An Elementary Text-book of the Microscope
by J. W. Griffith · English · Gutenberg #48016 · 396,357 chars · public domain (US)
The Project Gutenberg eBook of An Elementary Text-book of the Microscope
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: An Elementary Text-book of the Microscope
Author: J. W. Griffith
Release date: January 18, 2015 [eBook #48016]
Most recently updated: October 24, 2024
Language: English
Other information and formats: www.gutenberg.org/ebooks/48016
Credits: Produced by deaurider, Chuck Greif and the Online
Distributed Proofreading Team at http://www.pgdp.net (This
file was produced from images generously made available
by The Internet Archive)
*** START OF THE PROJECT GUTENBERG EBOOK AN ELEMENTARY TEXT-BOOK OF THE MICROSCOPE ***
[Illustration: Plate I.
_London, John Van Voorst._]
AN
ELEMENTARY TEXT-BOOK
OF
THE MICROSCOPE;
INCLUDING A DESCRIPTION OF THE METHODS OF
PREPARING AND MOUNTING OBJECTS, ETC.
BY
J. W. GRIFFITH, M.D., F.L.S., ETC.,
MEMBER OF THE ROYAL COLLEGE OF PHYSICIANS; CONJOINT AUTHOR OF
THE MICROGRAPHIC DICTIONARY, ETC.
WITH TWELVE COLOURED PLATES,
CONTAINING 451 FIGURES.
[Illustration]
LONDON:
JOHN VAN VOORST, PATERNOSTER ROW.
MDCCCLXIV.
[_The right of translation is reserved._]
PREFACE.
The object of this little work is to furnish an elementary course of
instruction in the use of the Microscope, and on its application to the
examination of the structure of plants and animals. Assuming that the
reader has had no previous acquaintance with the Microscope, or with the
study of natural history, I have attempted to render the descriptions of
the objects as simple as possible. At the same time, the technical terms
have been added and explained, in order gradually to render them
familiar to the reader, and thus facilitate the future study of larger
and more detailed works. The objects figured and described comprise the
principal structures and more minute forms of both the vegetable and the
animal kingdom, those having been selected which are common and readily
procurable.
A chapter has been given upon the optical principles on which the
action of the instrument depends (which will assist the reader to
understand the operation of its constituent parts), including a sketch
of the subject of polarized light. The order in which the subjects are
treated is scientific, and particular directions have been given for the
examination of the objects.
The small size of the work has necessitated the exclusion of figurative
descriptions, so that it is adapted rather for a worker than a reader;
at the same time, the matter forms a course, and must be taken as a
whole for the proper comprehension of the subjects. The technical terms
used are referred to in the Index, so as to furnish to some extent a
glossary of terms; and their derivation is given, to facilitate their
recollection. The figures, with very few exceptions, are drawn from
nature, and are coloured that the objects may be more easily recognized.
The magnifying powers under which they have been drawn are denoted by a
small number placed beneath each figure: and the particular attention of
the reader is requested to this point; otherwise the whole subject will
be utterly confused; so much does the appearance of objects vary under
different powers.
Directions are given for preparing and mounting objects, implying that
the reader will collect specimens for himself, which is to be strongly
recommended as the best method of acquiring a practical and useful
acquaintance with the objects. These will serve to furnish permanent
landmarks in the great ocean of structural forms, will probably recall
in after-years pleasant recollections of early excursions in search of
the beauties of nature, and, surely, deepen the conviction of the
existence of their All-wise Creator.
J. W. G.
CONTENTS.
CHAP. PAGE
I. THE MICROSCOPE 1
II. THE MOUNTING OF OBJECTS 10
III. VEGETABLE ELEMENTS AND TISSUES 19
IV. VEGETABLE ORGANS 31
V. FERNS 49
VI. MOSSES 54
VII. ALGÆ 64
VIII. LICHENS 91
IX. FUNGI 96
X. ANIMAL ELEMENTS AND TISSUES 113
XI. ARTICULATA 127
XII. RADIATA 153
XIII. PROTOZOA 155
XIV. OPTICAL PRINCIPLES 167
PLATE I. [FRONTISPIECE.]
VEGETABLE TISSUES, &c.
Fig.
1. Leaf of Geranium: cells, chlorophyll, and intercellular passages.
2. Cells of Apple.
3. Starch-granules: _a_, of Wheat; _b_, of Arrowroot; _c_, of Potato;
_d_, of Oat; _e_, of Lentil; _f_, of Rice.
4. Cells of Potato, containing Starch.
5. Garden Rhubarb-stalk: _a_, raphides; _b_, reticulated duct; _c_,
spiral vessel; _d_, woody fibre; _e_, annular vessel.
6. Wood-cells from stem of Chrysanthemum.
7. Deal, transverse section: _a_, glandular tissue; _b_, woody fibre.
8. Deal, longitudinal (radial) section of glandular tissue.
9. Deal, longitudinal section of woody fibre.
10. Deal, tangential section.
11. Holly, wood of: _a_, porous cells; _b_, _d_, _e_, wood-cells; _c_,
dotted ducts.
12. Hairs, vegetable: _a_, _b_, of Groundsel; _c_, of London Pride; _d_,
_e_, of Geranium; _f_, of Chrysanthemum.
13. Epidermis of Geranium-leaf.
14. Style of Crocus, with pollen-granule and-tube.
15. Pollen-grain of Crocus, with pollen-tube.
16. Pollen of Primrose.
17. Pollen of Sunflower.
18. Pollen of _Convolvulus major_.
19. Caraway-seed.
20. Needle-point.
21. Sting of Nettle.
22. Hair of Spiderwort.
23. Hair of Spiderwort, single cell.
24. Epidermis of Geranium-petal.
25. Petal of Chickweed.
26. Sepal of Chickweed.
27. Seed of Poppy.
28. Epidermis of _Deutzia_.
29. Seed of Mignonette.
30. Pollen of Chickweed, dry.
31. Pollen of Chickweed, in water.
32. Flower of Chickweed.
33. Epidermis of petal of Chickweed.
34. Hairs of calyx of Chickweed.
35. Hairs of seed of _Collomia_.
36. Stem of Dicotyledon, section of.
37. Stem of Monocotyledon, section of.
38. Seed of Shepherd’s Purse, transverse section.
39. Stamen of Chickweed.
40. Stigma of Chickweed.
41. Ovary of Chickweed.
42. Leaf of Chickweed.
43. Seed of Wallflower, section of.
44. Seed of Wallflower, radicle and cotyledons.
45. Embryo-sac of Chickweed.
46. Embryo-sac of Chickweed.
47. Embryo-sac of Chickweed.
48. Wheat, cotyledon and leaves of, section.
49. Mustard-seed, cotyledons and radicle.
50. Mustard-seed, transverse section.
51. Chickweed, seed of.
52. Seed of _Eccremocarpus scaber_.
53. Grain of Wheat: _a_, cotyledon; _b_, embryo; _c_, radicle; _d_,
albumen.
54. Ovule of Wallflower.
55. Cotyledons of Chickweed.
56. Plum-stone, section of.
A
TEXT-BOOK
OF
THE MICROSCOPE.
CHAPTER I.
THE MICROSCOPE.
The microscope (from μικρὁς, little, and σκοπέω, to see), so called
because it enables us to see objects which are too small to be seen with
the naked eye, consists of several parts, each of which has its special
use. As the proper management of these is of great importance in the
successful application of the instrument to minute investigations, we
shall commence with the consideration of their names and uses, including
those of the more important pieces of accessory apparatus.
_Microscope._--The foot of the microscope is that part which supports
the instrument upon the table; it is connected above with the stand, of
which it is often considered a part. The stand sometimes consists of a
single rod or pillar; but in the best microscopes it is composed of two
upright plates, between which, at the upper part, the rest of the
microscope swings stiffly upon an axle. Arising from this axle,
indirectly through the medium of parts which require no special mention,
is an arm, to which the body is fixed. The body is moveable up and down
by one or two large milled heads, connected with a grooved rod or
pinion, which works in the teeth of a rack fixed to the back of the
body, or of the arm which supports the body. The large milled heads form
the “coarse movement,” as it is called.
On the top of the arm, or on the front and lower part of the body of the
microscope, is placed the “fine movement,” consisting of a small milled
head, with a fine screw, for moving the body through very small
distances.
Next is the “stage,” or flat plate, upon which the objects to be viewed
are placed. This is often so arranged that, by turning two milled heads,
the object can be moved backwards and forwards, or from side to side; it
is then a “moveable stage.”
The eye-piece slides into the upper end of the body; and the
object-glass screws into its lower end.
Beneath the stage is the mirror, which reflects the light through the
object, the object-glass, and eye-piece to the eye.
_Object-glasses._--The object-glasses are the most valuable parts of the
instrument. There are generally three or more of them; and, by means of
an “adapter,” any object-glass can be made to fit any microscope. Great
care is required in their use, especially to avoid scratching the lower
surface of the glass, which is sometimes accidentally done by pressing
the surface against any hard body, or allowing such a body to fall upon
it. When not in use, the object-glasses should either be put away in the
brass boxes or covered with a small bell-glass, to prevent their
receiving any injury.
The object-glasses possess various magnifying powers, according to the
distance at which they require to be placed from the object for distinct
vision: this is not, however, absolutely correct, yet may serve as a
general expression. Thus we have a 1-inch, ½-inch, ¼-inch object-glass,
&c. The object-glasses, for brevity, are often called powers.
As a beginner may at first have some difficulty in distinguishing a high
from a low power, it may be remarked that the size of the lower glass is
larger the lower the power: but, in the case of the better
object-glasses, the focal distance is engraved on the box in which the
object-glass is packed when put away.
As the coarse movement raises or depresses the body and object-glass
through comparatively large distances, it must be used only with the
lower object-glasses, _i. e._ those of low or little magnifying power,
as the 2-inch, 1-inch, or ½-inch, or to bring the object-glass near the
focal distance with the higher powers; whereas the fine movement serves
to adjust the higher powers, as the ¼-inch, &c.
If the object-glasses should become soiled on the lower face, this
should be wiped very gently with an old silk handkerchief or piece of
very soft wash-leather, previously shaken to displace dust. The same
method will answer to cleanse the upper surface of the eye-piece.
Great care must be taken that a slide which has been warmed in any
experiment be not placed near the object-glass until quite cold.
