can explain the problem._
It is customary to regard the soil, between the particles of which the
root-hairs of plants are distributed, as if it were merely a dead matrix
of smaller or larger pieces of rock, such as sand, gravel, stones, etc.,
and organic remains, such as bits of wood, leaves, bones, etc., with
water and air in their interstices. As matter of fact, however, soil is
a much more complex body than was suspected until comparatively recent
times.
It is, of course, beyond the scope of this book to go into the different
varieties of soils, their structure or arrangement, and the chemical
nature of their constituent rocks and the debris mingled with the
latter. For the same reason I must pass over the curious properties of
soils in relation to the solutions they yield to water in contact, the
manner in which they retain some of these solutions and allow others to
pass easily, and the remarkable double decompositions which go on in
them. Moreover, I must assume as known the chief physical properties of
ordinary soils with respect to the phenomena of capillarity, absorption
of heat, action of frost, and so forth.
But all ideas as to the nature of soil based merely on the study of its
chemistry and physics are misleading, and it is in just the
establishment of this truth that modern discoveries in Agricultural and
Forest Botany have played so important a part.
From the facts that organic debris is found chiefly at the surface of
the earth, and that the smallest particles are held in suspension by the
water near the surface, it is comprehensible why such organic remains
abound in the upper parts of the soil, where the rootlets with their
absorbing root-hairs are also found, because they must have oxygen. The
rule is, therefore, that an ordinary soil consists of upper strata, rich
in organic materials and in oxygen, and a subsoil, poorer in these
substances.
Among these organic materials are countless myriads of living beings,
especially fungi and bacteria, which require oxygen and organic
materials for their subsistence, and it depends on the open or close,
moderately moist or damp, warm or cold nature of the soil, and on some
obviously connected factors, how far down these aerobic organisms can
thrive. As we go deeper down they become fewer and fewer, and gradually
disappear, and (neglecting certain anaerobic bacteria of putrefaction)
they are rarely found in marked abundance more than a few inches below
the surface soil.
These aerobic fungi and bacteria are the great agents of continued
fertility of a soil, and it is they which, living and multiplying in the
moist and well-aerated warm interstices of a rich open soil, carry out
the useful destruction of organic matter, breaking it up into mineral
and gaseous bodies, which are then dissolved in the water bathing the
root-hairs or escape into the atmosphere. In this work of destruction
they are aided by the oxygen of the air and the solar heat: their own
fermentative action is also accompanied by a marked rise of temperature,
and the carbon-dioxide and other products of their activity all go to
complicate the chemical changes going on in the soil around the roots.
Duclaux has calculated that _Aspergillus niger_, a common mould fungus,
can break down organic substances, such as carbohydrates, at such a rate
that a metre cube of the fungus would decompose more than 3000 kilogr.
of starch in a year, and this may serve as an example giving some idea
of the possibilities in soil.
Analyses of waters containing large quantities of organic matter, as
they enter such open soils as those referred to, compared with the
drainage water after passing through the upper strata, show that the
carbonaceous and nitrogenous materials are broken down to more or less
completely oxidised simpler compounds, and that the following chief
changes result. The ammonia and some other nitrogenous bodies remain
behind in the soil, as also do the phosphoric acid and much of the
potash; whereas large quantities of nitric and nitrous acids, together
with much sulphuric acid, chlorides, and calcium salts pass away in the
drainage. These facts are obviously highly important in agriculture.
Experiments on sewage farms have shown also that the upper soil retains
most of the bacteria of the sewage. Koch found at Osmont, near Berlin,
that whereas the different sewage waters contained numbers so enormous
that each cubic centimeter probably held 38,000,000 germs, the different
drainage waters held only 87,000 per c.cm.; and the whole process of
water-filtration through sandy soils depends on these well-known facts.
Recent experiments in connection with soil-filtration, however, bring
out the further facts that the oxidations which organic matters undergo
in the soil--and without which they are useless to the higher
plants--are enormously enfeebled if the upper layers of soil are
sterilised, so as to deprive them of the myriads of aerobic bacteria,
fungi and yeasts which they normally contain, and there can no longer be
any doubt as to the importance of the biology of the soil in connection
with the preparation of materials suitable for absorption in solution by
the root-hairs of agricultural and other plants.
The researches of the last ten years have brought to light a long list
of forms, comprising yeasts, such as Hansen's _Saccharomyces
apiculatus_, fungi and bacteria which live and grow in the soil, finding
their water and food supplies in the interstices, and under conditions
which we now know to be very diverse. They are usually more numerous, in
species and individuals, in cultivated farm and garden soils than in
woods, prairies, and untilled lands; but the geological nature of the
strata, the closeness and otherwise of the soil, its damp or dry
character and its average temperature (which depends on many things
besides latitude or altitude) and other factors co-operate to rule their
distribution and numbers. The fact that cultivated land is so well
supplied with manures, air, etc., is of great importance in relation to
their relative abundance there, and it is extremely probable that the
use of artificial manures lessens their numbers considerably as compared
with land on which stable and other animal manures are employed.
A list of the soil-bacteria which have been isolated and more or less
carefully cultivated and examined would comprise about fifty species;
but it is certain that, as at present classified and named, many more
species are to be discovered in any ordinary soil.
The fungi are apparently even more numerous than the bacteria, and we
may rest satisfied for the present with the general statement that the
life-actions of the myriads of individuals of these organisms in the
soil completely alter the question of soil-water as understood by the
last generation of agriculturalists.
But there is another aspect of this question of soil-organisms which has
grown in importance of late to such an extent that we are more than ever
justified in regarding the biology of soil as far more vital to the
interests of the plant than its physical or chemical properties. With
many of the fungi in the soil the roots of plants have to compete--just
as plant competes with plant--for water, salts, and other
food-materials. The toadstools which are so conspicuous in fields and
forests spring from mycelia which ramify in the ground, and are busily
breaking down the remains of other organisms, and just such fungi are
known to store up relatively large quantities of salts of potassium and
phosphorus--the very salts which are so valuable to crops and occur so
sparingly in most soils, but which the extensively spread fungus mycelia
can gradually accumulate. Some of these fungi, moreover, are more active
in their antagonism, and actually attack and pierce the roots as
destructive parasites, but I pass these by for the present, as they form
the subject for further consideration when we come to the diseases of
plants.
It is obvious that the competition of fungi with root-hairs for mineral
salts, oxygen, etc., may be at times acute, and it is extremely probable
that cases of so-called sterility of soil, where a particular soil is
found unsuitable for a crop, may sometimes be due to this
over-competition.
The researches of recent years, however, and especially those of Frank,
Winogradsky, Hellriegel, and Stahl, have brought to light a series of
relationships between certain of these soil-organisms and the higher
plants which place the matter of soil-biology in quite new lights.
On the one hand it has been discovered that groups of bacteria are the
active agents in bringing about the destruction of organic nitrogenous
matter with the formation of ammonia, in oxidising this ammonia to
nitrous and to nitric acids, which combine with bases in the soil to
form the corresponding salts; while, on the other hand, other forms can
decompose the nitrates and reduce them to nitrites, or set free ammonia
or even nitrogen from them. Moreover, there are certain species which
can fix the free nitrogen of the atmosphere, and start the cycle of
up-building of this inert element into the complex higher compounds we
term organic. It is impossible to over-estimate the importance of these
processes of nitrification and denitrification going on in the soil
about the root-hairs of the higher plants.
But, in addition to this circulation of nitrogen in the soil, it
turns out that the life-actions of bacteria, and not mere chemical
decompositions, are largely responsible for the circulation of
carbon, of iron, of sulphur and other elements formed from the
decomposition--also by bacterial and fungal agency--of animal and
vegetable remains in the soil.
Even more startling are the biological relations in the soil between
the absorbing roots of the higher plants and some of these bacteria and
fungi, for it has now been established beyond all doubt that certain
fungi enter the living roots and there flourish not as mere destructive
parasites, but as messmates not only tolerated by the plant, but even
indispensable to its welfare. It is probable that nearly half the plants
of our fields, moors, and forests entertain such fungi in their
root-tissues. The curious, and long-known nodules on the roots of
leguminous plants--peas, beans, clover, etc.--are filled with bacteria
which enable these plants to avail themselves of the free nitrogen of
the air, and so enrich the soil with nitrogenous substances.
The roots of most forest trees, orchids, and plants of the moorlands,
meadows and marshes are similarly occupied by fungi, which in some way
convey salts--probably especially phosphates and potassium compounds--to
the plant in return for the small tax of organic carbon-compounds it
exacts from the latter. In some cases at any rate, as Bernard has lately
shown, the very existence of the plant depends on its seedling roots
obtaining this advantageous attachment and co-operation (symbiosis) of
the fungus immediately on germination.
These remarks must suffice to illustrate this part of my subject, and to
emphasise the statement that the question whether a given plant can be
grown in a given soil, is by no means one of simply the physical and
chemical constitution of the latter. The plant will have to run the
gauntlet of a long series of vicissitudes brought about by the presence
or absence, relative proportions and vigour, and specific nature of the
organisms in the soil at its roots, and it is easy to see that many
cases of disease may be due to the absence of advantageous bacteria or
fungi, or to circumstances which disfavour their life, as well as to the
predominance of competing organisms.
It will now be evident that the old points of view must be abandoned,
and with them, especially, the widely prevalent notion that chemical
analyses of the plant and soil can explain the real problems of
agriculture.
It was of course an enormous advance in the science when, thanks to the
splendid labours of the chemists, at the end of the last century and the
beginning of this, we obtained that preliminary knowledge of the
constitution of the air, and of the composition of the water, acids and
salts, etc., which plants require for their food-materials and
life-processes. Much was gained by De Saussure's establishment of the
fact of oxygen respiration, though we now understand by the term
something very different from, and much more complex than, what he
understood by it, as, also, much had been gained by the previously
acquired knowledge of the gas-exchanges in carbon-assimilation: nor must
we forget the services of those who proved, by laborious analyses,
continued for long periods, what chemical compounds are found in the
tissues of plants, and in the soils at their roots and the atmosphere
which surrounded them. We must also remember many other contributions
which have been furnished, and are still being furnished by the chemist;
and I for one hope that his labours will continue to go hand in hand
with those of the physiologist.
But, when all due honour is paid to the scientific chemist, it must
still be allowed that his problems are different from the real problems
of agriculture. To take one set of instances alone. The chemist can
analyse a given soil or a given manure, and can even go a long way
towards making them, but his analyses do not tell us what conditions are
necessary in order that their ingredients may be presented to the roots
so as to be absorbed and become built up into the plant. Chemistry told
us that carbon was fixed from the air, but physiological experiments
determined how this meant the synthesis of certain definite
carbohydrates--this, too, in the face of the powerful authority of the
chemist Liebig, who supposed that the vegetable acids were the results
of the assimilation of carbon. Wolff, De Saussure, and other chemists
have done yeoman service in showing that different plants, growing in
the same soil, contain different proportions of mineral substances; but
it was by means of water-cultures, and other physiological researches,
such as those of Pfeffer on osmotic phenomena and of Schwarz and Molisch
on root-hairs, that the puzzling question of selective absorption, by
means of the living root-hairs, came into the arena of our knowledge.
In every case--and, as already said, I am not undervaluing the work
done--the chemist has left us only on the threshold of the real
problem. He has stood outside the factory in which the real work we want
to know about is being carried on, and has told us of so many tons of
this material being carried in at the gates, and of so many tons of that
coming out; he has even burnt down the factory, and all its contents and
machinery, and has then told us how many tons of the various materials
were there at the time; but this is not what we want, valuable as the
information is, and still more will be. What we want, and what we expect
to obtain, is more information regarding what is done with the materials
in the factory: what machinery they are put into, and how they are put
in: what stages they go through, and how the stages follow one another:
what wear and tear has to be endured, and how we can step in and stop
the working of the machine for our own benefit at the best possible
time.
The physiologist proceeds empirically, by experimenting with the living
machinery. He recognises the parts and their structure, and tries to
find out what they are doing: he knows that the laws of physics and
chemistry cannot be traversed, but he sees these laws at work under
special and very complex and peculiar conditions. He therefore, as the
results of his experiments, sets new questions--or old questions under
new conditions, if you like--and undoubtedly wants the help of both
chemist and physicist; or, if it is preferred, the chemist and physicist
may attack the problems, but they must familiarise themselves with the
peculiar mechanism of the organism concerned, and cannot hope to attain
success without experimenting with it. I confess it seems to me as
reasonable to look upon scientific agriculture as a branch chiefly of
chemistry as it would be to look upon horse-breeding or pigeon-rearing
from the same point of view; and why the professed chemist's advice is
regarded as so comforting and final in the one case and not in the other
is one of those mysteries which seem inherent in human nature.
The central point in agriculture is the plant: get the most out of
it--the energy-winning machine which alone can keep the animals and
everything else connected with the farm going--and all the rest follows.
The old agriculture has taken a gloomy view of things, and especially on
account of a large variable which it blames for many ills, namely, the
season or climate. Perhaps the old agriculture has not sufficiently
recognised that Nature grows plants in accordance with the fact that
variation is not peculiar to the weather: if the seasons vary, so do
fruit and other produce and the plants which yield them; and since man
cannot hope to control the one variable, possibly relief will be found
in doing more, within his limits, towards controlling others.
In any case he cannot hope to succeed without study of the physiology of
the plant.
NOTES TO CHAPTER VII.
An admirable short account of soil in its relation to
root-hairs is given in Sachs' _Lectures_, XV.; but for a more
exhaustive treatment of the subject of soil the reader is
referred to King, _The Soil_ (Wisconsin, 1895), or Warrington,
_Lectures on the Physical Properties of Soil_ (Oxford, 1900);
Larbaletrier, _L'Agriculture_ (Paris, 1888), chapters II. and
III. There is also a very good account in Bailey, _The
Principles of Agriculture_ (London, 1898), chapters I.-III.
With reference to the organisms in soils and the
decompositions they bring about, the student should consult
Kramer, _Die Bakteriologie in ihren Beziehungen zur
Landwirthschaft_ (Wien, 1890), and Lafar, _Technical Mycology_
(Engl. edition, 1898), sections V., VIII., and IX.
CHAPTER VIII.
HYBRIDISATION AND SELECTION.
_The crossing of varieties of wheat, etc.--The essentials of
fertilisation--Rimpau's experiments--Hybrids and selected
varieties._
In the more hopeful view of the case which the new agriculture will have
to take, it will recognise the physiological truth that since the living
plant is the important and variable machine which constructs the produce
looked for, and since that machine will work best in proportion as its
needs are properly satisfied; therefore in cases where the needs of a
given type of the machine cannot be efficiently provided for, it will be
well to select some other type which will take what supplies and
conditions can be offered. Of course, this is already recognised to a
certain extent, as is implied in the practices of "rotation of crops,"
selection of "pedigree wheats" and mixtures of "pasture grasses," and in
decisions as to the quality of land according to the kinds of weeds
found on it, and so forth; but I am convinced that the agriculturist of
the future--and the same applies to the horticulturist, planter and
forester--will have to concern himself more systematically with the
working and the variability of the plant, and particularly with what
Darwin termed Variation under Domestication, than has always been the
custom in the past. The subject of the plasticity of cultivated plants,
and especially of hybrids, is in one sense an old one; but much work is
being done which proves, as such work is apt to do, that very much more
may be done by well-planned experiments on the selection of new
varieties raised by hybridising and cultivation.
In illustration of this point, a short summary of some of the results of
crossing different species of wheat, barley, oats, peas, beet, etc., may
serve to show what has been gained and what may be hoped for in these
directions. It should be stated that much has been done and is being
done in this country as well as abroad, as witness English varieties of
corn, peas, and potatoes, and the recent experiments on crossing various
kinds of maize in America.
The hybridiser grows his cereals, etc., in pots until ready for
crossing, and then takes them into the laboratory, removes the weaker
spikelets, and takes out the young stamens from the flowers left on the
plant. The female plant is then ready, and the flowers covered with
paper caps. The pollen, obtained by a clean wet brush from the plant
chosen as the father, is then carefully placed in position on the
stigmas, and the caps replaced. The pollination is repeated
occasionally, and care taken that no uncrossed flowers develop later. In
this way a few seeds or grains are got to start with.
This would be the place to introduce an account of the enormous advances
made by the botanists of the last decade or two in the study of the
microscopic phenomena of fertilisation. Without going into
details--which would more than occupy all the space at command--I may
recall the discoveries of Strasburger and his pupils, and of Guignard,
which have supplemented the earlier discoveries of De Bary, Cohn, and
Hofmeister, by establishing the facts that the essential point in
fertilisation is the fusion of two nuclei, and the bringing together in
the fused mass of two extremely minute thread-like coiled bodies, the
so-called chromatosomes or filaments, one of which is derived from the
male and the other from the female parent. The particulars as to the
marvellous adaptations to secure the union of these two infinitesimally
minute threads, their behaviour immediately before and after union, and
many other points must be passed over, as I have only space to emphasise
the one crowning discovery that these tiny filaments of nuclear
substance are the material carriers of all the hereditary properties of
the parents to the young plant which their union initiates.
It must not be supposed that the above statements are based on any
meagre foundation of facts. The attraction of the fusing nucleated
masses had been demonstrated over and over again by Tulasne, De Bary,
Strasburger and others; but Pfeffer brought the matter to a crisis by
discovering the attractive (chemotactic) substance emitted in given
cases, and by collecting the fertilising bodies by its means into
artificial tubes.
The fusion of the nucleated bodies in the sexual act was observed by
Strasburger in the living plant a few years ago, and numerous later
observers have confirmed it. Meanwhile all the stages of approach and
contact of the essential filaments of the nuclear substance have been
traced, as also all the stages of the transference of half of each
filament, male and female, into each of the first two cells of the very
young embryo-plant.
Moreover, the essentials are found to be the same in the animal kingdom
also, and the bearing of all these discoveries on the phenomena of
reproduction, variation, and heredity in living organisms has been and
is of the highest importance, for they support, control, explain and
correct so many of the splendid results of Knight, Koelreuter, Sprengel,
Hildebrand and Hermann Mueller, and in every direction throw side-lights
into the crevices of that magnificent structure, the theory of Natural
Selection, erected for all time by our countryman, Charles Darwin.
To return now to experiments on crossing. It is found that the first
products of the crossing appear exactly alike; they may have characters
intermediate between those of the father and mother, or they may
resemble one more than the other, but all the seeds of the same cross
do it in the same way.
On then sowing the seeds of the plants produced from this first cross,
variations begin to appear. Most of the progeny revert to one or other
of the parent forms, others show all conceivable combinations of their
characters, and a few may give rise to entirely new characters. In
succeeding generations the reversions are preponderant, and, supposing
no care is taken to prevent it, the whole of the offspring gradually go
back to the ancestral type.
Some important consequences result, however, if systematic care is
brought to bear on the matter. This tendency to variation in the second
generation of crossed plants has often been noted, and it bears out very
distinctly the conclusions to which Darwin came.
The hybridiser takes advantage of this variation, as others have done,
to select some forms and rigidly suppress others, in order to obtain
well-marked varieties of the plants he experiments with. In
illustration, I may take the following from Rimpau's account of his
experiments on crossing wheat: By crossing a white English long-eared,
dense wheat, and celebrated as a heavy cropper, with a red, looser
German wheat, remarkable for its resistance to winter cold, Rimpau hoped
to obtain a variety uniting both the above qualities. As regards the
property of resistance, he failed, and he eventually gave up the
attempts in face of the advantages offered by the so-called
_Square-heads_, which then came into the market. His experiments, even
with the above varieties, are worth noting, however, for they show how
promising the results of carefully conducted crossing and selection may
be.
The crossing was done in 1875, in both directions. In 1876 the few
grains obtained were found to yield plants almost all alike, with the
long loose ear of the German parent, but the paler colour of the English
wheat.
In 1877 the plants, obtained by sowing the finest grains, were found to
consist of pure white, pure red, and of forms which appeared to vary and
revert in all possible degrees as regards colour, density, and other
characters intermediate between these.
By carefully separating the closest and densest white wheats from the
closest and densest red ones, he got in 1878 a large number of each
coming nearer to the type sown than did the mongrel forms intermingled
with them: these reversions and intermediate forms were then rigidly
eliminated, and only the deepest coloured and densest red and white
forms again sown.
In 1879 these two chosen varieties were constant, so far as concerned
those selected from the crossing of female English white with male
German red wheat, and the following year proved the constancy of the red
variety in the reciprocal cross. In 1886 all four varieties--_i.e._ the
two reds and the two whites of both the crossings--had become constant.
Still more instructive are the results of the cross between the same
white English non-bearded wheat and a red German bearded wheat.
The first results of the crossing in 1875 showed the loose ear of the
German mother, but was paler in colour; while the influence of the
English father was shown by the absence of beard.
From the reversions and mixtures of the mongrels showing reminiscences
of the parents in all degrees in 1877, rigid selections and re-sowings
were made as before, and Rimpau eventually got four very distinct
varieties, two red and two white, a bearded and a beardless form of
each, and these were declared fixed and constant in 1879-1882.
Passing over many similar results, and merely noting a very successful
variety got from a cross between a very early ripening loose red
American wheat and the dense heavy cropping English Square-head--the
crossed variety which has proved very suitable for certain light soils
and dry climates on the Continent, which demand very rapid ripening, and
are therefore of great physiological and technical interest--I must pass
on to note the curious result of the successful hybridisation of wheat
and rye. This cross has been effected several times, and first in this
country according to reports from Edinburgh (1875), New York (1886), and
elsewhere, and Rimpau's careful experiments seem to leave no doubt on
the matter.
First I must remind you that wheat (_Triticum_) differs from rye
(_Secale_) in several marked characters, such as the breadth and shape
of the glumes, the number of flowers in the spikelet, etc.; and that the
cultivated rye differs from cultivated wheats in the characters of the
straw, in having long ears, and in its flowering glumes remaining widely
divaricated for some days when in flower.
In 1888 Rimpau removed the young stamens from the German wheat referred
to, and pollinated the stigmas with pollen from a long-eared rye. Four
sound grains were obtained, looking like wheat-grains.
The history of one of these grains was as follows: In 1889 it yielded
ears which were peculiarly narrow and long, and its stalks were also
much longer than the wheat: the flowers remained exposed, with widely
open paleae, for several days, and the grains were very peculiar, though
wheat-like.
Fifteen of the best grains were selected, and in 1890 three of the
resulting plants proved to be a wheat of the Square-head type and one
quite sterile. The others retained the elongated, narrow, brownish-red
ears, the flowering glumes again opening wide for some days. This last
is a characteristic of rye, but not of wheat.
A long series of natural hybrids of wheat, barley, and oats are also
described and discussed by Rimpau, as well as artificial crosses--some
very remarkable--of barleys, but they must be passed over here.