_Mirror._--The mirror has sometimes one silvered face only, at others
two--one flat, the other concave. The flat surface is used to reflect
the light upon the object when the light is too great with the concave
surface.
Beneath the stage, in most microscopes, is a circular moveable
“diaphragm,” perforated with holes of various sizes, to allow more or
less of the light reflected by the mirror to pass through, as may be
required.
When opake objects are viewed, the mirror should be turned aside, so as
not to reflect any light through the stage.
_Eye-pieces._--With all microscopes, two or more eye-pieces are
supplied. These possess different magnifying powers, and are lettered or
numbered accordingly; the lowest power with the earliest letters of the
alphabet, or with the smallest numbers, thus: A, B, C; or 1, 2, 3, &c.
_Forceps._--These are fine pincers, for holding minute bodies to be
viewed as opake objects. In use, they are inserted by a stem connected
with a joint, in an aperture, generally in the stage; and are moveable
in all directions.
_Live-box._--This is a brass slide, perforated in the middle, to the
aperture in which is soldered a short piece of brass tube, closed at the
top with a circular plate of thin glass. A rather wider and longer piece
of brass tube slides over the former; this is also closed at the top by
a thin glass plate, so as to allow of an object being confined or
compressed between the two glass plates. It is used for examining living
objects in water.
_Knife, &c._--For cutting slices or sections of objects, a very sharp
knife with a thin back will be found useful; or a razor may be used for
the same purpose. And for picking minute objects to pieces, or
dissection, fine needles, cut off short with pliers, the blunt ends
being thrust into hair-pencil sticks, will be requisite.
A pair of fine surgical forceps will also be required, for taking up
minute objects. These should be without teeth, and the spring-action so
weak that the points can be very easily approximated.
_Dipping-tubes._--For removing minute objects from water, two or three
narrow glass tubes, of different lengths, are very useful. These are
called “dipping tubes,” and are used thus:--the tube being held upright
between the second finger and the thumb, the fore finger is placed at
the top of the tube to close it; the tube is then put into the water
until the lower end is close to the object, when, on suddenly removing
the fore finger, the water will rise in the tube, carrying the object
with it. The fore finger is then again applied to the tube, and, as thus
held, the water will not run out. The tube is then held over a
watch-glass, or a slide, upon which the water and object will fall on
removing the fore finger.
A small glass spirit-lamp will be found very useful. The spirit for
burning should be methylated alcohol, or wood-naphtha. As these spirits
are inflammable, great care should be taken to keep the stock-bottle
away from a candle or other flame, when filling the lamp.
_Achromatic condenser._--A very important piece of apparatus, when high
powers are used, is the achromatic condenser; it is not, however,
usually supplied with the cheaper microscopes. It consists of a brass
fitting, placed beneath the stage, into which an object-glass is
screwed, in an inverted position, _i. e._ the small end of the
object-glass being placed uppermost. It serves to condense the light to
a focus upon the object, so as to illuminate it more brightly; and as it
can be elevated or depressed by a milled head and rack-work, the object
can be viewed by either converging or diverging rays.
_Simple microscope._--For examining the larger kinds of objects, and for
dissection, a simple microscope is very useful. This consists of a
stand, a stage, and an arm supporting a simple lens or combination of
lenses, but without the body of the compound microscope (as the ordinary
microscope is distinctively called). For most purposes, common
plano-convex or doubly convex lenses are sufficient to form the
object-glasses of a simple microscope. With the best microscopes, an
“erector,” or tube containing a pair of lenses, fitted within the body,
renders the compound microscope capable of answering most of the
purposes of a simple microscope.
_Polariscope._--An expensive but interesting and useful addition to a
microscope is a polarizing apparatus, or polariscope. This consists of a
Nicol’s prism, or a plate of tourmaline, placed beneath the stage, and
another in the body of the microscope or above the eye-piece; both in
brass fittings. The former is called the polarizer, and the latter the
analyzer.
_Rotating disk._--Another most useful piece of apparatus, for moving
opake objects whilst under the microscope, in all directions, is Smith
and Beck’s “rotating disk.”
_Slides._--The slides upon which objects, especially those to be viewed
as transparent objects, are to be placed, should be made of crown or
plate glass. They are usually 3 inches long, and 1 inch wide; but I
prefer them 2½ inches long, and 1 inch wide, simply because they take up
less room in a cabinet, and because they do not project beyond the stage
on either side. They should not be more than 1/20th of an inch thick,
and as colourless and clear as possible. The edges should be ground or
filed, to prevent their scratching the stage.
_Covers._--The covers are square pieces of very thin glass, less in
breadth than the slides, so as not to reach their margins; and of
various thicknesses, the thicker and stronger being used to cover large
objects for examination under the lower powers, and the thinner serving
to cover very delicate objects requiring the higher powers.
_Side condenser._--For illuminating opake objects, a large plano-convex
or doubly convex “bull’s-eye” lens, or side condenser, is used; this is
fixed to an arm, which slides on a stand, so as to be capable of being
raised or lowered to a suitable height. This is placed between the
source of light and the stage, and at such a distance from the latter
that the light may be brought to a focus upon the object. Sometimes a
“Lieberkuhn” or concave silver reflector is used for this purpose.
These are the most important pieces of apparatus required in examining
microscopic bodies. But the beginner will do well, if he have the
achromatic condenser and the polarizing apparatus, to lay these aside
until he has had considerable practice in examining objects simply with
the mirror and the lower powers.
_General method of observation._--In the ordinary use of the microscope,
the object to be examined is laid upon the middle of a slide, which is
placed upon the stage. The object is then brought under the centre of
the object-glass, the mirror inclined half towards the light and half
towards the object, until the object is seen to be illuminated, when,
upon looking through the eye-piece and adjusting the coarse and fine
movements, the object as it comes into focus will be seen, as it were,
drawn upon a white disk, which is called the “field.”
When the object is wet, it cannot be viewed without the application of a
cover, because the water evaporates and condenses upon the under surface
of the object-glass.
To avoid the danger of injuring the object-glass, or crushing the object
by lowering the body and object-glass too much in adjustment to focus,
the best plan is to lower the body by means of the coarse movement until
the object-glass appears near the object to the eye placed on one side
of the stage, and then to apply the eye to the eye-piece, and turn the
milled head so as to raise the body and object-glass until the object is
brought into focus.
In the examination of an object, it is best to begin with a low power,
so as to obtain a view of the general arrangement of its parts, and
then to apply the higher eye-pieces and powers, so that the more minute
structural details may be observed.
_Illumination._--The illumination or proper management of the light in
using the microscope is of very great importance. The best light,
especially with the low powers, is daylight, particularly that reflected
from white clouds; this is least injurious to the eyes. But as daylight
cannot always be used for microscopic investigations, and as cloud-light
is insufficient with the higher powers, some kind of artificial light
must be supplied. That mostly used is the light of a reading oil lamp or
of a gas-burner, a candle being quite useless, on account of the
flickering of the flame with the slightest draught. A moderator lamp has
the defect of too great height; otherwise this would be better than any
other oil lamp. A powerful and excellent light for the highest powers is
afforded by a short paraffine-oil lamp.
As intently looking at strongly illuminated objects is injurious to the
sight, the amount of light allowed to pass the diaphragm should be no
more than is agreeable, and sufficient to show the object distinctly.
In using the higher powers, the field is much less bright with the use
of the same light than in the case of the lower powers; and difficulty
is often found in obtaining sufficient light for the distinct vision of
the object. The achromatic condenser is of most important service here;
but it is sometimes requisite, even when this is used, to condense the
light upon the mirror by a shallow bull’s-eye; or a large common
metallic reflector may be used for the same purpose.
The most convenient manner of proceeding in regard to the use of the
individual eyes is to apply the left eye to the eye-piece, so that the
right eye may be used in finding the stage-movements, or in moving the
slide, without removing the eye from the eye-piece. If this arrangement
be adopted, the light should be placed towards the left-hand side of
the microscope. But the best way to avoid injuring the sight would be to
use both eyes for viewing the objects in turn, although most microscopic
observers make use of one eye only for this purpose.
The structure of many transparent objects can be best seen when the
mirror is turned more or less obliquely to one side, so as to view them
by oblique light, as it is called: we shall refer to this point
hereafter.
As a rule, objects are best seen by transmitted light, or as transparent
objects, although it is well to examine objects under both kinds of
illumination, _i. e._ by transmitted and reflected light.
If during the use of the microscope, after removing the eye from the
instrument, the impression of the light remains perceptible to the
sight, the light used has been too strong, or its action too long
continued; and the instrument should be at once laid aside for a time.
CHAPTER II.
THE MOUNTING OF OBJECTS.
The mounting or “putting up” of microscopic objects signifies their
preparation in such way that they may be preserved for future reference
and observation.
As a general rule, objects should be mounted in that manner by which
their structure is best and most clearly shown; but in certain instances
the objects are mounted so as to make their structure difficult of
detection, that they may form test-objects of the power and quality of
the microscope.
Some objects require to be mounted in the dry state, while others are
best mounted in liquid; some again as opake, others as transparent
objects: these must be considered separately.
_Dry opake objects_ were formerly mounted by gumming them upon small
coin-shaped pieces or disks of cork, blackened upon the surface with a
mixture of fine lamp-black and thin warm size, laid on with a
hair-pencil. They were kept in a drawer, to the bottom of which a sheet
of cork was glued, the disk being transfixed by the pin, so that the
free or projecting pointed end of the pin could be thrust into the
sheet-cork. This plan may still be adopted in the case of common
objects, as seeds, &c.; but it is objectionable, on account of the
facility with which the bare objects are knocked off or injured by dust.
Hence dry opake objects are usually mounted in such manner as to be
enclosed in a cell, the sides being formed by a ring of glass-tube or
cork, or a square piece of leather, cardboard, or paper, with a hole cut
or punched out of the middle. The glass rings are best; but as they are
expensive, some of the other substances are generally used. The size
and thickness of the material from which the rings are made must
obviously vary according to the size and depth of the object. The rings
are cemented to the middle of ordinary slides; and it is best to keep a
number of them ready prepared. The cementing material must vary
according to the nature of the ring used. If this consists of glass,
Canada-balsam or marine glue is best. In using the former, the ring is
gently heated over the flame of the spirit-lamp, and a thin layer of the
balsam applied to its upper or under surface, by means of an iron wire
with a little balsam on its end; it is next warmed over the spirit-lamp,
so that the surface is entirely and evenly coated. A clean slide is then
slightly heated, the ring laid upon it, and gentle pressure is used to
squeeze out the excess of balsam; and the slide is kept at a gentle
heat, until on cooling the balsam becomes so hard as not to be indented
with the finger-nail. Marine glue is applied in the same way as the
balsam, except that prolonged heat is not required to harden it, for it
becomes hard on cooling. The balsam may also be replaced by black japan
or asphalte.