Peas rarely become hybridised naturally. According to Darwin, H. Mueller,
and Focke, the flowers are little visited by insects in our countries,
though the mechanism points to their adaptation for pollination by large
bees.
Rimpau confirms Darwin, H. Mueller, and Ogle as to the self-fertilisation
of our cultivated peas. Nevertheless, as is well known, marked varieties
have been obtained by artificial crossing by Gaertner, Knight, Laxton,
and others, especially in this country.
At the same time experiments show that while it is very easy to obtain
artificial hybrids of such plants, and there is no fear of natural
inter-crossing, the forms are remarkably unstable as yet. Similarly
unsatisfactory results were obtained with beet. As experiments are still
going on, however, we may expect to hear more about these and other
results.
It is probable, from recent experiments by De Vries, Correns, and
others, that a remarkable regularity, expressed by Mendel in the form of
a law, obtains in the variations which result from hybridising.
In considering these illustrative cases, it is necessary to thoroughly
apprehend that two procedures are involved. In the first place we have
the cross-pollination leading to the formation of the hybrid plant by
cross-fertilisation. But experience shows that this would lead to very
uncertain results if the plant-breeder did not supplement them by the
second and extremely important process of rigid selection--_i.e._ by
choosing the best of the progeny and breeding from them apart from the
parent-forms, and gradually intensifying, as it were, the variations in
certain directions which have been started by the crossing.
It is by selection, careful culture, and repeated selection that so much
has been done in obtaining the innumerable new varieties of roses,
sweet-peas, orchids, orchard fruits, cereals, grapes, strawberries,
melons, tomatoes, early potatoes, etc., brought forward by numerous
breeders of plants in all countries, as will readily be understood if
reference be made to the work of Hays and Webber in America; Saunders in
Canada; Garton, Sutton, Veitch, Bateson, and others in this country.
Nor is it necessary that the new materials for selection to work upon
should be started by hybridisation. Grafting, change of conditions, and
even variations so vaguely understood that we term them "spontaneous,"
may supply the starting-points for changes in the characters of plants,
so remarkable after intensification by breeding that people find it
difficult to believe they can have come from one stock.
Here, however, I must conclude, merely remarking that the above sketch
is a mere outline of the subjects modern agriculture and horticulture
concern themselves with. There are hundreds of problems connected with
the germination of seeds, on which valuable recent work has been done by
Klebs, Green, Horace Brown, and others; with the resistance of seeds
and seedlings to high and low temperatures, a subject opened out by
Sachs, Kny, De Vries, Krasan, Just, Hoehnel, Dewar, Dyer, and others;
with the conditions of vegetation which affect the various functions of
growth, respiration, assimilation, transpiration, and so forth, on which
I cannot even touch in these pages.
Meanwhile I hope I have succeeded in impressing upon you the grand fact
that the plant is a living and very complex engine, driven by the
radiant energy of the sun, and capable of doing work thereby, and this
just as truly as any heat-engine is driven by chemical energy gained by
means of the sun's rays, or as a water-mill is driven by power which
must be referred to the energy of potential in the head of water placed
in position by the sun's work in evaporation. Fundamentally the whole of
life and work on our planet is to be referred to the one great source of
energy which renders possible the establishment of differences of
potential.
This machine, then, doing work in various ways, adapts itself--or goes
to the wall--to the conditions of its work among competing organisms or
opposing circumstances. Curiously enough, while in some cases it suffers
from the competition, in others it is benefited by its life-actions
fitting in between those of other organisms, which in their turn
supplement it. In other words new types of this engine, capable of doing
the work in various ways, are obtainable; some are good types for the
conditions afforded, others are bad ones.
Examples of both will occur in the further exposition of the subject.
Man's position in regard to the struggle is that of an intelligent being
who steps in at certain stages and protects, fosters, and in every way
favours the agricultural plant--the living machine--and sees that every
opportunity is given it to do its best work in the best way--from his
points of view!
NOTES TO CHAPTER VIII.
The foundation of any course of reading on hybridisation
and selection should be Darwin's _Effects of Cross and
Self-Fertilisation in the Vegetable Kingdom_, which, with his
books _On the Origin of Species by means of Natural Selection_
and _The Variation of Animals and Plants under Domestication_,
will prepare the student for the long course of reading
necessary for a full appreciation of what has been done in
this department of science.
From the numerous works which followed these I should select
Bailey's _Survival of the Unlike_, London, 1896, and
_Evolution of our Native Fruits_, New York, 1898, as
especially useful for the reader of this book, to which may
also be added _Plant Breeding_, New York, 1896, by the same
author, as giving numerous facts and practical directions of
value. Further, the "Hybrid Conference Report," _Journ. Roy.
Hort. Soc._, 1900, abounds in facts and information. Rimpau,
_Landw. Jahrb._, vol. xx., 1891, p. 239. The student who
wishes to get towards the root of the matter will hardly be
able to dispense with Strasburger's _Neue Untersuchungen ueber
die Befruchtungsvorgang bei den Phanerogamen_, Jena, 1884. An
interesting summary of recent work on _Xenia_ and "double
fertilisation" will be found in _Bull. No. 22, U.S. Dept. of
Agric._, 1900. See also _Nature_, Mar. 15, 1900, p. 470.
If he wishes to explore the vast region of controversial
literature that opens up from these points, and which is far
beyond the purpose of this book, he may consult the literature
collected in Kassowitz' _Allgemeine Biologie_, Wien, 1899, B.
II., and the references in the works quoted; also,
Strasburger, "The Periodic Reduction of Chromosomes in Living
Organisms," _Ann. Bot._, viii., 1894, p. 281. For "Mendel's
Law," see Correns in _Ber. d. deutsch. bot. Gesellsch._, vol.
xviii., 1900, p. 158.
_PART II._
DISEASE IN PLANTS.
CHAPTER IX.
PHYTOPATHOLOGY. DERIVATION AND MEANING.
_History. References in the Bible--Greeks and Romans--
Shakespeare--Rouen law--Superstitions--Malpighi and Grew--
Hales--Unger--Berkeley--De Bary, etc. Physiology and Biology
--Diagnosis--Etiology--Therapeutics. Study of causes._
Phytopathology, from Greek words which signify to treat of diseases of
plants, comprises what is known of the symptoms, course, and causes of
the diseases which threaten the lives of plants, or bring about injuries
and abnormalities of structure. As a distinct and systematised branch of
botany it is a modern study, the history of which only dates from about
1850, though the subject had been treated more or less disjointedly by
several authors during the preceding century, and isolated records of
diseased crops, fruit-trees, etc., exist far back in the history of
Europe. The existence of mildews and blights on cereals indeed was
observed and recorded by the writers of the older books of the Bible,
half a dozen references to such blights being found in the Old
Testament, as well as others to blasted fig trees, etc., in the New
Testament. Aristotle, about 350 B.C., noticed the epidemic nature of
wheat-rust. The Greeks and Romans were so well acquainted with such
diseases that their philosophers speculated very shrewdly as to causes,
while the people dedicated such pests to special gods. As regards the
Middle Ages, we know little beyond the fact that blights and mildews
existed, but Shakespeare's reference in _King Lear_ (Act III., Sc. 4)
leaves no doubt as to his acquaintance with mildew in the 17th century,
and other authorities bear out the same. Even the law took cognisance of
the danger of wheat-rust in 1660 in Rouen (Loverdo). Prior to the 18th
century, however, only meagre notes on the subject occur scattered here
and there among other matters, and much superstition existed then and
later regarding these as other diseases.
Malpighi, in 1679, gave excellent figures of leaves rolled by insects
and of numerous galls, the true nature of which he practically
discovered by observing the insect piercing the tissues; previous
observers--Pliny knew that flies emerge from galls, but thought the
latter grew spontaneously--having nothing but superstitions and
conjectures to offer. Grew, in 1682, also gave a capital figure and
description of a leaf mined by "a small flat insect . . . which neither
ranging in breadth nor striking deep into the leaf, eats so much only
as lies just before it, and so runs scudding along betwixt the skin and
the pulp of the leaf, leaving a whitish streak behind it, where the skin
is now loose, as the measure of its voyage"--a by no means inadequate
description of the injury and its cause.
During the eighteenth century several academic treatises or
dissertations dealing with diseases of plants appeared.
But as a rule we only find disjointed notes. Hales (1727-33) discusses
the rotting of wounds, canker, and a few other matters, but much had to
be done with the microscope ere any substantial progress could be made.
With the nineteenth century, and the founding of the modern theories of
nutrition by Ingenhousz, Priestley, and De Saussure, we find a new era
started. As the discoveries of the microscopists continued to build up
our knowledge of the anatomy of plants and began to elucidate the
biology of the fungi and other cryptogams, while the chemists and
physiologists laid the foundations of our modern science of plant life,
it gradually became possible to tabulate and classify plant diseases,
and discuss their symptoms and causes in a more scientific manner. Even
in 1833, however, Turpin, and a far better observer, Unger, regarded
parasitic fungi as due to diseased outgrowths of chlorophyll-corpuscles
and parenchyma cells, views shared by Meyen (1837) and Schleiden (1846).
We may pass over the various treatises of Wiegmann (1839), Meyen (1841),
Raspail (1846), Kuehn (1859), and a number of other works of the period,
merely referring with emphasis to Berkeley's admirable papers in the
_Gardener's Chronicle_ (1854) for a summary of what was then known. All
these works antedate De Bary's _Morphologie und Physiologie der Pilze,
etc._ (1866), in which he brought together the results of his researches
during the decade, proving the real nature of parasitic diseases and
infection as worked out by experiments between 1853 and 1863.
This work put the whole subject of parasitic diseases of plants and
animals on a new footing, and paved the way for the modern treatment of
plant pathology as elaborated in the treatises of Frank (1880 and 1895),
Sorauer (1886), Kirchner (1890), and others, to which the reader is
referred for further details. I will merely quote the following passage
from Raspail's _Histoire Naturelle de la Sante et de la Maladie_, 1846
(vol. ii., p. 176), in illustration of the views entertained by high
authorities just prior to De Bary's work: "L'insecte qui produit les
_erineum_, _uredo_, _aecidium_, _xyloma_, _puccinia_, n'est donc plus
pour nous un insecte inconnu, mais un _acarus_ (grise), un _aphis_
(puceron) ou un _thrips_, qui produit au printemps une deviation, etc."
And this view, that fungi already well known to mycologists were called
forth by the punctures of insects, was regarded as not out of harmony
with the idea that the fungus itself was an abnormal outgrowth of the
tissues of the host.
The proper study of plant pathology presupposes and involves a knowledge
of the physiology of plants, of the normal relations of the latter to
their environment, and of the biology of those animals and plants
(principally insects and fungi) which are parasitic on them. It is of
the first importance to understand that a disease is a condition of
abnormal physiology, and that the boundary lines between health and
ill-health are vague and difficult to define. As with the study of the
diseases of man and other animals, so with those of plants, the practice
resolves itself into the accurate observation and interpretation of
symptoms (_Diagnosis_) on the one hand, and of causes (_Aetiology_) on
the other, before any conclusions of value can be drawn as to preventive
or remedial measures (_Therapeutics_). In plants, however, symptoms of
disease are apt to exhibit themselves in a very general manner, or at
any rate it may be that our perceptions of them differentiate symptoms
due to very different reactions imperfectly, probably because the
organisation of the plant is less specialised than that of animals. The
turning yellow and premature falling of leaves, for instance, is a
frequent symptom of disease; but it may be due to a long series of
different causes of ill-health--_e.g._ drought, too high or too low a
temperature, light of insufficient or of excessive intensity, a
superfluity of water at the roots, the presence in the tissues of
parasitic fungi, or that of worms or insects at the roots or elsewhere,
poisonous gases in the air, soil, etc., and so forth. Consequently the
science of plant pathology is much concerned with the direct action of
external causes, which are probably less obscure than in the case of
animals, though by no means always obvious. Such considerations at any
rate seem to account for the fact that most authorities on plant
pathology base their classification on the causes of disease, there
being few noteworthy exceptions.