The pieces of cork, leather, or paper are best fastened to the slides
with solution of shellac or sealing-wax in methylated alcohol, or with
white hard varnish.
When the ring or piece has been firmly fixed to the slide by either of
the above cementing materials, so as to form the sides of the cell, the
bottom is to be covered with a piece of black paper, cut to fit it
exactly, and fastened to the surface of the slide with a little gum, or
of either of the above varnishes. As soon as this is thoroughly dry, the
upper surface of the cell-wall, whether of glass or cork, &c., is thinly
covered with varnish, and a clean thin-glass cover laid upon it, and
very lightly pressed; the object is then permanently preserved.
The main points to be observed are, that the object and varnish are
completely dry, and that the cell is thoroughly closed. If the latter be
not the case, more varnish must be applied to any little openings which
may have been left; and it is better to apply the varnish in very small
quantities at a time, the application being renewed as soon as the
previous layer is quite dry.
_Dry transparent objects_ are usually small and delicate; for, unless
they are so, their structure cannot be well seen. In mounting these, a
square piece of note-paper or tracing-paper, with the centre cut out,
may be fastened to a clean slide with a little paste, gum, or shellac
varnish. When this is thoroughly dry, the object is placed in the vacant
space, a clean dry cover laid on, and the varnish applied by means of a
hair-pencil to the edges in very small quantities. This will run in
between the under surface of the edges of the cover and the upper
surface of the paper, and when dry will cement the two together.
Supposing that the object is so delicate that it cannot be removed from
the surface of a slide, if it will not be injured by heat, a good plan
is to draw a square or circle around the object with a little black
japan, then to heat the slide gradually until the japan is not indented
with the finger-nail when cold. A clean slide is then laid upon the ring
of japan, the whole again gently warmed, until the varnish is softened,
and the cover lightly pressed so as to be in contact all round with the
varnish. The slide must then be rapidly cooled, by being laid upon a
piece of metal, which prevents the varnish from running in so as to
spoil the object.
Many dry transparent objects can be preserved by mounting in Canada
balsam. This is the best process for mounting objects in general; but
only those can be so preserved which are not injured by drying, and
which are not rendered too transparent by the balsam. If the object to
be mounted in balsam be small, it is thoroughly dried, and then a drop
of oil of turpentine added to it upon a slide; the slide is then gently
warmed, which causes the turpentine to evaporate. When this has nearly
all evaporated, a drop of balsam is allowed to fall upon the object from
the end of a wire held at a distance above the flame of a spirit-lamp. A
warmed cover is next laid upon the balsam, and gentle pressure applied
until the cover is sufficiently depressed. The slide is then kept at a
gentle heat until the balsam is quite hard when cold. The superfluous
portions may be removed with the point of a knife, and any residues
cleaned off with turpentine or a little benzole on a cloth.
Another way consists in laying a cover upon the dry object on a slide,
adding a drop of turpentine, and warming the whole over a spirit-lamp
until all air-bubbles are displaced, then continuing the application of
the heat until most of the turpentine has evaporated. A drop or more of
the balsam may next be applied to the edge of the cover, when it will
run in and mix with the turpentine. The whole is then gently heated
until the balsam is hard when cold, more balsam being added if
necessary, to replace the turpentine which has evaporated.
When the objects are large, they should be pressed as flat as possible
without injury between two slides, being retained until dry by enclosure
between the prongs of an American clothes-peg; or the slides may be
fastened at the ends by sealing-wax. When perfectly dry, the object
should be immersed in turpentine, kept in a common gallipot, until all
the air-bubbles have been entirely displaced, and the object appears
very transparent. It is then removed from the turpentine with forceps,
drained, laid upon a slide, and melted balsam dropped upon it until it
is quite covered. A clean dry slide is then laid upon its surface, and
the two slides gently pressed together, the two slides fixed at the ends
by sealing-wax, and the whole allowed to cool and dry. If requisite,
more balsam is added to fill up any vacuities. When the balsam has
become hard, the excess is cleaned away with a knife and turpentine, and
the object is permanently mounted.
If the object should be spoiled by the presence of air-bubbles, the
slides and object should be immersed in turpentine or methylated
alcohol, until the whole of the balsam is dissolved; the remounting may
then be proceeded with as at first. If the slides have been immersed in
the alcohol (which is the quickest method), the object must be soaked in
turpentine before the balsam is reapplied.
If, after an object has been mounted in balsam, on applying heat,
bubbles resembling air-bubbles should be formed, the object must not be
considered as spoiled; for these are merely bubbles of the vapour of
turpentine, and will disappear spontaneously after a little time.
A quick way of mounting in balsam is to drop the melted balsam at once
upon the dried object; but as air-bubbles are very apt to be produced in
this way, the beginner had better previously apply the turpentine.
As balsam is very viscid, and adheres firmly to everything with which it
comes in contact, some care is required in its use. Young microscopists
very generally manage to soil the microscope, tables, chairs, papers,
books, and even their clothes with it. It may be easily cleaned off,
however, with turpentine or benzole.
_Moist objects_ are best preserved, whenever practicable, in glycerine.
There are, however, two important objections to its use: one is, that it
makes objects very transparent; the other is, that it often wrinkles and
distorts them, by withdrawing their watery contents. Hence only those
objects can be preserved in glycerine, which are not too transparent,
and which are sufficiently firm to resist the tendency to collapse.
When the objects are tolerably flat, and sufficiently firm to bear the
pressure of the cover, they may be mounted by adding a small quantity of
glycerine to them lying on a slide; the cover is then applied, and a
little of the cement mentioned below applied warm with a hair-pencil
around the edges of the cover to fasten it to the slide. Care is
required that the glycerine applied be no more than sufficient; for
wherever it has touched the cover or the slide, the cement will not
adhere. Superfluous portions may be sucked up with a piece of clean
moist sponge or a corner of blotting-paper.
When it is required to mount a large number of objects in a short time,
the cement need only be applied to two opposite sides of the cover,
leaving the other two sides open.
When the objects require to be protected from the pressure of the cover,
the sides of a cell must be made with the cement or black japan upon the
slide before the cover is applied, a further quantity being used to
close the cell as usual.
A very strong solution of chloride of calcium may be used for the same
purposes and in the same way as the glycerine. It has the advantage of
not making the object so transparent; but it has the disadvantage of
crystallizing slightly in a dry atmosphere. In most cases, I prefer it
to glycerine.
A large number of interesting objects cannot, however, be preserved in
either glycerine or chloride of calcium, without their value being
impaired by the cause mentioned above. Many kinds of liquid have been
recommended for preserving these, all agreeing mainly in being
inefficient. The objection to them is, that they are evaporable; and
after the object has been mounted for some time, the liquid creeps
between the cement and the slide or cover, at some spot, and evaporates;
and if the cement be not quite hard, the inner and more liquid portion
of it runs into the cell, and spoils the object. A solution, containing
a grain of salt and a grain of alum to the ounce of distilled water, is
as good as any other; or simply distilled water in which a piece of
camphor has been kept. In use, the cell is first formed by making a
circle or outline square on the slide with black japan, and heating this
carefully until it becomes solid when cold. The object is then laid in
the cell, the liquid added, and the cover applied, any excess being
removed with blotting-paper. The cell is to be closed with old black
japan or gold-size, applied round the margins of the cover with a hair
pencil. A second and a third layer of the varnish may be applied upon
the first, when it has become hard outside. Although black japan and
gold-size are generally used for the cement, I prefer that mentioned
below.
When large preparations are mounted in liquid, the cell-walls are either
formed of glass rings, or they are built up with four oblong pieces of
glass, cemented to the slide and to each other with marine glue. A very
good preservative liquid for large specimens is a solution of chloride
of zinc, in the proportion of 20 grains to the ounce of distilled water.
A mixture of spirit of wine and water, in the proportion of 1 part to 2,
or 1 to 4, is often used for the same purpose.
When preparations are mounted, the cement and the adjacent parts of the
slide and cover should be coated with a solution of sealing-wax in
spirit, which hardens the exterior of the cement.
The cement above alluded to is made by melting together 5 parts of
rosin, 2 parts of balsam, 1 part of bees’-wax, and 1 of red ochre. The
cement is best kept in a little metallic cup, and melted over a
spirit-lamp when used. It should be applied while hot, with a hair
pencil; and cools very quickly.
The preservative liquids should be kept in corked bottles, a hair pencil
being fixed into the under part of the cork; or, what is better, in
stoppered bottles, the stopper being prolonged to a point nearly
reaching the bottom of the bottle.
As soon as the preparations are mounted, they should be labelled, the
labels being kept ready gummed. The balsam should be kept in a capped
bottle, such as is used for holding solutions of gum, with an iron wire
for removing portions as required. By keeping, the balsam becomes
thicker; it may be thinned by the addition of oil of turpentine, and the
application of a gentle heat.
The black japan, &c., may be procured at any oil-shop; the glass rings,
cell-sides, covers, &c., from Mr. Norman, 178 City Road, or of the
microscope-makers.
Mounted objects should be kept in shallow drawers, and be laid flat--not
standing on edge.
_Magnifying power._--Before entering upon the consideration of the
objects themselves, a word or two must be said upon the magnifying
powers. In the plates of this work, the serial number of each figure is
expressed by large numerals placed above the objects, while the number
of times the object is magnified is indicated by small numerals placed
beneath. The latter must be understood to express the number of times
the drawing is larger than the object in one dimension. Thus,
considering fig. 13, Plate I. to be an inch in width (for it is really
somewhat less), being magnified 150 times in the direction of the width,
the object itself is about 1/150th of an inch in size; and it is
represented magnified 150 times linear, or 150 diameters, as it is
called.
A knowledge of the number of times the object is magnified is of the
greatest importance in making use of the drawings; for, without it, the
observer will be unable to apply such a magnifying power of the
microscope as will enable him to see the structural appearances figured
in the drawings.
The observer must also be acquainted with the magnifying powers of his
microscope with the various object-glasses and eye-pieces. These are
usually given when the instrument is purchased. Or they may be
determined approximatively thus:--An ivory scale, with 1/100th of an
inch engraved upon it, is placed on the stage, and viewed as an opake
object, both eyes being kept open; and the size of the image of one of
the gradations is measured with compasses, upon the stage as seen with
that eye which is not applied to the eye-piece. The number of 1/100ths
of an inch contained in the measure obtained with the compasses
represents the magnifying power. Thus, supposing the image of the
1/100th of an inch on the scale appears magnified to the length of 1
inch on the stage; the magnifying power is 100 diameters, or 100 times
linear. This proceeding is difficult to any one unaccustomed to the use
of the microscope, yet by practice it becomes very easy. Other methods,
which require the use of the camera lucida, are given in the
Micrographic Dictionary.
CHAPTER III.
VEGETABLE ELEMENTS AND TISSUES.
We may now enter upon the consideration of the microscopic structure of
objects, beginning with those which are derived from the vegetable
kingdom, as they are more easily procured and prepared for examination
than those belonging to the animal kingdom; moreover they are not so
transparent, and hence are more readily distinguished under the
microscope, which is of importance in the case of an unpractised
observer.
_Cells._--The elements of which all plants consist are cells. Cells, in
their simplest condition, are microscopic, rounded, colourless, closed
sacs or vesicles, resembling small bladders (Plate I. fig. 2), and
consist of a thin, transparent, colourless, vegetable skin or membrane
(_a_) called the cell-wall. The cells are well seen in a little of the
pulp of an apple (fig. 2), or in a section of almost any soft part of a
plant. A high power is usually required to show them distinctly, on
account of their minute size. The outline of the cells is seen to be
double, one line indicating the inner, the other the outer, surface of
the cell-wall, the space between the two lines corresponding to the
thickness of the cell-wall.
In the pulp of the apple, the cells are loosely connected, and so retain
their rounded form; but in most parts of plants, the cells become
crowded and squeezed together, from their ordinary or normal expansion
being limited in certain directions, so as mutually to alter each
other’s shapes. The sides then lose their originally rounded form and
outline, becoming more or less straight (Pl. I. figs. 1 & 4),--the
cells at the same time mostly adhering to each other, so as to be
separated with difficulty.
The forms thus produced are various and interesting, and have all
received names by which they are distinguished. They are described in
works on botany in two ways--according to the outline (which is the most
common, as this expresses the appearance usually presented in sections
and on the surfaces of vegetable structures), or according to the entire
or solid form, which it is often a difficult matter to determine.
_Cellular tissue._--Cells aggregated thus form a tissue, which is called
cellular tissue or _paren´chyma_ (παρά, among, and ἕγχυμα, poured
substance), because it fills up the interstices of the other tissues of
plants.
In technical descriptions, the cell-structure is often left out of
consideration; and bodies composed of parenchymatous tissue are
described as being reticulated or netted, because the united sides of
the cell-walls appear as a network covering the surface.
It must be understood that parenchymatous cells are such only as have
the three dimensions of solidity (viz. the length, breadth, and depth)
nearly equal.
_Intercellular passages._--The observer will not have examined many
sections of cellular tissue, without noticing certain irregular black
lines running between the cells, as in a piece of a Geranium-(_Pelargo´nium_-)
leaf (Pl. I. fig. 1). These lines arise from the existence of passages
between the cells, containing air; and they are called intercellular
passages. By gently warming a section containing them in water over a
spirit-lamp, or by moistening the section with a drop of spirit, the
passages will be filled up with the liquid, so as to become transparent.
When the intervals between the cells are larger and broader, they are
called _intercellular spaces_.
So far, cells have been considered simply in regard to their form, as
vesicles, either rounded or altered in shape by mutual pressure. We have
now to notice the matters contained within the cells, or the
cell-contents.
_Cell-contents._--In most cells, especially when young, a minute,
rounded, colourless body may be seen, either in the middle or on one
side, called the _nucleus_; this is very distinct in a cell of the pulp
of an apple (Pl. I. fig. 2 _b_). And within this nucleus is often to be
seen another smaller body, frequently appearing as a mere dot, called
the _nucle´olus_.
The nucleus is imbedded in a soft substance, which fills up the entire
cell (Pl. I. fig. 2 _c_); this is the pro´toplasm (πρῶτς, first, πλἁσμα,
formative substance). As it is very transparent, it is readily
overlooked; but it may usually be shown distinctly by adding a little
glycerine to the edge of the cover with a glass rod, when it contracts
and separates from the cell-walls, as in the lower cell of fig. 2. The
protoplasm in some cells is semisolid and of uniform consistence, while
in others it is liquid in the centre, the outer portion being somewhat
firmer and immediately in contact with the cell-wall. In the latter
case, it forms an inner cell to the cell-wall, and is called the
_primordial utricle_. The terms “protoplasm” and “primordial utricle”
are, however, used by some authors synonymously.
The protoplasm is the essential part of the cell, and it forms or
secretes the cell-wall upon its outer surface in the process of
formation of the cell considered as a whole. It is also of different
chemical composition from the cell-wall, being allied in this respect to
animal matter.
_Chlor´ophyll_ (χλωρὀς, green; φὐλλον, leaf).--On examining a section
of any green part of a plant, as the green substance of a
Geranium-(_Pelargonium_-) leaf, it will be seen that the green colour
does not arise from the whole substance being coloured, as appears to
be the case to the naked eye, but from the presence of little grains or
granules of a green colouring-matter in the protoplasm of the cells.
This green matter is called chlorophyll. If the cells be crushed, the
granules will escape, and can be examined in the separate state.
Chlorophyll is most abundant in those parts of plants which are exposed
to the light.
_Starch._--In many cells of plants, particularly those which have
attained their full growth, other granules, larger than those of
chlorophyll, and colourless, are met with; these are the
_starch-granules_ (Pl. I. fig. 3). They are usually rounded or oblong,
and exhibit on the surface a number of rings, one within the other, or
concentric, as it is called. In the centre of the innermost ring is a
black dot or streak, arising from the presence of a little pit or
furrow, and called the _hilum_.
The starch-grains may be readily seen within cells in a thin section of
a potato (Pl. I. fig. 4); here they are very numerous, and larger than
in most other plants. A separate grain is represented in fig. 3.
The appearance of rings in the separate grains arises from the
starch-granules being composed of numerous concentric coats or layers,
like those of an onion.
A very simple and striking method of determining whether any granule is
composed of starch or not, consists in adding to it, when placed in
water on a slide, a drop of solution of iodine. As soon as this touches
the granule, it assumes a beautiful purple colour, the depth of tint
depending upon the quantity of the iodine-solution; if this be very
considerable, the granule appears almost black. The section of potato
forms a very interesting object when moistened with the iodine-solution,
the starch-granules becoming beautifully coloured, whilst the cell-wall
remains colourless, and the protoplasm becomes yellow.
The form of the starch-granules differs in different plants, so that
the kind of plant from which starch has been derived may be
distinguished by attention to the size, form, and structure of its
starch-granules. Thus, the granules represented in Pl. I. fig. 3, which
it will be noticed are all drawn under the same power, are derived from
different plants,--_a_ being those of wheat-flour, in which the hilum is
obscure, and the rings faint; _b_ is a granule of West Indian arrowroot,
in which the hilum forms a transverse crack; _c_ is a granule of
potato-starch, in which the hilum is a dot, and the rings are very
distinct; _d_ represents the compound granules of the oat, the separate
granules being figured below; _e_ is a granule of lentil-starch, with
its long dark hilum and elegant oval concentric rings; and _f_
represents a compound and separate granule of rice-starch. It will be
noticed that the granules of oat-and rice-starch are angular, as it is
called.
The knowledge of the peculiar forms of the starch-granules is important
in a practical point of view, for it enables us to recognize them when
mixed as an adulteration with other substances, and also to distinguish
the different kinds of starch from each other. Thus table-mustard, as it
is called, is principally composed of the cheaper wheat-or pea-flour,
which is easily recognized by the structure of the starch-grains.
Arrowroot is considerably dearer than potato-starch; hence in trade the
latter is fraudulently sold for the former, the adulteration being
detected with difficulty by the eye, but easily under the microscope.
Again, rice is largely mixed with wheat-flour, as it makes inferior
flour into very white bread; and this may also be readily detected under
the microscope. The reader can now understand how valuable the
microscope is in detecting adulterations, with a knowledge of the
various forms and structures of substances, especially with the aid of a
few chemical tests.
Starch-grains are altered by boiling in water, becoming swollen and
often changed into curious forms, the rings becoming faint or
disappearing. If a piece of boiled potato be examined, the
starch-granules will seem to have vanished from the cells, which are
swollen and covered with an irregular kind of network. The network
consists of parts of the protoplasm situated in the interstices of the
starch-granules, and solidified or coagulated by the heat. On crushing
the cells by pressing upon the cover, the starch-granules will escape,
swollen and partly fused together; but they may easily be recognized as
consisting of starch by the iodine test.
The granules of “tous les mois” starch are particularly well adapted for
showing the concentric rings, the granules being about twice as large as
those of the potato.
Starch-granules are best examined in water; and a small quantity only of
the starch must be placed on the slide, if the structure of the granules
is to be seen clearly. They may be mounted in glycerine, although this
makes them very transparent.
To those who possess a polariscope, starch-granules are particularly
interesting, as they exhibit a black cross, and, with a plate of
selenite laid beneath the slide, a beautiful play of colours.
In addition to the starch and chlorophyll, the cells of plants contain
other matters, as gum, sugar, &c.; but as they are dissolved in the
cell-liquid, they are not visible. In the cells of certain plants,
however, spherical globules, with light centres and black outlines, will
be met with: these consist of oil.
_Raph´ides._--Lastly, occurring in the cells of plants, especially such
as are soft and juicy (succulent), will be found minute, hard,
colourless crystals, called _raphides_ (ῥαφὶς, a needle). These are most
frequently needle-like or acicular (_acus_, a needle), but sometimes
prismatic or rod-like with flat sides; they are also not unfrequently
grouped into little tufts. They may be readily found in a piece of the
stem of garden-rhubarb (Pl. I. fig. 5 _a_), or of the common balsam.
_Porous and spiral cells._--The walls of the cells of cellular tissue
are sometimes covered with little dots (Pl. I. fig. 11 _a_), or
slit-like markings; the cells are then called porous cells. A specimen
of them may be obtained from a section of the pith of the elder
(_Sambúcus nígra_).