NOTES TO CHAPTER IX.
The bibliography here quoted will be found in Berkeley,
"Vegetable Pathology," _Gardener's Chronicle_, 1854, p. 4;
Plowright, _British Uredineae and Ustilagineae_, 1889; Eriksson
and Henning, _Die Getreideroste_, Stockholm, 1896; De Bary,
_Comparative Morphology and Biology of the Fungi_, etc., 1887;
Frank, _Die Krankheiten der Pflanzen_, 1895-96, and scattered
in the works referred to in them and in the text.
CHAPTER X.
HEALTH AND DISEASE.
_Variation--Disease--Comparison to a top. Health--Extinction
of species--Natural demise. Examples of complex interactions
in health--Interference, and tendencies to ill-health._
When we come to enquire into the causes of disease, it appears at first
an obvious and easy plan to subdivide them into groups of factors which
interfere with the normal physiology of the plant. Scientific experience
shows, however, that the easy and the obvious are here, as elsewhere in
nature, only apparent, for disease, like health, is an extremely complex
phenomenon, involving many reactions and interactions between the plant
and its environment. If we agree that a living plant in a state of
health is not a fixed and unaltering thing, but is ever varying and
undergoing adaptive changes as its life works out its labyrinthine
course through the vicissitudes of the also ever-varying environment,
then we cannot escape the conviction that a diseased plant, so long as
it lives, is also varying in response to the environment. The principal
difference between the two cases is, that whereas the normal healthy
plant varies more or less regularly and rhythmically about a mean, the
diseased one is tending to vary too suddenly or too far in some
particular directions from the mean; the healthy plant may, for our
present purposes, be roughly likened to a properly balanced top spinning
regularly and well, whereas the diseased one is lurching here, or
wobbling there, to the great danger of its stability. For we must
recognise at the outset that disease is but variation in directions
dangerous to the life of the plant. Health consists in variation also,
but not in such dangerous grooves. That the passage from health to
disease is gradual and ill-defined in many cases will readily be seen.
In fact we cannot completely define disease. Mere abnormality of form,
colour, size, etc., is not necessarily a sign of disease, in the usual
sense of the word, otherwise the striking variations of our cultivated
plants would suggest gloomy thoughts indeed, whereas we have reason to
believe that many cultivated varieties are more healthy--in the sense of
resisting dangerous exigencies of the environment--than the stocks they
came from. Strictly speaking, no two buds on a fruit-tree are alike, and
the shoots they produce vary in position, exposure, number, and vigour
of leaves, and so forth. The minute variations here referred to are not
seen by the ordinary observer, but those who bud, graft and multiply by
cuttings on a large scale know that such bud-variations are important,
quite apart from more extensive "sports" which occasionally occur.
On the other hand, we have reason to believe that many species have died
out gradually as the environment altered. These plants died because they
did not vary sufficiently, or did not vary in the right directions; they
became diseased with respect to the then prevailing conditions of normal
physiology or health.
Disease, therefore, may be said to be variation of functions in
directions, or to extents, which threaten the life of the plant, the
normal in all cases being the state of the plant characteristic of the
species.
Even now, however, we have not obtained a complete definition, because,
since all plants die sooner or later, we have not excluded the natural
demise of the individual or its parts, and no one would call the
autumnal fall of leaves, or the withering of an annual after flowering,
death from disease. Clearly then the idea of disease implies danger of
premature death, and probably this is as near as we shall get to a
satisfactory definition. Since this matter is of primary importance for
our present theme, I will add the following instances for consideration.
A plant in perfect health and in the fullest exercise of all its
functions, has its roots in a soil which is suitably warmed and aerated,
contains the right quantities of water which dissolve just the proper
proportions of all the essential mineral salts, but nothing poisonous,
while the soil itself has a texture such that the roots and root-hairs
can extend and do their utmost in absorbing.
The leaves above are exposed to just the right intensity of light, in
air which is not too dry, and is of suitable temperature and
composition, containing no poisonous exhalations, etc.; and as the
foliage is gently moved by the breeze, it manufactures carbohydrates at
the optimum rate in the chlorophyll, and the so-called "elaborated sap"
containing the dissolved organic food-supplies is prepared in the
tissues in maximum quantities and of just the right degrees of
concentration and quality for use in the buds, stem, roots, etc., for
which it is destined as they draw on the supplies.
Between these assimilating organs, the leaves, and the absorbing roots,
we have in the stem the wood, with its vessels adapted in quantity and
calibre to convey the water containing dissolved salts from the
absorbing roots to the leaves (to say nothing of other parts) and,
separated from this wood by the cambium, we find the sieve-tubes and
cortical tissues in suitable quantity conveying the "elaborated
sap"--the solutions of organic food-materials from the leaves down to
the roots, up to the buds, and elsewhere. Joining these cortical and
wood tissues are adapted series of medullary rays which, apart from
other connections, bring about the necessary interchanges of water and
"elaborated sap" with the cambium, the formative tissue which has to be
fed and served by them, and which by its growth supplies new vessels and
sieve-tubes, etc., to carry the continually increasing quantities of
water and food substances as the roots and leaves increase in number and
area, and thus enables this ideally correlated system to go on working
at maximum energy.
Now suppose the same plant with its roots in an unsuitable soil--too dry
or too poor in mineral supplies, for instance--the transpiring leaves
above cannot obtain sufficient water and salts to supply their needs,
but we will suppose hypothetically that they still assimilate under the
same ideal conditions as before. The supplies now coming to the cambium
are diminished, since the want of water and minerals compels the leaves
to put aside any excess of carbohydrates (_e.g._ as stored
starch-grains), and the plastic materials which do pass to the cambium
so deficient in water cannot be directly utilised, and a starvation
period sets in. Consequently the cambium forms less wood, and this will
contain fewer and smaller vessels, and so reduce the conducting
passages: fewer sieve-tubes also are constructed, and the paths of the
water current and food supplies narrowed, which of course reacts on the
tissues everywhere. The reserve substances may slowly be dissolved and
distributed, however, and considerable quantities be passed in course of
time into the roots, which, as opportunity offers, gradually employ them
in making new roots, and if the disturbance has not gone too far and
the conditions do not become unfavourable, an increased root-supply may
by its larger absorbing area gradually establish the former state of
equilibrium of functions. But this at the expense of the plant, which is
smaller, has fewer leaves and narrower water channels, etc., than a
plant not thus checked, and it may take a long time to make up for the
loss of time and stature thus incurred. Indeed if the plant is an annual
no recovery at all may occur, the reserves passing into fruit and seeds
instead of slowly supplying the roots as described.
If it be asked, can such a condition of affairs as that described really
occur, we have only to think of a transplanted specimen with its roots
maimed and put into unsuitable soil, or of plants in the open with
feeding roots gnawed by an insect, etc., or of a tree hitherto in
equilibrium with its fellows in a plantation suddenly set free by
thinning and so forth.
Now take the case where the roots are maintaining their maximum
functional activity, but the leaves--owing to want of light, too much
moisture or too low a temperature of the air--are functionally
depressed. Here we get a state of over-saturation with water set up, the
tissues are turgid to bursting point, what supplies do traverse the
sieve-tubes, cortex, etc., do so slowly and are excessively diluted, and
the cambium again forms less wood, but the lumina of the vessels are
larger and the lignification less complete. Growth in length is
excessive, but more leaves are formed, though they are apt to be
abnormally thin and may be small. Little or no reserves are stored
anywhere, and the watery tissues contain dangerously diffusible
substances which may render them an easy prey to parasitic fungi. Here
again, however, if the disturbance of equilibrium has not gone too far,
and if the season permits, the new leaves may come into full activity
and the situation be saved by transpiration and assimilation gradually
increasing and restoring the equilibrium. But, as before, the plant has
suffered, and shows the effect in its weak shoots, retarded flowering,
and other ways.
Such plight as is here described may actually be attained in greenhouses
where over-watering is the fault, and even in the open it is not
uncommon in rainy summers, or in plantations where dominant trees get
the upper hand and partially shade more slowly growing species, or in
fields where rank grass is allowed to overwhelm crops of lower stature.
Now it will be evident that either of these typical cases of temporary
disturbance of functional equilibrium may be carried too far: in the
first case the plant may wilt and wither, in the second it may rupture
and rot, to take these eventualities only. And yet it is difficult to
call these indispositions diseases: they are rather examples of extreme
departures from the normal standard of health, just on the borderland
between health and disease. A step further, as it were, and disease
supervenes: certain tissues die from want of water, and a necrotic area
is formed, or the cortex bursts and a wound is formed in another way, or
some fungus gets a hold, and so on. These abnormal states are
particularly apt to predispose the plant to disease--insects revel in
such semi-wilted leaves and shoots crammed with reserves, and fungi in
the water-logged leaves of the second case, while a cold dry wind is
peculiarly fatal to such tissues.
NOTES TO CHAPTER X.
The reader may consult Hartig, _Diseases of Trees_, Eng. ed.,
1894, Introduction; Sorauer, _Pflanzen Krankheiten_, pp. 1-12,
and Frank, _Die Krankheiten der Pflanzen_, B. 1, p. 5, for
definitions of disease.
CHAPTER XI.
CAUSES OF DISEASE.
_A. External causes--I. Non-living environment: soil,
atmosphere, temperature--II. Living environment: plants,
animals--Complex interactions--Predisposing causes--No one
factor works alone--Tangled problems of natural selection
involved. B. So-called internal causes._
It is customary to classify the causes of disease in plants into two
principal groups--(1) those due to the action of the non-living
environment--soil, atmosphere, physical conditions such as temperature,
light, etc.; and (2) those brought about by the activities of living
organisms--plants and animals of various species. Before passing to
further subdivisions under these two heads, however, it is necessary to
observe that no disease can be efficiently caused by an organism alone,
since its powers for injury as a parasite, or otherwise, are affected by
its non-living environment as well as by the host-plant. For instance,
the spores of a parasitic fungus which would infect and rapidly destroy
a potato plant in moist warm weather may be showered on to such a plant
with impunity if the air remains dry and cool--or on to a cabbage under
any circumstances as far as we know.
Again, probably no one factor of the non-living environment ever
suffices to induce a disease, possibly because no such thing as only one
change at a time ever occurs. For instance, it is difficult to say, when
a soil becomes sodden with water, whether the excess of water and
dissolved matters, the want of air displaced by the water, the lowering
of the temperature, or the accumulation of foul products, etc., is the
principal factor in causing the damage which results, and we have to
determine by the balance of experimental evidence which is the dominant
factor in all such cases.
The study of aetiology of disease is in fact only a particular case of
that of aetiology in general. Plants at high altitudes in the Alps
acquire very different characteristics from the same species in the
plains. Is this due to the low temperature, the rarer atmosphere, the
more intense illumination, the changes in moisture, etc., etc.? The
question is more difficult than it appears at first sight, and we must
remember that, complex as are the factors working on the host, they are
equally complex in their actions on a parasite attacking the host,
whence the resulting disease becomes indeed a tangled problem of natural
selection.
Finally it remains to say a few words about a numerous class of cases
where no external cause of disease can be discovered. It was formerly
the custom to group such cases of "Internal Causes" by themselves, but
apart from the fact that many of these mysterious diseases have
subsequently been shown to be due to the action of external agencies,
the whole question of internal causes resolves itself into one of
relations between the plant and its surroundings, and it becomes evident
that no inherited or internal disease can be regarded as explained until
we know the external causes which have so modified the structure and
working of the living cells as to make them abnormal in their reactions
to other parts of the plant. "Internal causes" of disease, therefore, is
a phrase expressing our ignorance, but somewhat more emphatically than
usual. If this is clearly understood there seems no reason against its
employment for the time being in the artificial scheme of classification
we require. With regard to external causes due to the non-living
environment, excess or deficiency of materials in the soil, water, or
atmosphere plays an important part, and--since we may neglect purely
aquatic plants--it is customary to speak of diseases due to unsuitable
soils or to injurious atmospheric influences. For instance, any
deficiency in the supplies of the necessary mineral salts (compounds of
calcium, magnesium, potassium with sulphuric, nitric and phosphoric
acids, etc.) leads to pathological changes, as also does the lack of the
necessary traces of iron. But it is equally true that the presence of
such ingredients in excess or in combinations unsuited to the plants
also leads to disaster, as also does the presence of minerals or other
compounds which poison the root-hairs--_e.g._ products of decomposition,
soluble salts of copper and other poisons. That these matters are bound
up with the whole question of manuring and of proper soil-analyses will
be evident.
Another essential factor is the nature and quantity of organic materials
in the soil, whether leaf-mould and decomposing vegetable remains,
stable manures, or other animal matters, all of which affect different
species very differently, and produce very different results in
different soils. It is necessary to apprehend in this connection what
has been stated above: that soil is not a mere dead structureless
medium, and that the root-hairs of ordinary plants cannot deal with
large quantities of putrefying organic matter: that a good soil must
abound in useful bacteria and fungi to render such substances
available--and in very various ways--and that it must be open and
aerated, of proper temperature and suitably supplied with water, and so
forth, or disaster will result. Here, again, then we are brought into
close contact with all that is known of fermentation, nitrification, and
the various biological changes going on in soil, and the application of
such knowledge to the practice of manuring and tillage in all its forms.
In view of the above remarks, the danger of "over-feeding," in this
sense, has a real meaning for horticulturists, though it must not be
forgotten that no substance is really a food until it is assimilable
into the protoplasm: manures, etc., are food-materials, not food. The
futility of mere chemical analyses to prove what a plant requires is now
well known, and it is only on the basis of long and carefully conducted
experiments that we can ever discover what a particular plant in a
particular soil, situation, and climate requires for healthy
development. Again, the quantity of water in soil may be too great or
too small for given species, and this either on the average for the
year, or during critical periods only; and it is obviously important
whether the excess or deficiency is due to improper supplies of water,
the depth or shallowness of the soil, its retentive powers, or the
nature of the sub-soil and so on, again bringing the whole matter into
connection with our understanding of the physical constitution and
structure of soils, and the nature of soil-drainage.
For instance, a common way of killing ferns is to keep the roots and
soil wet and the air and fronds dry, whereas the natural habitats
provide for wet and shaded fronds and well-drained soil.
It may be noted here that in most cases where gardeners speak of plants
being killed under the "drip" of trees--_e.g._ Beech, the injury is due,
not to the effects of water but to the shade: the loss of light is so
great that the shaded plants die of inanition because their leaves are
not able to provide sufficient carbohydrates.
Closely bound up with this is the question of the gases in soils. Apart
from the disastrous effects of poisons--_e.g._ coal gas escaping from
pipes under pavements in towns, etc., diseased conditions often result
from deficiency of oxygen at the root-hairs, due to imperfect aeration
of soils, brought about by stagnant water, excess of animal matter, and
so forth.
Unsuitable constitution of the atmosphere is also a fruitful source of
disease, though its effects are commoner in closed stoves and
greenhouses than in the open. Nevertheless the continual exhalation of
sulphurous fumes, chlorine, and other poisonous gases in the
neighbourhood of manufacturing centres or of large smoky towns,
volcanoes, etc., play their part in injuring plants; and excessive
moisture in the form of mist, rain, etc., is also important. All these
matters bring us at once into the region of physiology, and only an
intelligent appreciation of what is known about the action of the
atmosphere on the soil and the plant will save the peasantry of a
country from a hopeless mysticism but little removed from that of the
Middle Ages, when blights and other evils were vaguely referred to the
river-mists, thunder clouds, and easterly winds.
If we summarise the above as the material factors of the environment, we
may classify another set of external non-living causes of disease as the
non-material factors. Such are principally the following:
The space at the disposal of plants greatly affects their welfare. The
crowding of roots in the soil and of foliage in the air, resulting in
the loss of light to the leaves, involves deficiency of all the
materials referred to above--minerals, organic materials, gases, and
water--and no better illustration of the intense struggle for existence
among these apparently passive and motionless beings, plants, can be
given than an over-crowded seedbed or plantation. If left to themselves
such over-stocked areas exhibit to the keen eye of the trained observer
all the phases of starvation, weakness, wounding, rot, and, so to speak,
brutal dominance of the stronger over the weaker which it is the object
of cultivation to prevent. Here, then, we are brought face to face with
the true significance of thinning and weeding out, pruning, and similar
processes.
Unsuitable temperature is one of the commonest of all sources of
disease, for every plant is adapted to certain ranges of temperature,
and best adapted to a given optimum somewhere between the maximum and
minimum temperature for each function. Consequently any serious
departure from the mean may bring about physiological disturbances of
the nature of disease, and this in very various ways, as exemplified by
the results of frost, sun-scorching, drought, hail-storms, forest fires,
and so forth.
As a predisposing factor to disease abnormal temperature effects play a
great part. Many wound-fungi gain their entrance through frost-cracks,
bruises due to hailstones, or into tissues chilled below the normal.
No less remarkable are the diseases primarily due to insufficient or
improper exposure to light, which affects the chlorophyll-apparatus and
the process of carbon-assimilation and through these the whole
well-being of the plant. Every plant is adapted to certain ranges of
light intensity, and most cultivators know how impossible it is to grow
shade plants in fully exposed situations, and how easily plants which
live in open sunny situations are "drawn" and killed by shade. It is
equally important to have the right kind of light, as disastrous
experiences with greenhouses glazed with glass which cut off certain
rays of light have taught. Here, again, it is important to notice that
the optimum intensity or quality of light may differ for different
functions and organs of the plant, as is shown by many adaptations on
the part of species growing in natural situations--_e.g._ bud
protection, orientation of leaves, etc.--and it may be taken as a rule
that etiolated plants are peculiarly susceptible to other diseases.
As regards other factors of the inorganic environment, disasters which
come within the scope of our subject may be brought about by many
agencies, the mechanical effects of snow and hail, wind, avalanches,
etc., the effects of lightning, and so forth, being a few of them.
NOTES TO CHAPTER XI.
For other detailed classifications of the causes of disease
the reader is referred to the works of Sorauer and of Frank
referred to in the last chapter. Also Kirchner, _Pflanzen
Krankheiten_, Stuttgart, 1890.
Of more historical importance are the older classifications of
Berkeley, _Gardeners' Chronicle_, 1854, and Re, _Gardeners'
Chronicle_, 1849-50. These latter are interesting as showing
the very different views held by the earlier workers, and
comparison of these with the modern views helps to mark the
progress of physiology during the half century which has
intervened.
CHAPTER XII.
CAUSES OF DISEASE. THE LIVING ENVIRONMENT.
_Causes due to animals--Vertebrata--Wounds, etc.--Invertebrata
--Insects, etc.--Plants as causes of disease--Phanerogams,
weeds, etc.--Cryptogams, fungi--Epidemics, etc._
Passing now to those causes of disease which are connected with the
living environment, we may obviously divide them into two groups of
agents, animals and plants.
Among animals, the various vertebrata, including man, are especially
responsible for the larger kinds of wounds and wholesale destructive
processes due to breakage, stripping of leaves and bark, cutting and
biting, and so forth. Cattle, rabbits, rats and mice, squirrels and
birds of various kinds stand out prominently as enemies to trees and
other plants, to which they do immense injury in various ways by their
horns, teeth, claws, and beaks; and the damage which an ignorant
gardener or forester can do with his ill-guided footsteps, axe, spade,
and knife can only be appreciated by one who knows the habits of plants.
It is among the invertebrata, however, especially insects and worms,
that the most striking agents of disease in plants are to be found, for,
with the exception of certain rodents--and we may logically include also
human invasions--vertebrate animals do not often appear in such numbers
as to bring about the epidemics and scourges only too commonly caused by
insect pests.
Insects injure plants in very various ways. Some, such as locusts,
simply devour all before them; others, _e.g._ caterpillars, destroy the
leaves and bring about all the phenomena of defoliation. Others, again,
eat the buds--_e.g._ _Grapholitha_; or the roots--_e.g._ wire-worms, and
so maim the plant that its foliage and assimilation suffer, or its roots
become too scanty to supply the transpiration current. Many aphides,
etc., puncture the leaves, suck out the sap, and produce deformations
and arrest of leaf-surface, as well as actual loss of substance, and
when numerous such insects induce all the evils of defoliation. Others,
such as the leaf-miners, tunnel into the leaves, with similar results on
a smaller scale.
It must be remembered that a single complete defoliation of a herbaceous
annual, or even of a tuberous plant like the potato, so incapacitates
the assimilatory machinery of the plant, that no stores can be put aside
for the seeds, tubers, etc., of another year, or at most so little that
only feeble plants come up.
In the case of a tree the case is different, and since most large trees
in full foliage have far more assimilatory surface than is actually
necessary for immediate needs, a considerable tax can be paid to
parasites or predatory insects before the stores suffer perceptibly.
Still, it should be recognised that the injury tells in time, especially
in seed years.
Many larvae of beetles, moths, etc., bore into the bark and as far as
the cambium or even into the wood or pith of trees, the local damage
inducing general injuries in proportion to the number of insects at
work: moreover, the wounds afford points of entrance for fungi and other
pests.
Galls and similar excrescences result from the hypertrophy of young
living tissues pierced by the ovipositors of various insects, and
irritated by the injected fluid and the presence of the eggs and larvae
left behind. They may occur on the buds, leaves, stems, or roots, as
shown by various species of _Cynips_ on oak, _Phylloxera_ on vines,
etc., in all cases the local damage being relatively small, but the
general injury to assimilatory, absorptive, and other functions is great
in proportion to the number of points attacked.
Many grubs--larvae of flies, beetles, etc.--bore into the sheaths or
internodes of grasses, or the pith of twigs, or into buds, fruits, and
other organs of plants, and do harm corresponding to the kind and amount
of tissues injured.
Various species of so-called eelworms--Nematodes--also cause gall-like
swellings on young roots, or they invade the grains of cereals.
Finally, various slugs and snails cause much injury by devouring young
leaves and buds and diminishing the assimilatory area.
Plants as agents of disease or injury fall naturally into the two main
categories of flowering plants (Phanerogams) and Cryptogams, among which
the fungi are the especially important pests.