Sometimes cells exhibit the appearance of a spiral line marking their
walls, as if a little bell-spring were coiled up in them (Pl. III. fig.
2 _a_). These are called spiral cells, or spiral fibrous cells, and the
tissue formed by them is called fibro-cellular tissue.
We now leave the cells of ordinary cellular tissue, to examine those in
which the dimension of length predominates, so that they form tubular
cells; and first of those required to possess strength and firmness,
combined with flexibility. These qualities are met with in the cells
constituting
_Woody tissue._--Of this there are two forms, called respectively
wood-cells and woody fibres.
The _wood-cells_ are moderately long, more or less tapering and
overlapping at the ends; and the cell-walls are thickened, so as to
possess considerable firmness. These cells are found in the wood of
stems, as in the white woody portion of an ash stick, that of a
lime-tree, the stem of a Chrysanthemum, &c. (Pl. I. fig. 6). They are
closely packed, and the tissue formed by their union is called
_prosen´chyma_ (πρὸς, close, ἔλχυμα, tissue).
In the other kind of woody tissue the cells are very long and slender,
strong, yet flexible, gradually tapering at the ends, where they overlap
each other; and they have thick walls, so that, when divided
transversely, the cavity appears almost filled up (Pl. I. figs. 5 _d_,
9, & 7 _b_). This tissue is called _woody fibre_ or _pleuren´chyma_
(πλευρἀ, rib, ἔγχυμα), from its strength.
The walls of the cells of woody tissue are often covered with dots,
either simple or with an inner dot (Pl. I. fig. 6 _b_, fig. 11 _b_), or
with streaks (Pl. I. fig. 6 _a_) or with a spiral fibre (fig. 11 _b_,
_c_), either alone or with dots also.
This tissue is of great importance in plants, from its strength and
flexibility; it forms a considerable part of the veins of leaves, the
inner bark (_liber_), and of the wood of the stems of trees. It is also
very useful to man: for it constitutes hemp, of which rope and string
are made; flax, of which linen is made; cocoanut fibre; bast, used by
gardeners for tying up plants, which is the inner bark of the lime; and
jute, which is the inner bark of an Indian lime-tree.
In the white woody part of the stems of trees belonging to the fir-order
(Conif´eræ), as a piece of deal or pine, which is mainly composed of
wood-(prosenchymatous) cells, the cells exhibit rows of minute circular
markings (Pl. I. fig. 10). These were formerly supposed to be solid
bodies or glands; hence the tissue is still sometimes called glandular.
Within the outer ring of each marking is an inner central dot, or
sometimes an oblique streak. The side view of the cells (Pl. I. fig. 8
_a_), which is seen in a tangential section, shows that the markings are
minute pits, each being opposite to one of an adjacent cell, and sunk
inwards towards the centre of the cell, the inner dot or streak being a
thinner portion of the cell-wall. This glandular tissue of the Coniferæ
is interesting as forming a test-object for the defining power of the
microscope, which should show the two rings sharply and free from
colour; the section of the wood should be examined as a dry transparent
object.
The difference between the woody fibre and the wood-cells of coniferous
wood may also be seen well in a piece of deal, as cut up for fire-wood.
If the end of a stick of this be examined with the naked eye, parts of
brown rings will be seen traversing the whiter portion of the wood.
These brown rings consist of woody fibre; the white portion of
wood-cells. On making a very thin transverse section, the interior of
the woody fibres is seen to be almost entirely filled up (Pl. I. fig. 7
_b_), while the cavity of the wood-cells is much more open (Pl. I. fig.
7 _a_); the former also contain globules of turpentine.
It must be remarked here that some botanical authors include both forms
of woody tissue under the term prosenchyma. But, as we shall see
hereafter, the form of the prosenchymatous cells being sometimes used as
a character for distinguishing the cells of leaves, to which the term
pleurenchymatous cells would be inapplicable, the above distinction will
be found important.
_Vessels, vascular tissue._--In the next form of tubular cells, these
are broader and softer than the cells of woody tissue, thin-walled, and
the ends pointed; and their walls exhibit spiral or ring-like markings,
or rows of dots (Pl. I. fig. 5 _c_, _e_, _b_), indicating the existence
of one or more spiral fibres or rings. When the vessels contain spiral
fibres, they are called spiral vessels (Pl. I. fig. 5 _c_); when they
contain ring-shaped portions of fibre, they are called annular
(_an´nulus_, a ring) vessels (Pl. I. fig. 5 _e_); and when the spaces
between the fibres are partly filled up, leaving only dots, the deposit
forming a kind of network, we have a reticulated (_réte_, a net) vessel
(Pl. I. fig. 5 _b_). This tissue can easily be obtained from a piece of
cooked rhubarb, the stem of a balsam, or from any soft-stemmed plant.
Vessels very frequently contain air.
_Ducts._--The tubular cells forming ducts (Pl. I. figs. 5 _b_, 11 _c_)
are large, more or less flattened or blunt at the ends (truncated); and
the cell-membrane at first closing the ends is often removed or
absorbed, so that the ducts communicate with each other, to allow of
the free passage of the sap through them. Their walls are invariably
covered with markings, consisting of either simple or bordered dots,
resembling those met with in the preceding forms of tissue. The ducts
are often easily recognizable with the naked eye, in transverse sections
of stems, by the large pores which they form in the wood. These may be
well seen in a section of a piece of cane. The tissue composed of dotted
ducts is called bothren´chyma (βὁθρος, pit); but the term is principally
applied to those ducts in which the dots are simple, _i. e._ have no
inner dot.
The structure of the above forms of tissue may be best understood in
relation to their development. It has been stated that the essential
part of the cell is the protoplasm. As cells grow older, new matter is
deposited by the protoplasm upon the inner surface of the cell-wall,
either to a small extent, evenly and uniformly, as in ordinary
parenchyma, or unevenly, in the form of spiral layers, forming fibres or
bands, leaving bare spaces, where the original cell-wall exists alone.
The matter thus deposited is called secondary deposit, the original
cell-wall being the primary deposit. When the secondary deposit covers
the interior of the cells except at certain slit-like spaces, we have
the appearance figured in Pl. I. fig. 6 _a_. When the deposit forms a
spiral fibre, or a series of rings, we have the spiral or annular vessel
or duct. And when the interspaces between the coils of a close spiral
fibre are filled up except at certain spots, we have the dotted or
reticulated vessel or duct.
In many instances, these deposits are present together: thus, sometimes
the outermost deposit leaves rounded pits or dots, while an inner
portion forms a spiral fibre (Pl. I. fig. 11 _b_); or one layer leaves
simple rounded pits, while the other leaves smaller slits or dots placed
opposite the former (Pl. I. fig. 6 _b_).
In some cells the cavity is almost entirely filled up by secondary
deposit, which leaves minute canals radiating from a small cavity in the
centre to the circumference, as seen in the transverse section of a
plum-stone (Pl. I. fig. 56); here the canals appear as dark lines. In
others, again, the secondary deposit forms several distinct layers,
leaving channels very similar to those of the last; an example is met
with in the gritty tissue of the pulp of a pear.
The obvious use of the pits and channels in the above tissues is to
preserve the permeability of the walls of the elements, which would be
destroyed if the walls were equally thickened all over.
_Cell-formation._--New cells are formed by the division of old or parent
cells. The actual process of division is difficult to observe, as it
requires prolonged observation; but cells are often met with in all
stages of division, of which some instances will be pointed out
hereafter. The cell-division takes place in two ways, either according
to the endogenous (ἔνδον, within, γεννἁω, to produce), or the exogenous
(ἔξω, outside, γεννἁω) method. The manner in which the division takes
place in the former is this:--At first a slight indentation or
constriction of the protoplasm occurs at the line of division; this
deepens until the protoplasm is completely divided. The freshly divided
surfaces then become coated with a new portion of cell-wall, so as to
make two or more new cells, which either remain in contact or separate
from each other. In some cases, the divided portions of protoplasm
become coated all over with new cell-walls.
In the exogenous process, a portion of the protoplasm protrudes from the
surface of the cell, carrying the cell-wall before it, so as to form a
little bud-like body; this is next cut off at its point of junction with
the parent-cell, and coated, as in the first case, with a new cell-wall,
so as to form a new cell.
_Preparation._--In examining the vegetable elements and tissues, very
thin sections must be made with a razor or thin sharp knife; these are
then to be placed in a little water on a slide. As the structures are
all minute, the distinctness with which they are seen will mainly depend
upon the proper thinness of the sections. When sections of dry stems are
to be examined, the black margins of the air-bubbles contained in the
cells often render the structure indistinct; these must therefore be
displaced by first wetting the tissue with methylated alcohol, and then
adding water to it in a watch-glass or on a slide; or the tissue may be
soaked in warm water for some hours: and this is mostly requisite in
preparing thin sections of dry tissues.
Attention must also be paid to the manner in which the section is made,
or the direction in which the portion of the plant is cut. There are
three important directions which must be distinguished, producing
transverse, longitudinal, and tangential sections. If the cuts be made
across the length of a stem, for instance, the section is called
transverse. If the cuts be made in the direction of the length, through
the centre, the section is longitudinal; and if the cuts are made in a
direction parallel to a line running down the centre of the stem, but
nearer its margin, it is a tangential section. It is scarcely necessary
to mention that an oblique section is intermediate between a transverse
and a longitudinal section.
CHAPTER IV.
VEGETABLE ORGANS.
The vegetable elements and tissues which have been described form,
either separately or by their combination in various ways, the organs of
plants. To these we shall now pass, and consider the structure of the
principal organs of the members of the vegetable kingdom.
_Leaves._--Leaves in their simplest form consist of a single sheet or
layer of parenchymatous cells or cellular tissue, an example of which
may be found in almost any moss (Pl. III. fig. 30). The granules of
chlorophyll will often be very distinctly seen in these cells. The first
addition to this form of leaf is a row or two of prosenchymatous cells
running longitudinally down the middle of the leaf, so as to form a
rudimentary vein or nerve. In other and more highly developed leaves,
the layers of cells are numerous, and traversed by bundles of
wood-cells, vessels, and ducts (fibro-vascular tissue), forming the
veins,--the entire surface being covered with a skin or membrane, called
the epidermis.