Beginning with weeds, we find a large class of injurious agents. Weeds
damage the plants we value by crowding them out in the struggle for
existence, as already stated, and when the weed-action is simply due to
superfluous plants of the same species, we speak of overcrowding. But it
must not be overlooked that the competition between crowded plants of
the same species--where every individual is acting as a weed to the
others--may be more dangerous than between plants and weeds belonging to
other species and genera, because in the former case they are struggling
for the same minerals and other necessary food-materials: a matter of
importance in connection with the rotation of crops.
The question of allowing grass to grow at the foot of fruit trees, as in
orchards, is a good case in point. Such grass may increase the damp and
shade, thus favouring fungi at one season, and dry up the moisture of
the soil to the injury of the fine superficial roots at another, as well
as exhaust the soil, owing to the competition of the roots for salts
and other materials. On the other hand, the checking of surface roots by
competition with the grass has been claimed as advantageous. In this
connection probably the whole question of the composition of the turf
arises, as well as that of possible cropping for hay, and manuring.
As regards any particular weed, the cultivator should learn all he can
respecting its duration, seeding capacity, method of dissemination, the
depth and spread of its root-system, and any other particulars which
enable him to judge when and how to attack it. It is only necessary to
see the victory of such drought-resisting weeds as _Hieracium
pilosella_, Plantains, _Hypochaeris_, on lawns to realise how weeds may
win in the struggle for existence with the finer grasses.
Many so-called weeds are, however, partially parasitic, with their roots
on the roots of others--_e.g._ _Rhinanthus_, _Thesium_, etc., and much
damage is done to meadow grasses and herbage by the exhaustive tax which
these semi-parasites impose.
This is carried still further in the case of such root-parasites as
_Orobanche_, where the host-plant is burdened with the whole support of
the pest, because the latter, having no chlorophyll, is entirely
dependent on the former for all its food.
Even ordinary climbing plants may injure others by shading them, either
by scrambling over their branches--_e.g._ Bramble, or twisting their
tendrils round the twigs--_e.g._ Bryony, or twining round them--_e.g._
Woodbine, _Convolvulus_, etc. The principal direct injury is in these
cases owing to the loss of light suffered by the shaded foliage, but
the weed-action is often increased by the competition of their
roots--_e.g._ briars; and in the case of woody climbers the gradually
increased pressure of the woody-coils round the thickening stems
compresses the cambium and cortex of the support and induces strictures
and abnormalities which may be fatal in course of time.
Epiphytes, or plants which support themselves wholly on the trunks,
branches, or leaves of other plants, also injure the latter more
especially by shading their foliage--_e.g._ tropical Figs, Orchids,
Aroids, etc.; and similar damage is done by our own Ivy, the main roots
of which are in the soil, but the numerous adventitious roots of which
cling to the bark.
When the climber or epiphyte is also parasitic, as in the case of the
Dodder, _Loranthus_, Mistletoe, etc., the direct loss of substance
stolen from the host by the parasite comes in to supplement any effect
of shading that the latter may bring about if it is a leafy plant.
Of Cryptogams, apart from a few epiphytic ferns, and the intense
weed-action of certain Equisetums, the rhizomes and roots of which are
as troublesome as those of twitch and other phanerogamic weeds, it is
especially the fungi which act as agents of disease, and which, as we
now know, are _par excellence_ the causes of epidemics.
The action of fungi may be local or general; and restricted, slow and
insidious, or virulent and rapidly destructive.
Examples of local action are furnished by _Schinzia_, which forms
gall-like swellings on the roots of rushes; _Gymnosporangium_, which
induces excrescences on the stems of junipers, and numerous leaf-fungi
(_Puccinia_, _AEcidium_, _Septoria_, etc.), which cause yellow, brown, or
black spots on leaves, as well as by _Ustilago_, which attacks the
anthers or the ovary of various plants, and so forth. In such cases the
injury done by a few centres of infection is very slight, but prolonged
action may bring into play secondary effects such as the gradual
destruction of the cambium round a branch, when, of course, the effect
of ringing results; or if the fungus becomes epidemic and myriads of
leaf-spots are formed, the destruction of foliar tissue, gradual taxing
of the assimilatory cells, etc., may end in rapid defoliation, and
renewed attacks soon exhaust the plants and lead to sterility and death,
as often occurs with Uredineae--_e.g._ the coffee leaf-disease.
It is highly probable that such fungi are particularly exacting owing to
their exhausting demands for compounds of potassium, phosphoric acid,
and other bodies.
Examples of virulent and rampant general action are afforded by finger
and toe in turnips, etc., where the roots are invaded by
_Plasmodiophora_, which induces hypertrophy and rotting of the roots;
and by the damping off of seedlings, where the fungus _Pythium_ rapidly
invades all parts of the seedlings and reduces them to a water-logged,
putrefying mass; or the potato-disease, which is due to the rapid
spread of _Phytophthora_ in the leaves and throughout the plant, which
it blackens and rots in a few days.
Many fungi not in themselves very virulent or aggressive do enormous
harm owing to the secondary effects they induce. Some of the
tree-killing hymenomycetes, such as _Agaricus melleus_, for instance,
penetrate the wood of a pine at the collar, and the result of the large
flow of resin which results is to so block up the water passages that
the tree dies off above with all the symptoms of drought. Similarly, the
_Peziza_ causing the larch disease, having obtained access to the stem
about a foot or so above the ground, will gradually kill the cambium
further and further round the stem, and so girdle the tree as
effectually as if we had cut out the new wood all round. In all such
cases--and the same applies to the leaf-diseases referred to above--the
fungus may be compared to an army which is not strong enough to invade
the whole territory, but which, by striking at the lines of
communication, cuts off the supplies of water, food, etc., and so brings
the struggle to an end. Indeed we might compare the cases of fungi which
attack the root and collar, and so strike at and cut off the water
supply, to a compact army which at once cuts off the enemy from his
narrow base; whereas the innumerable units which bring about an epidemic
attack on the leaves, and so surround the enemy and cut off his food
supplies all round, is rather like a much larger army which cannot get
in beyond the natural barriers of the tissues, and so puts a _cordon_
all round the territory and seizes the multitudes of food-stuffs at the
frontiers. The end result is similar in both cases, but the methods of
warfare differ.
Many fungi, however, though they make their presence noticeable by
conspicuous signs, cannot be said to do much damage to the individual
plant attacked. The extraordinary malformations induced by parasites
like _Exoascus_, which live in the ends of twigs of trees and stimulate
the buds to put out dense tufts of shoots, again densely
branched--Witches' brooms--are a case in point. Also the curious
distortions of nettle stems swollen and curved by _AEcidium_, of maize
stems and leaves attacked by _Ustilago_, and of the inflorescences of
_Capsella_ by _Cystopus_, etc., are not individually very destructive;
it is the cumulative effects of numerous attacks, or of large epidemics,
which tell in the end.
Some very curious effects are due to fungi such as _AEcidium elatinum_,
which, living in the cortex of firs, stimulate buds to put out shoots
with erect habit, and with leaves which are radially disposed, annually
cast, and differently shaped from the normal--characters quite foreign
to the species of fir in its natural condition.
Equally strange are the shoots of _Euphorbia_ infested with the aecidia
of _Uromyces_, those of bilberries affected with _Calyptospora_, etc. In
all these cases we must assume a condition of toleration, so to speak,
on the part of the host, which adapts itself to the altered
circumstances by marked adaptations in its tissue developments, mode of
growth and so forth.
This toleration is perhaps most marked in the case of those cereals
which, though infected by the minute mycelium of _Ustilago_ while still
a seedling, nevertheless go on growing as apparently healthy green
plants indistinguishable from the rest, although the fine hyphae of the
parasite are in the tissues and keeping pace with the growth of the
shoots just behind the growing points. As the grains of the cereal begin
to form and swell, however, the hyphae suddenly assume the part of a
dominant aggressor, consume the endosperm of the enlarging seed, and
replace the contents of the grain with the well-known black spores known
as Smut.
NOTES TO CHAPTER XII.
The reader will find a summary of such fungi as are here
concerned in Massee, _A Text-Book of Plant Diseases_, 1899, or
Prillieux, _Maladies des Plantes Agricoles_.
For further details the student should consult the works of
Frank and Sorauer referred to in the notes to Chapter IX., and
Tubeuf, _The Diseases of Plants_, Engl. ed. 1897, pp. 104-539.
For experiments on the effects of grass on orchard trees, see
_Report of the Woburn Experimental Fruit Farm_, 1900, p. 160.
For the further study of weeds, the interesting bulletins of
the Kansas State Agricultural College, 1895-1898, will show
the reader what may be done in the matter of classifying them
according to their biological peculiarities.
In regard to insects, the reader will find the following list
embraces the subject: Somerville, _Farm and Garden Insects_,
1897; Theobald, _Insect Life_, 1896; Ormerod, _Manual of
Injurious Insects_, 1890, and _Handbook of Insects Injurious
to Orchards, etc._, 1898.
The admirable series of publications of the U.S. Department of
Agriculture under the editorship of Riley and Howard, and
entitled _Insect Life_, 1888-1895, also abounds in
information.
Further, Taschenberg's _Praktische Insektenkunde_, 1879-1880,
and Judeich and Nietsche, _Lehrbuch der Mitteleurop. Forst.
Insektenkunde_, 1889.
For an elementary introduction to the study of fungus
diseases, see Marshall Ward, _Diseases of Plants_, Soc. for
Promoting Christian Knowledge, London.
CHAPTER XIII.
NATURE OF DISEASE.
_General and local disease--General death owing to cutting-off
supplies, etc.--Disease of organs--Tissue-diseases, e.g.
timber--Root-diseases--Leaf-diseases, etc.--Diseases of
Respiratory, Assimilatory, and other organs--Physiological and
Parasitic diseases--Pathology of the cell--Cuts--Cork--Callus
--Irritation--Stimulation by protoplasm--Hypertrophy._
On going more deeply into the nature of those changes in plants which we
term pathological or diseased, it seems evident that we must at the
outset distinguish between various cases. A plant may be diseased as a
whole because all or practically all its tissues are in a morbid or
pathological condition, such as occurs when some fungus invades all the
parts or organs--_e.g._ seedlings when completely infested by _Pythium_,
or a unicellular Alga when invaded by a minute parasite; or it may die
throughout, because some organ with functions essential to its life is
seriously affected--_e.g._ the roots are rotten and cannot absorb water
with dissolved minerals and pass it up to the shoot, or all the leaves
are infested with a parasite and cannot supply the rest of the plant
with organic food materials, in consequence of which parts not directly
affected by any malady become starved, dried-up, or poisoned or
otherwise injured by the results or products of disease elsewhere.
In a large number of cases, however, the disease is purely local, and
never extends into the rest of the organs or tissues--_e.g._ when an
insect pierces a leaf at some minute point with its proboscis or its
ovipositor, killing a few cells and irritating those around so that they
grow and divide more rapidly than the rest of the leaf tissues and
produce a swollen hump of tissue, or gall; or when a knife-cut wounds
the cambium, which forthwith begins to cover up the dead cells with a
similarly rapid growth of cells, the callus. Numerous minute spots due
to fungi on leaves, cortex, etc., are further cases in point, the
mycelium never extending far from the centre of infection.
Many attempts have been made to classify diseases on a basis which
assumes the essential distinction of the above cases, and we read of
diseases of the various organs--root-diseases, stem-diseases,
leaf-diseases, and so forth; or of the various tissues--timber-diseases,
diseases of the cambium, of the bark, of the parenchyma, and so on.