_Epider´mis_ (ἑπἱ, upon, δἑρμα, skin).--This membrane is composed of one
or more layers of colourless, closely packed cells (Pl. I. figs. 13 &
28), the colour it occasionally exhibits usually arising from some of
the underlying cells of the leaf being seen through it, or remaining
adherent to it when stripped from the leaf. It is easily separated, by
making a cut in a soft leaf, and peeling it off with a fine pair of
forceps, or by soaking a leaf for some time in water and then stripping
it off. It must be remarked that the epidermis covers not only the
leaves, but every part of the plant.
_Hairs._--Arising from the epidermis are the hairs of plants. These are
thread-like or filamentous prolongations of the epidermis beyond the
surface of the leaf (Pl. I. fig. 12), consisting of cells arranged end
to end. They are often branched, sometimes star-shaped (stellate) (fig.
28), and present great varieties in form, as shown in the figures, the
plants from which these were drawn being mentioned in the Description of
the Plates. Sometimes hairs terminate in a little head (Pl. I. figs. 12
_c_, _d_, _e_), the cell or cells composing which secrete a colouring or
a viscid substance; they are then termed glandular. The hairs of plants
are particularly interesting to the microscopic observer, not only on
account of their curious forms, but in connexion with the remarkable
phenomenon of the circulation of the cell-contents, or _rotation_, as it
is called, observable in them. This is difficult to be perceived by any
one unaccustomed to microscopic observation, because the particles by
which the motion of the cell-contents becomes evident are exceedingly
minute; but practice in the use of a high power will overcome this
difficulty. The hairs which exhibit the phenomenon best are those of the
American Spiderwort (_Tradescan´tia Virgin´ica_), which is to be found
in every garden. It may, perhaps be recognized thus:--The plant is about
a foot and a half high; the leaves are sword-shaped and channelled, and
the flowers are purple, in heads, and 1½ inch in diameter. The hairs are
attached to the sides of the stamens, towards the lower part or base.
The stamens should be carefully picked off with forceps, and placed on a
slide in a drop of water; the hairs should then be separated with the
mounted needles, and a cover applied. Under a low power, the hairs are
seen to be beaded or monil´iform (_moníle_, a necklace), and of a fine
purple colour (Pl. I. fig. 22). On applying a high power, as the
¼-inch, the individual cells will come distinctly into view, and the
nucleus will be seen very clearly as a roundish granular mass (Pl. I.
fig. 23 _a_). On carefully examining the cell-contents, delicate lines
will be observed radiating irregularly from the nucleus, some passing to
the top of the cells, while others run towards its base, as in the
figure; and on very close inspection, the portions of protoplasm of
which these lines consist, will be found to move slowly and steadily,
the motion becoming perceptible by means of the minute granules of which
the protoplasm consists. The currents return at the ends of the cell,
there being no passage of the contents of one cell into the cavity of
either of those adjacent. During this examination, it will be noticed
that the surface of the cell-wall is striated with fine wrinkles.
It may be remarked that the hairs should be taken from flowers which
have only just opened; for this curious and inexplicable rotation is
connected with the growth of the cell; and when this has attained
maturity, it no longer occurs. The phenomenon may be observed in many
other hairs of plants, as those of common groundsel (_Senécio vulgáris_)
(Pl. I. fig. 12 _a_, _b_), and in the cells of the leaves of some
water-plants; but I must refer to the article “Rotation” in the
Dictionary for further information.
The most important variety of hair is that derived from the Cotton-plant
(a kind of Mallow), and forming the cotton of commerce. These hairs
spring from the epidermis of the seeds. The cells composing it are very
long and soft, becoming flaccid and easily bent when dry (Pl. IX. fig.
13).
_Stings._--Stinging hairs or stings may be well illustrated by reference
to the common large nettle (_Urtíca dioíca_). In this plant they consist
of a thick-walled cell, bulbous at the base, which is imbedded in the
epidermis (Pl. I. fig. 21), the pointed end being terminated by a very
minute dilatation or knob. The sting contains an acrid liquid, which
escapes when the little knob is broken off in wounding the skin, and
produces the well-known irritation. By the side of the figure of the
sting is represented the point of a fine needle (fig. 20), showing that
the expression “sharp as a needle” has no force when microscopic bodies
are in question.
_Stom´ata_ (στὁμα, mouth).--On viewing a strip of epidermis, the
observer will be sure to notice certain oval or roundish bodies (Pl. I.
fig. 13 _a_), composed of mostly two kidney-shaped cells in apposition
but leaving a chink between them; these are the stomata. They
communicate beneath with the intercellular passages, of which they may
be considered the mouths; and by their agency a direct communication is
established between these passages and the air. The two cells which
guard the orifice are termed the “guard cells.”
Stomata are most numerous on the under surface of leaves; they are
entirely absent in plants growing under water, and in most of the lower
plants. In many of the stomata, viewed in the ordinary way, the air
situated between the guard cells is indicated by the black spot or dot
present; but after a time, or by the application of a gentle heat to the
slide, the air becomes displaced by the water, and their structure
becomes very distinct.
In certain plants, the epidermis is imbued with flint or sil´ica; so
that even when burnt to an ash the stomata are still quite distinct.
Examples of this may be found in the stalk or culm of grasses, as in
straw, the shining epidermis of which is siliceous; or the epidermis of
canes. Among the lower plants, this peculiarity is especially curious in
the species of _Equisétum_, or mares’-tails.
The manner in which the veins of leaves are arranged is worthy of
special attention, as it forms one of the characters by which the two
leading divisions of the Vegetable Kingdom are characterized. Thus in
one of these divisions the veins are branched, so as to form a network
throughout the leaf; the plants with these netted veins, to which belong
our trees, shrubs, and most herbs, are the Dicotylédons, or Ex´ogens;
while in the second division, the veins run parallel to each other,
being little or not at all branched, and not forming a network. The
plants with parallel veins, among which are our grasses, lilies, &c.,
are the Monocotylédons or En´dogens.
_Stems._--In the stems of plants, the tissues are arranged round a
centre; otherwise, in the simpler and lower plants, they agree in
structure with leaves, the centre being occupied by some element of
fibro-vascular tissue, as simple wood-cells, a few vessels or ducts.
In the higher or flowering plants, the stem exists in two distinct
forms, corresponding to the differences above noticed in the arrangement
of the veins of the leaves; these must be considered separately.
In the _Dicotyledons_ or _Exogens_ (Pl. I. fig. 36), the centre of the
stem, in a transverse section, is seen to be occupied by the pith or
_medulla_, which is represented in the figure by the innermost circle.
Immediately outside and around this is a narrow ring, indicating the
section of a sheath to the pith, and called the _medullary sheath_. Next
comes a broad ring of wood of the first year’s growth (fig. 36 _a_),
traversed, from the pith to the bark, by wedge-shaped paler rays, termed
the medullary rays. Outside the first year’s wood is the newer and paler
wood of the second year (_b_); and so on, a new ring of wood being added
outside the preceding layer for each year of growth of the stem.
On the outer side of the wood is the inner bark or _liber_ (fig. 36
_c_); and outside this is the spongy outer bark (_d_), covered by its
epidermis.
These structures are of different composition, as may be best seen in
longitudinal sections. The pith and the medullary rays consist of
cellular tissue, the cells being mostly rounded in the former, and more
closely pressed together and squarish in the latter. The medullary
sheath consists of vascular tissue; and the wood, of wood-cells
traversed longitudinally by bundles of vascular tissue and ducts, the
latter being larger and more distinct towards its outer boundary. The
liber is composed of woody fibre, and the outer bark of cellular tissue.
The new woody matter being deposited outside the old, between the bark
and the previously formed layer, gives origin to the term exogen (ἔξω,
outside, γεννἁω, to produce). These structures may be examined in the
section of a branch of the lime-tree or lilac.
In the _Monocotyledons_ or _Endogens_ (Pl. I. fig. 37), there is no
distinct bark, nor pith, nor medullary rays--the entire stem consisting
of cellular tissue with isolated bundles of fibro-vascular tissue
scattered through it. Moreover the new substance is added to the centre
of the stem, or within the old; hence the term endogen (ἔνδον, within,
γεννἁω). A section of a piece of cane will exhibit this structure.
To examine the structure of stems, sections must be made in various
directions. The relative position of the component parts of a stem are
best seen in a transverse section; but the structure of the tissues is
most evident in longitudinal sections, and under the higher powers. The
annual rings of the Exogens are best observed in transversely sawn-off
pieces of perfectly dry stems, which have been polished with sandpaper,
and varnished with spirit varnish.
_Roots._--The structure of roots is very similar to that of stems; there
is, however, no distinct pith, nor are there stomata on the epidermis;
and the vessels are replaced by ducts. The very fine rootlets or
radicles of water-plants often show the rotation of the protoplasm very
distinctly.
_Flowers._--The various parts of flowers, being each a modified leaf,
present the same general structure as the latter. As the reader may not
be acquainted with the names of these parts or organs in the higher
plants, and as we shall have to compare them with their representatives
in the lower forms of vegetable life, it will be well briefly to
indicate them. A common and beautiful yet despised flower (Pl. I. fig.
32) may serve for illustration; this is chickweed (_Stellária média_),
which can be found everywhere. The outermost circle of flower-leaves,
which forms a kind of cup to the rest of the flower (_a_), is the
_calyx_; the separate leaves being called the _sepals_. The row within
this, in most flowers consisting of brilliantly coloured pieces, forms
the _corolla_ (_b_); the individual pieces being the _petals_. When the
two kinds are equally coloured, or not distinguishable, the whole is
called the perianth, as in a tulip. When the segments of the _perianth_
are dry and chaffy, as in the flowers of grasses, the outermost are said
to constitute the _glumes_, and the innermost the _paleæ_. Within the
ring of petals are certain thread-like organs called _stamens_ (_c_);
and these consist of a _filament_ (fig. 39 _a_), surmounted at the top
or apex by the _anther_ (fig. 39 _b_), which is usually coloured, and
consists of two lobes. The anthers when ripe burst, and discharge a
coloured dust; this is the _pollen_. Lastly, within the stamens is the
central organ of the flower, the _pistil_, and sometimes there are
several of them. The pistil consists of three parts, viz. a swollen
base, the _ovary_ (fig. 41 _b_), surmounted by a column or _style_ (fig.
41 _a_), and which is crowned by a viscid and often hairy summit, the
_stigma_ (fig. 40*). In chickweed there are 3 styles.