Furthermore, attempts have been made to speak of general functional
disease, of diseases of the respiratory organs, of the absorptive
organs, and so forth, as opposed to local lesions.
Critical examination, however, shows that no such distinctions can be
consistently maintained, partly because the organs and functions of
plants are not so sharply marked off as they are in animals, the
diseases of which have suggested the above classification, and partly
because all disease originates in the cells and tissues, and it is a
matter of detail only that in some cases--_e.g._ severe freezing or
drought of seedlings, or when some ingredient is wanting in the
soil--the diseased condition affects practically every cell alike from
the first, while in others it spreads more or less rapidly from some one
spot.
Even the distinction into physiological diseases _versus_ parasitic
diseases cannot be maintained from the standpoint of the nature of the
disease itself. All disease is physiological in so far as it consists in
disturbance of normal physiological function, for pathology is merely
abnormal physiology, no matter how it is brought about. This is not
saying that no importance is to be attached to the mode in which disease
is incurred or induced: it is merely insisting on the truth that the
disease itself consists in the living cell-substance--the
protoplasm--not working normally as it does in health, and this, whether
want of water, minerals, or organic food be the cause, or whether the
presence of some poison or mechanical irritant be the disturbing agent,
as also whether such want or irritation be due to some defect in soil
or air, or to the ravages of a fungus or an insect.
This being understood I need not dwell on the common fallacy of
confounding the fungus, insect, soil or other agent with the disease
itself, or of making the same blunder in confusing symptoms with
maladies. In this sense, wheat rust is not a disease: it is a symptom
which betrays the presence of a disease-inducing fungus, the Rust
fungus. Similarly, chlorosis is not a disease: it is a symptom of
imperfect chlorophyll action, and the best proof of the truth of both
statements is that in both cases the fundamental disease-action is the
starvation of the cell-protoplasm of carbohydrates and other essential
food matters--in the one case because the fungus steals the
carbohydrates as fast as the leaves can make them, in the second because
the leaf is unable to make them.
The foundation of a knowledge of disease in plants therefore centres in
the understanding of the pathology of living cells.
If a suitable mass of living cells is neatly cut with a sharp razor the
first perceptible change is one of colour: the white "flesh" of a potato
or an apple, for instance, turns brown as the air enters the cut cells,
and the microscope shows that this browning affects cell-walls and
contents alike. The cut cells also die forthwith; and the oxygen of the
air combining with some of their constituents forms the brown colouring
matter which soaks into the cell-walls. The uninjured cells below them
grow longer, pushing up the dead debris, and divide across by walls
parallel to the plane of the wound, and so form series of tabular cells
with thin walls, which also soon turn brown and die, the cell-walls
meanwhile undergoing changes which convert them into cork. The living
cells deeper down are now shut off from the outer world by a skin, of
several layers, of cork-cells, which prevent the further free access of
air or moisture. During the period of active cell-division which
initiates the cork, the temperature of the growing cells rises: a sort
of fever (wound-fever) is induced, evidently owing to the active
respiration of the growing cells.
This healing by cork occurs in any tissue of living cells exposed by a
cut--leaf-tissue, young stem or root, fruit, cambium, etc.; and the same
applies to any other kind of cutting or tearing injury--such as a prick
with a needle or the proboscis of an insect, a stripping, or even a
bruise.
Such healing is prepared for and carried out very thoroughly in the case
of falling leaves and cast branches, the plane of separation being
covered by a cicatrix of cork.
If the cell-tissue under the wound is actually growing at the time,
however, a further process is observed when the wound-cork has been
formed. The uninjured cells below go on growing outwards more vigorously
than ever, the pressure of the overlying tissues taken off by the cut
having been removed, and, lifting up the cork-layer as they do so, they
rapidly divide into a juicy mass of thin-walled cells which is of a
cushion-like nature and is termed a _Callus_. This callus is at first a
homogeneous tissue of cells which are all alike capable of growing and
dividing, but in course of time it undergoes changes in different parts
which result in the formation of tracheids, vessels, fibres and other
tissue-elements, and even organs, just as the embryonic tissues of the
growing points, cambium, etc., of the healthy plant give origin to new
growths. Such wound-wood, however, is apt to differ considerably in the
arrangement, constitution and hardness of its parts as compared with
normal wood, and its peculiar density and cross-graining are often
conspicuous.
If instead of a simple tissue, the cut or other wound lays bare a
complex mass such as wood, the resultant changes are essentially the
same to start with. The living cells bordering the wound form cork, and
then those deeper down grow out and form a callus. The exposure of the
wood however, entails alterations in its non-living elements also. The
lignified walls of tracheids, fibres, etc., turn brown to a considerable
depth, and this browning seems to be--like all such discolorations in
wounds--due to oxidation changes in the tannins and other bodies
present: the process is probably similar to what occurs in humification
and in the conversion of sap-wood into heart-wood in trees. Such wood is
not merely dead, but it is also incapable of conveying water in the
lumina of its elements, which slowly fill with similarly dark-coloured,
impervious masses of materials termed "wound-gum," the nature of which
is obscure, but which slowly undergoes further changes into resin-like
substances.
The exposure of wood by a wound results also in another mode of stopping
up the vessels and so hindering the access of air, loss of water, etc.,
for the living cells of the medullary rays and wood-parenchyma grow into
the lumina of the larger vessels through the pits, forming _thyloses_,
again a phenomenon met with in heart-wood. In Conifers the stoppage of
the lumina is increased by deposition of resin, which also soaks into
the cell-walls and the wounded wood becomes semi-translucent owing to
the infiltration.
Every living cell in an active condition is irritable, and one of the
commonest physiological reactions of growing tissues is that of
responding to the touch of a resistant body, as is vividly shown by the
movements of the Sensitive plant, _Dionaea_, etc., and by those of
tendrils, growing root tips, etc., on careful observation. We have
reason for stating that if a minute insect, too feeble to pierce the
cuticle, cling on to one side of the dome-shaped growing point of any
shoot, the irritation of contact of its claws, hairs, etc., would at
once cause the protoplasm of the delicate cells to respond by some
abnormal behaviour; and, as matter of experiment, Darwin showed long ago
that if a minute piece of glass or other hard body is kept in contact
with one side of the tip of a root, the growth on the side in contact is
interfered with. Moreover we know from experiments on heliotropism,
thermotropism, etc., that even intangible stimuli such as rays of light,
etc., impinging unsymmetrically on these delicate cells cause
alterations in their behaviour--_e.g._ arrest or acceleration of growth.
Perhaps the most remarkable class of stimulations, however, is that due
to the presence of the entire protoplasmic body of one organism in the
cell of another, each living its own life for the time being, but the
protoplasm of the host cell showing clearly, by its abnormal behaviour,
that the presence of the foreign protoplasm is affecting its physiology.
A simple example is afforded by Zopfs' _Pleotrachelus_, the amoeboid
protoplasmic body of which lives in the hypha of _Pilobolus_, causing it
to swell up like an inflated bladder, in which the parasite then forms
its sporangia. The _Pleotrachelus_ does not kill the _Pilobolus_, but
that its protoplasm alters the metabolic physiology of the latter is
shown by the hypertrophy of the cells, and by the curious fact that it
stimulates the _Pilobolus_ to form its sexual conjugating cells,
otherwise rare, an indication of very far-reaching interference with the
life-actions of the host.
An equally remarkable example is that of _Plasmodiophora_, the amoeboid
naked protoplasm of which lives and creeps about in the protoplasm of a
cell of the root of a turnip, to which it gains access through the
root-hairs. It does not kill the cell, but stimulates its protoplasm to
increased activity and growth and division, itself dividing also and
passing new amoebae into each new daughter-cell of the host. Here the
processes of stimulation, hypertrophy and further division are repeated,
until hundreds or thousands of the turnip root-cells are infected. The
externally visible result is the formation of distorted swellings on the
root (Finger and Toe), most of the cells of which are abnormally large
and filled with amoeboid _Plasmodiophora_ protoplasm, which finally
devours the turnip-protoplasm and itself passes over into spores. Here
we have most convincing proof of the stimulation of protoplasm by other
protoplasm in direct contact with it; and that the metabolism of the
host-cells is profoundly altered is shown not only by the abnormal
growth of the cells, but also by the starvation of the rest of the
turnip plant as the _Plasmodiophora_ gets the upper hand. We have here,
in fact, a local intracellular parasitic disease, gradually invading
large tracts of tissue and eventually inducing general disease resulting
in death--a state of affairs reminding us of cancer in animals.
Irritation and hypertrophy of cells, however, may be induced by
parasites which never bring their protoplasm into direct contact with
that of the host. Many Chytridiaceae penetrate the cells of plants, and
grow inside them as short tubes, vesicles, etc., the protoplasm of which
is separated by their own cell-walls from that of the host-cell;
nevertheless hypertrophy and abnormal cell-divisions and secretions are
induced, and the effect even extends to neighbouring cells--_e.g._
_Synchytrium_--showing that some influence is exerted through cells
themselves not directly affected. This latter point need not surprise us
now we know that the cells of plant-tissues are connected by fine
protoplasmic strands passing through the separating cell-walls.
But the invading plant need not actually enter the cells, and may still
stimulate them through both its own and their own cell-walls to abnormal
growth. This is well shown by the intercellular mycelium of _Exoacus_
and _Exobasidium_, and the latter affords an excellent illustration of
the far-reaching effects of hyphae on the cells (of _Vaccinium_) into
which they do not penetrate. Not only are the cells stimulated to grow
larger and divide oftener than normally, thus producing large gall-like
swellings, but the chlorophyll disappears, the cell sap changes colour
to red, the numerous compound crystals normally found in the tissues
diminish in number and are different in shape, large quantities of
starch are stored up, and even the vascular bundles are altered in
character. All these changes indicate very profound alterations in the
physiological working of the protoplasm of the cells of the host, and
yet the fungus has done its work through both its own cell-walls and
those of the host.
Even harmless endophytic algae in the intercellular spaces of plants may
stimulate the cells in their immediate neighbourhood to increased
growth, _e.g._ _Anabaena_ in the roots of Cycads.
NOTES TO CHAPTER XIII.
With reference to cork-healing and wound-fever the student may
consult Shattock "On the Reparative processes which occur in
Vegetable Tissues," _Journal of the Linnean Society_, 1882,
Vol. XIX., p. 1; and Shattock "On the Fall of Branchlets in
the Aspen," _Journal of Botany_, 1883, Vol. XXI., p. 306. Also
Richards, "The Respiration of Wounded Plants," _Annals of
Botany_, Vol. X., 1896, p. 531; and "The Evolution of Heat by
Wounded Plants," _Ann. of Bot._, Vol. XI., 1897, p. 29.
For details and figures respecting callus, see Sorauer,
_Physiol. of Plants_, p. 175.
In respect to the irritable movements referred to see Darwin,
_The Power of Movements in Plants_, 1880, chapter III. The
recent work of Nawaschin, _Beobachtungen ueber den feineren
Bau u. Umwandlungen von Plasmodiophora_, Flora, Vol. LXXXVI.,
1899, p. 404, should be read for details and literature
concerning "Finger and Toe."
CHAPTER XIV.
NATURE OF DISEASE (_Continued_).
_Actions of poisons in small doses--Results of killing a few
cells--Malformation--Enzymes--Secretions and excretions--
Acids, poisons, etc.--Chemotactic phenomena--Parasitism--
Epiphytes and endophytes--Symbiosis--Galls._
Physiological research has shown that the respiratory activity of cells
may be increased by small doses of poisons, and even that growth may be
accelerated by them--_e.g._ chloroform, ether--and, still more
remarkable, that fermentative activity may be enhanced by minute doses
of such powerful mineral poisons as mercuric chloride, iodine salts,
etc., and that the cells may be gradually accustomed to larger doses
without injury. Unfertilised eggs of insects have been started into
growth by treatment with acids and those of frogs with mercury salts,
and the germination of beans quickened by various poisonous alkaloids.
In other words, graduated doses of poison may alter the physiological
activity of living cells, inducing pathological phenomena, while larger
doses kill them.
Now we know at least one parasitic fungus which poisons the cells of its
host, and kills them, with similar symptoms to those resulting from
excessive doses of the above-named toxic agents. _Botrytis_ hyphae,
living in the cell-walls of plants, but not entering the cells, excretes
a poison which kills the protoplasm, and the fungus then feeds on the
debris. Numerous other fungi form powerful poisons, but we do not know
whether or how they employ them--_e.g._ Ergot.
It is obvious that if all the young cells of a root-tip or of the apex
of a shoot, or those of a young leaf, are growing and dividing
regularly, the killing of one or a few cells at one point on the side of
the organ must result in irregularities--in malformation--of the adult
organ. This has been proved experimentally by destroying a few cells
with a needle. It can also be done by planting a minute mycelium of
_Botrytis_ laterally on a young organ--_e.g._ a very young lily-bud. The
fungus adheres to the surface, kills a few epidermis cells, and forms a
foxy-red spot, which becomes concave as the dead cells lose water and
dry. Since the rest of the bud goes on growing, however, while this dead
point remains stationary, the latter gradually becomes the centre of a
concavity, the growing tissues having grown round it: the bud is
deformed. Numerous cases of malformed organs are explained in this way;
a minute insect has bitten or pierced the young tissue, or a fungus has
killed a minute area, or a drop of acid condensed from fumes in the air
is the lethal agent, and so forth. And even on a much larger scale we
see the same kinds of agents at work. Wherever a patch of cells is
killed whilst those around go on growing, there must result some
deformation of the resulting organ, since had the injury been withheld
the number and sizes of the cells now fixed in death would have
increased and covered a larger area: they now serve to pull over to
their side the still living and growing cells. The same results follow
on any lateral wound: the killed spot of tissue serves as a point round
which the continued growth of other parts of the organ turns. Hence the
malformation is in these cases a secondary effect, and not, as in simple
hypertrophy, a direct effect of the action of the cells involved in the
injury.
There is another class of bodies secreted by fungi, however, which act
directly on cells, viz. enzymes--that is, soluble bodies which are able
to dissolve cellulose (_cytases_), starch (_diastases_), proteids
(proteolytic enzymes), and other substances, by peculiar alterations in
their constitution. It is by means of its _cytase_ that _Botrytis_
hyphae pierce the cellulose walls of plants, and no doubt in all cases
where fungi pierce cell-walls it is by the solvent action of such a
cytase, and similarly when haustoria penetrate into the cells. It is
also by means of these starch-dissolving enzymes (diastases) and
proteolytic enzymes, etc., that the hyphae inside the cells are enabled
to make use of the starch, proteids, etc., they find there.
All living cells form materials, resulting from the activity of the
protoplasm, which we may compare with the refuse or by-products formed
in any great manufacturing industry: these by-products have to be got
rid of if they are injurious or noisome (_excretions_), and if
not--_i.e._ if they are capable of further use (_secretions_)--they have
to be stored away till required. Some of the most prominent of these
bodies excreted by fungi are, as we have seen, poisonous acids, such as
oxalic acid, enzymes, and organic poisons, such as those in ergot. But
similar enzymes, acids, poisons, etc., to those found in fungi are also
found in the cells of other plants and animals; for only by means of
their solvent actions can processes like digestion and assimilation of
the starchy and other materials into the body-substance be accomplished,
and we have seen that it is a general property of living cells to form
acids, and other excretions and secretions.
Now we know very little about what may happen when an organism--say a
fungus--secreting especially one kind of enzyme or poison or other
active substance, comes into intimate contact with another--say a
leaf-cell--which secretes predominantly others, but what we do know
points to the certainty that various complications will occur.
For instance, if certain bacteria which prefer an alkaline medium, and
yeasts which prefer an acid environment are mixed in a saccharine
solution, it depends on the reaction of the liquid which organism gains
the upper hand: if the liquid is acid the yeast may dominate the
bacteria; if alkaline it may be suppressed by them.
That a parasite may be prevented from successfully attacking a
particular plant is shown by the failure of _Cuscuta_ to establish its
haustoria in poisonous plants such as _Euphorbia_, _Aloe_, etc., and it
has been pointed out that poisonous secretions in the cells of the plant
protect them against the penetration of fungi. This cannot be taken as
meaning that any poison protects against any parasite, however, for
_Euphorbia_ is itself subject to attacks of Uredineae, and _Pangium
edule_, which contains prussic acid and is extremely poisonous to most
animals, is eaten with avidity by several insects, while nematode worms
can live in its tissues. This is no more remarkable, however, than the
fact that _Fontaria_, a myriapod, secretes prussic acid in its own
tissues, or than that certain glands of the stomach secrete free
hydrochloric acid, and _Dolium_ forms sulphuric acid in its glands.
There is yet a further point to notice here. It has been proved that
certain substances formed in plant-cells, not necessarily nutritive,
attract the hyphae of parasitic fungi or repel them, according to the
kind and degree of concentration. So clear has this proof been made that
it was possible in experiments conducted apart from a host plant, to
make the hyphae on one side of an artificial membrane--_e.g._
collodion--penetrate it by placing one of these attractive
(_chemotropic_) substances in suitable proportions on the other side.
The hyphae dissolved holes in the membrane by means of enzymes and
plunged into the attractive substance on the other side.
The foregoing sketch gives us a glimpse into the causes at work in
parasitism.
Suppose a fungus on the outside of the epidermis of a young organ--say a
leaf. It may be unable to penetrate into the plant, and finding no
suitable food outside it dies: or it may be satisfied with the traces of
organic matter on the epidermis and then lives the life of a saprophyte.
Or it may be able to establish a hold-fast on the tender epidermal
surface, but without entering the cells, and irritate the developing
organ by contact stimulation, inducing slight abnormalities; if in its
further, purely superficial growth such an epiphyte covers large areas
of the leaf, and especially if the hyphae are dark coloured--_e.g._
_Dematium_ and other "Sooty Moulds"--injury may be done to the leaf
owing to the shading action which deprives the chlorophyll below of its
full supply of solar energy. Some epiphytes, however, are able to fix
their hyphae to the epidermis by sending minute peg-like projections
into the cuticle--_Trichosphaeria_, _Herpotrichia_--while others send
haustoria right through the outer epidermal walls--_e.g._
_Erysiphe_--and thus supplement mere contact-irritation and shading by
actual absorption from the external cells. Here the fungus is a
parasitic epiphyte.
A stage further is attained in those fungi which enter the stomata and
live in the intercellular spaces--_e.g._ many Uredineae and
_Phytophthora_--and many such intercellular endophytes increase their
attack on the cells by piercing their walls with minute (_Cystopus_) or
large and branched (_Peronospora_) haustoria, or even eventually pierce
the cells and traverse them bodily (_Pythium_). In all these cases it is
clear that conflicts must occur between poison and antidote, acid and
alkali, attractive and repellent substances, enzyme and enzyme, etc., as
was hinted at above; and the same must take place when the parasite is
endophytic and intracellular from the first, as in Chytridiaceae, etc.,
the zoospores of which pierce the outer cell-walls and forthwith grow
into the cells. There are also fungi which, while able to pierce the
outer cell-walls, and grow forward in the thickness of the wall itself,
cannot enter the living cells themselves--_e.g._ _Botrytis_. In the
example mentioned, the fungus excretes a poison, oxalic acid, which
soaks into and kills the cells next its point of attack: into these dead
cells it then extends, and, invigorated by feeding on them, extends into
other cell-walls and excretes more poison, and so on.
On the basis of the foregoing it seems possible to sketch a general view
of the nature of parasitism. In order that a fungus may enter the cells
it must be able to overcome not only the resistance of the cell-walls,
but that of the living protoplasm also: if it cannot do the latter it
must remain outside, as a mere epiphyte, or at most an intercellular
endophyte. If it can do neither it must either content itself with a
saprophytic existence or fail, so far as that particular host-plant is
concerned. Its inability to enter may be due to there being no
chemotropic attraction, or to its incapacity to dissolve the cell-walls,
or to the existence in the cell of some antagonistic substance which
neutralises its acid secretions, destroys its enzymes or poisons, or is
even directly poisonous to it.
Moreover when once inside it does not follow that it can kill the cell.
The protoplasm of the latter may have been unable to prevent the fungus
enemy from breaking through its first line of defence--the cell-wall,
but it may be quite capable of maintaining the fight at close quarters,
and we see signs of the progress of the struggle in hypertrophy,
accumulation of stores, and other changes in the invaded cells and their
contents.
Finally, the invested or invaded cell may so adapt itself to the demands
of the invader that a sort of arrangement is arrived at by which life in
common--_Symbiosis_--is established, each organism doing something for
the other and each taking something from the other. In this latter case,
which is often realised--_e.g._ lichens, leguminous plants and the
organisms in their root-nodules, mycorrhiza, etc.--we leave the domain
of disease, which supervenes indeed if the other symbiont is lacking.
Some interesting facts bearing on the matters here under discussion,
have been obtained from the study of _Galls_, the curious outgrowths
found on many plants and due to the action of insects.
A typical gall exhibits three distinct and characteristic layers of
tissue surrounding the hollow chamber in which the larva of the insect
lies, viz., an outer layer of soft cells forming a parenchyma covered
with an epidermis, and frequently also with a layer of cork; an inner
stratum consisting of very thin-walled delicate cells filled with
protoplasmic and reserve food-materials on which the larva feeds; and
between the two a more or less definite layer of thick-walled
sclerenchyma cells which serve as a protection against accidents to the
larva as the outer layer shrivels or rots, or if it is exposed to the
attack of marauders. This layer may be absent from galls which have a
short life only. Vascular bundles run into the outer layer from the
leaf-veins or the stele of the shoot, etc. Such galls abound in tannin,
and are frequently of use in the arts on this account: they also contain
starch, and proteid substances and crystals of calcium oxalate. When the
larva has consumed the stores of food material and reached the adult
stage it eats its way out and escapes.
The growth of such a gall is preceded by the laying of an egg on or in
the embryonic tissue of a leaf, stem, or other young part, and it is
interesting to note that only organs in the meristematic stage can form
galls, and that it is by no means necessary that the tissues should be
wounded. Moreover, the egg as such is incapable of stimulating the plant
tissues, but when it hatches, the resulting larva, beginning to feed on
the cells, irritates the tissues and rapid growth and cell-division
occur, as in the case of other wounds or of fungus attacks. The actual
wound made by the ovipositor heals up at once. It is evident from
numerous recent researches that these true galls are not due to any
poisonous or irritating liquid injected by the parent, but that the
stimulus to the tissue formation is similar to that exerted by a wound.
The young gall is in fact a callus enclosing the living larva, and it is
the continued irritation of the latter which keeps up the stimulation.
The final shape and constitution of the gall depend on mutual
reactions--not as yet explained in detail--between the species of p
Showing first 200,000 characters. Full text: gutenberg.org/ebooks/39011