It must be remarked that, in the flowers of some plants, stamens alone
are present, while others contain pistils only, although most flowers
contain both organs. When the stamens and pistils occur in separate
flowers on the same plant, the plant is said to be _monœcious_ (μὁνος,
single, οἶκος, family); when all the flowers of distinct plants contain
either stamens only or pistils only, the plant is _diœcious_ (δις,
twice, οἶκος); and when the stamens and pistils occur together in all
the flowers of the same plant, the plant is said to be _hermaphrodite_.
These terms had their origin in the idea that the differences of plants
in respect to these organs were analogous to those of the sexes in
animals. All the parts of a flower have their special uses: thus the
calyx and corolla protect the delicate organs enclosed by them, until
they attain maturity. The petals also, by their brilliant colours,
attract insects which feed upon or collect the honey of the flowers;
these at the same time conveying the pollen which adheres to their
bodies from one flower to the stigma of another. The stamens and pistils
are organs of fructification, it being essential for the fertilization
of the flowers that the pollen should come into contact with the stigma.
We will now consider some interesting points of structure in these
organs.
_Petals._--The petals often form most beautiful microscopic objects, on
account of the curious shape and structure of the cells of their
epidermis, and the splendid tints of the colouring matters contained in
them. As petals are mostly too thick to allow of the cells being
distinctly seen in the entire state, a little cut should be made in them
while gently stretched on the finger, and the epidermis carefully
stripped off with forceps; the strip should then be laid on the slide in
water as usual: in this way the curious patterns of the epidermic cells
will become very distinct. The petals of a red geranium (_Pelargónium_)
may be used to illustrate them (Pl. I. fig. 24). The structure may be
best understood by reference to the epidermis of the leaf of a geranium
(Pl. I. fig. 13), in which the cells present wavy or undulate walls. In
the petal (fig. 24), the walls are inflexed at tolerably regular
distances, so as to give rise to the appearance of a row of teeth lining
the cell. If the strip of petal be folded, so as to exhibit the side
view, it will also be seen that the cells project outwards from the
surface to form a bluntish point or papilla, or the petals are papillose
as it is called; and the surface of the membrane around the papillæ is
finely wrinkled, so as to present the appearance of very delicate
radiating lines or striæ. Intermediate degrees of this inflexion may be
found in various flowers, between the slight condition seen in fig. 13
and the extreme state of fig. 24, as in the snapdragon (_Antirrhínum
május_).
_Anthers._--The cavities of the anthers are lined with fibro-cellular
tissue, the fibres of which aid in discharging the pollen; this may be
seen by dissecting an anther of London pride (_Saxif´raga umbrósa_), or
of a wallflower (_Cheiran´thus cheíri_) in water. It also exists in
chickweed.
_Pollen._--The pollen consists of minute grains called the
_pollen-granules_. They may be viewed either in the dry state as opake
objects, or when immersed in water as transparent objects. As it is
often difficult to moisten them, they may be touched on the slide with a
little spirit, and then a drop of water added. Their forms are very
varied and curious, but they are difficult of observation from their
minute size. They consist of one or more coloured cells, and these cells
are remarkable for their surfaces exhibiting spines, networks, folds,
and markings of various kinds. Thus in the primrose the pollen-granules
are cylindrical, the surface being furrowed (Pl. I. fig. 16); in the
sunflower the granules are spherical, and covered with tubercles
surmounted by spines (fig. 17); in the garden convolvulus the surface of
the spherical granules is covered with an elegant network, in the
meshes of which are also situated spines (fig. 18); and in the granules
of chickweed the surface presents pits, with minute tubercles in the
centre (figs. 30 & 31). The pollen-granules are often considerably
altered by immersion in water; so that, in judging of their structure
when examined in water, the resulting alteration must be taken into
account.
When ripe pollen-granules have been immersed in water for a short time,
one or more minute tubes will be seen protruding from their surface;
these are the _pollen-tubes_, and the granular protoplasm contained in
them is called the _fovil´la_. In the process of fertilization of the
flower, the pollen-granules fall upon the viscid stigma; the
pollen-tubes are then protruded, and, passing down the intercellular
spaces of the style (Pl. I. fig. 14), enter an aperture in the ovule or
young seed, which is thus endowed with the power of growing into a new
plant. The pollen-tubes are often very long, and they do not exist fully
developed in the pollen-granules, but grow down the style, just as the
little rootlet of a seed grows into the soil. The style of a crocus will
serve for dissecting out with mounted needles the long and very slender
pollen-tube (Pl. I. fig. 15).
_O´vary._--The ovary by its growth and enlargement becomes the fruit.
There are many interesting microscopic structures to be found in fruits
and the seeds they contain, a few of which may be noticed here.
On examining the surface of the rind or pericarp (περἰ, around, καρπὀς,
fruit) of an orange, little dots will be seen, paler than the rest of
the surface. These are receptacles of secretion, or glands, containing
the evaporable or volatile oil upon which the fragrance of the orange
depends. They consist of loose cells, surrounding a central cavity, and
are imbedded in the rind.
Other receptacles of secretion, called _vittæ_ (_vitta_, a band), occur
in the wall (pericarp) of the fruit of the Umbelliferæ, or Parsley Order
of plants, and their arrangement forms characters for distinguishing the
genera. They may be well seen in caraway-seeds; for the caraway-plant is
one of the Umbelliferæ. It must be observed that a caraway “seed” is not
really a seed, but consists of half the fruit; for, on careful
examination, one side of it will be found to be flattened, the
flattening resulting from the mutual pressure of the two half-fruits at
that part; moreover the dried style exists at its summit. In the figure
(Pl. I. fig. 19), the flattened part of the seed is next the observer.
The seed has five evident longitudinal ridges, one at each corner or
angle. The vittæ are dark-coloured (fig. 19 _a_), and placed one between
each pair of these ridges; and they consist of long flattened spaces in
the substance of the pericarp, with transverse markings, indicating
internal cross partitions. In botanical works, the presence of five
ridges, with single vittæ in the intervals, is given as a character by
which the half-fruits (carpels) of the caraway are to be distinguished.
But on closely inspecting the flattened surface, another ridge is seen
running down its middle; so that the seed really has six ridges, one of
which is smaller than the rest from the pressure of the other half.
Hence the character of five ridges with single vittæ is incorrect.
The vittæ contain the volatile oil to which the fragrance and pungency
of the fruit is owing, although some of the oil exists also in the cells
of the kernel or albúmen, which forms the white and greater part of the
seed.
The skin of a reddish apple, peeled off in the manner described for
petals, exhibits beautifully the red colouring matter of different tints
in adjacent cells, while the pulp displays the cell-contents, as already
mentioned. The latter may also be easily examined, from their large
size, in most of the softer fruits, as that of the snowberry or the
cucumber.
As the ovary or fruit approaches maturity, the petals and stamens wither
and fall off, the calyx often remaining, and being sometimes adherent to
the ovary, at others free or unattached to it.
_Seeds._--During the ripening of the fruit, the seeds contained within
it are gradually becoming further developed.
The seeds themselves are covered outside by a skin or coat called the
_testa_ (_testa_, a shell). This is remarkable for frequently displaying
various kinds of figured patterns, consisting of raised networks,
ridges, little knobs or tubercles, &c. Examples of these may be found in
the seeds of the poppy (Pl. I. fig. 27), mignonette (fig. 29), and
chickweed (fig. 51).
Some seeds are winged, as it is called, _i. e._ furnished with an
extension of the testa beyond the margin of the seed. This not
unfrequently consists of aggregated fibre-cells, the spiral fibre being
very distinct, as in the seeds of _Eccremocar´pus scáber_ (Pl. I. fig.
52). In the seeds of another curious plant in this respect, viz.
_Collómia grandiflóra_, the fibre-cells are separate, so as to resemble
hairs, and very mucilaginous, and in the dry seed are closely pressed to
its surface. If a portion of the testa of these seeds, which can be
procured at the seed-shops, be cut off, laid on a slide, a cover
applied, and when the object is in focus, a drop of water be added, in a
short time water softens the mucilaginous walls of the cells, the power
of the spiral fibres comes into play, and the cells expand so as to form
a very interesting object; the cells, in their expansion, apparently
writhing like so many minute worms (Pl. I. fig. 35).
The seed itself, which is contained within the testa or seed-coat,
consists essentially of the young plant or embryo. This is composed of
three parts, viz. the _plúmule_ (_plumula_, a little feather), or the
young stem; the _rad´icle_ (_radicula_, a little root), or the young
root; and one or two, rarely more, imperfectly developed or rudimentary
leaves, the _cotyle´dons_ (κοτυληδὠν, a cup).
These structures are closely packed in the seed, and are not easily
recognized at first. By keeping seeds moist for a day or two until they
begin to grow, or _germinate_ as the seed-growth is called, they are
readily detected, and may then be more easily found in the dry seed.
When somewhat advanced in growth, they are familiar to every one,
although they may not be recognized by their names. In table “mustard
and cress,” the whole consists of these organs of the two plants; the
white stalk directed downwards being the radicle, the two green
leaf-like lobes the cotyledons, and between the latter directed upwards
is the very minute plumule, which is more easily seen when the plants
have been allowed to grow larger. This structure of the seed is
important to be known, because the absence or presence and the number of
cotyledons afford characters, corresponding with those already mentioned
in respect to the veins of the leaves and the structure of the stem, for
distinguishing the great divisions of the Vegetable Kingdom. Thus, the
Exogens are Dicotyledons (δις, twice), their seeds having two
cotyledons; while the Endogens are Monocotyledons (μὁνος, single),
having one only; and the Cryptogam´ic plants are Acotyledons (α,
without), their seeds (spores) having none of these organs.
Some seeds consist entirely of the embryo, surrounded by the testa. But
in many others there is also present a usually whitish, firm cellular
substance, called the _albúmen_ (_albumen_, white of egg).
The albumen of seeds often affords good specimens of secondary deposit,
the cells being almost entirely filled with it. An example may be found
in a section of vegetable ivory, of which ornaments are sometimes made;
its structure resembles essentially that of the plum-stone. In other
instances the cells contain secreted matters, as starch, oil, &c.; and
sometimes the cotyledons also contain starch and oil. An example of the
former exists in the albumen of wheat; and of the latter, in the
horse-chestnut, the filbert, and mustard-seed.
The albumen and cotyledons serve to supply the embryo with nutriment
until the roots have grown sufficiently to enable them to absorb it from
the soil; the cotyledons also serve as temporary leaves.
The form and relative position of the radicle and cotyledons serve to
distinguish certain groups of plants. This may be illustrated by the
natural order Cruciferæ, or that containing the mustard, wall-flower,
&c.
Thus, in one group, which may be represented by the wall-flower, the
cotyledons are flat or plane (Pl. I. figs. 43 & 44), the radicle being
applied to their edges. This is best seen in a transverse section (fig.
43). They are then called _accum´bent_ (_accumbo_, to lie against); and
the botanical sign is O=. In the second group, the cotyledons are plane
(Pl. I. fig. 38), with the radicle applied to the back of one of them,
as in the seed of the common shepherd’s purse (_Capsel´la bur´sa
pastóris_) (Pl. VII. fig. 19). They are then termed _in´cumbent_
(_incumbo_, to lie upon), and the sign is O||. While in the third group
the cotyledons are folded in the middle, like the leaves of a book (Pl.
I. figs. 49 & 50), and the radicle is enclosed between them, as in the
white mustard (_Sina´pis alba_). The cotyledons are then called
_condu´plicate_ (_conduplico_, to fold); and their sign is O> >.
The plants above-mentioned are evidently all Dicotyledonous, or their
seeds have two cotyledons; and they contain no albumen.
In the Monocotyledonous division, which may be represented by a grain of
wheat (Pl. I. fig. 53), the single cotyledon forms a minute sheath
(_a_), enclosing the plumule (_b_), the radicle (_c_) being here but
little developed at first, the greater part of the grain consisting of
the albumen (_d_); the grain should be softened in water before
examination. In the germinated grain the cotyledon appears as a pale
sheath, surrounding the convolute green leaves of the plumule; which may
be best seen in a transverse section (Pl. I. fig. 48).
_Fertilization._--A few words must now be said regarding the formation
of seeds, and the action of the pollen-tubes in the process of
fertilization.
In the earliest stages of growth, the young seeds, or _ovules_ as they
are called, appear as little buds, arising from the inner wall of the
ovary; and the part from which they arise is called the _placen´ta_
(_placenta_, a cake). In chickweed (Pl. I. fig. 41 _c_), the placenta
forms a central column; and when the ovules are a little older, they are
found to have separated somewhat from the placenta, but retaining a
connexion by means of a little cord or stalk, termed the _funic´ulus_
(_funis_, a cord). The ovules may be readily found in the ovary or young
pod of a wall-flower, the placentas forming four lines, running
longitudinally down the interior of the pod.
In this early condition the ovule consists of a mass of cellular tissue;
and as new formations are soon added to it, it is termed in this state
the _núcleus_. Around the nucleus are then formed two coats, an outer,
called the prímine, and an inner, termed the secun´dine. These coats or
membranes are open at one end, so as to leave a passage down to the apex
of the nucleus; the opening is called the _forámen_. These structures
are well seen in the ovule of the wall-flower (Pl. I. fig. 54), the
foramen in the figure being indicated by a *; it will be noticed also
that the funiculus runs down one side of the ovule, so as to terminate
at the bottom or base of the nucleus. In ripe seeds, the spot at which
the funiculus has been attached is mostly perceptible in the form of a
scar. The slight prominence of the foramen can also often be
distinguished, as in the seed of chickweed (Pl. I. fig. 51*); in the
ripe seed the foramen is termed the _mícropyle_, and towards it the
radicle of the embryo is always directed.
One of the cells of the nucleus near its apex then enlarges, so as to
form a sac, called the _embryo-sac_. This is excessively thin and
transparent (Pl. I. figs. 45 _b_ & 47); and in it, also at the end next
the foramen, one or more (in the chickweed one) smaller cells are formed
from the cell-contents of the embryo-sac, which are called the
_embryonal vesicles_ (Pl. I. fig. 45 _a_).
Thus far developed, the embryo exists prior to the expansion of the
flower and the discharge of the pollen. The embryo-sac is not figured in
this early condition, the embryonal vesicle being then smaller than that
in fig. 45 _b_, although occupying the same position.
When the pollen has escaped from the anthers and fallen upon the stigma,
the pollen-tubes growing down the intercellular passages of the style,
enter the foramen of the ovule, and so reach the apex of the nucleus, at
which the embryonal vesicle contained in the embryo-sac is situated. The
end of the pollen-tube then adheres to the embryonal vesicle, and such
interchange of cell-contents takes place between them as effects
fertilization.
The process of cell-formation in the fertilized embryonal vesicle then
takes place rapidly, new cells being formed by the division of its
cell-contents (Pl. I. fig. 45 _a_); and it will be noticed that the new
cells are formed at the end of the embryonal vesicle, opposite to that
situated at the apex of the embryo-sac. As the cell-division and
formation proceed further, a mass of new cells is produced (Pl. I. figs.
46 _c_ & 47), forming the rudimentary embryo; and from this, by further
growth, the perfected embryo (fig. 55) results; or, to use a fashionable
technical term, the simply cellular embryonic mass becomes
differentiated into the radicle, cotyledons, and plumule, forming the
embryo. It will be remarked that the position of the embryo in fig. 55
is the reverse of that in figs. 46 & 47, the radicle in the former being
directed downwards, whilst that of the embryo in the figure of the
embryo-sac (fig. 47) is directed upwards.
The embryonal structures are very difficult of detection; but it happens
that in our little chickweed they are more easily dissected out than in
most other plants. For this purpose, the ovules, placed on a slide and
lying in water, should be picked to pieces with the mounted needles,
under the simple microscope. They may be preserved in chloride of
calcium or glycerine.
A clear distinction must be drawn between seeds, which result from the
process of fertilization, and buds, which are formed independently of
this process. Both consist essentially of embryo plants; but while the
former originate from a single cell, the latter are outgrowths of a
parent stem, from which their tissues are derived; and while the former
propagate the species, the latter increase the individual.
The obvious use of seeds is the distribution of the species by the
formation of new individuals.
In the general outline which has been given of the elements, tissues,
and organs of plants, they have been examined principally as existing in
the higher groups, or those of more complex structure; and to enter
further upon a description of these plants would involve the
consideration of variations in the form and arrangement of the organs of
which they are composed. As these can mostly be investigated without the
use of the microscope, we must pass to those in which the entire plant
consists of little more than simple or parenchymatous cells, and in
which the representatives of the flower are so inconspicuous, or are
reduced to so elementary a condition, that the plants included in the
Division have been termed _Cryptogam´ic_ (κρυπτὀς, concealed, γἁμος,
union--figuratively for reproductive organs) or Flowerless Plants. The
reproductive organs of the Cryptogamia are usually termed the
fructification, implying that they produce fruit, but not flowers.
PLATE II. [PAGE 49.]
FERNS AND LICHENS.
Fig.
1. _Chlorococcum vulgare._
2. _Parmelia parietina._
3. _Parmelia parietina_, section of a saucer (apothecium).
4. _Parmelia parietina_, gonidia and asci.
5. _Parmelia parietina_, asci and paraphyses; 5 _a_, spores.
6. _Calicium clavellum._
7. _Calicium clavellum_, stalked apothecia.
8. _Calicium clavellum_, apothecium.
9. _Polypodium vulgare_, frond.
10. _Polypodium vulgare_, lobe of frond.
11. _Polypodium vulgare_, group of capsules (thecæ).
12. _Polypodium vulgare_, capsule and spores.
13. _Polypodium vulgare_, spore germinating.
14. _Polypodium vulgare_, prothallium.
15. _Polypodium vulgare_, portion of prothallium, with archegonia.
16. _Aspidium filix mas_, frond.
17. _Aspidium filix mas_, pinnules with sori.
18. _Aspidium filix mas_, single pinnule.
19. _Scolopendrium vulgare_, frond.
20. _Scolopendrium vulgare_, portion of frond.
21. _Cladonia coccifera._
22. _Cladonia cornuta._
23. _Cladonia pyxidata._
24. _Cladonia rangiferina._
25. _Cladonia rangiferina_, ends of podetium.
26. _Graphis scripta._
27. _Graphis scripta_, lirellæ.
28. _Graphis scripta_, asci and spores.
29. _Graphis scripta_, spore.
30. _Opegrapha betulina._
31. _Opegrapha betulina_, lirellæ.
32. _Opegrapha betulina_, lirella.
33. _Opegrapha betulina_, ascus with spores.
34. Scalariform ducts of Brake (_Pteris_).
[Illustration: Plate II.
W Bagg sculp
_London: John Van Voorst._]
CHAPTER V.
FERNS, OR FIL´ICES.
The general appearance of plants belonging to the class of Ferns is so
well known that it need scarcely be described, especially since the
introduction of the glass plant-cases, by means of which the air can be
kept so damp that ferns are now grown in the very heart of our cities.
Their bright green and finely cleft leaves (PI. II. figs. 9 & 16) or
fronds (_frons_, a leaf) as the leaf-like organs of the lower plants are
called, arising in tufts from the stems, give them the elegant
appearance we are called upon to admire, whenever they are met with. The
brownish spots or stripes seen upon the back or under surface of the
fronds, and consisting of the fructification, form also a simple
character by which they may generally be distinguished; although in a
few of them the fructification is placed upon a distinct stalk. The stem
or rhi´zome (ῥἱξωμα, a root) of a fern is mostly situated just beneath
or at the surface of the ground, and is commonly mistaken for the real
root, which is buried in the earth. It is brownish outside, and covered
with scurfy scales or _ramen´ta_ (_ramentum_, a shaving). These scales
are interesting microscopic objects, from the distinctness with which
they exhibit the cellular network.
A section of the rhizome exhibits the fibro-vascular tissue arranged
differently from that in the stems of either Exogens or Endogens. It
forms curiously curved longitudinal plates, a very abundant component of
which is the scalar´iform (_scala_, a ladder) duct (Pl. II. fig. 34).
The walls of the scalariform ducts are angular, and the secondary
deposit is arranged in the form of transverse bars, somewhat resembling
the steps of a ladder; which structure is best seen in a transverse or
slightly oblique section. The fronds are usually cleft nearly down to
the main vein or midrib (fig. 9), or pinnatifíd (_pinna_, a feather,
_findo_, to cleave); sometimes the segments are similarly cleft, so that
the fronds are bipinnatifid. The manner in which the veins usually
branch is also peculiar, each branch separating from the point of
division at an acute angle with the original direction of the vein (fig.
20), so as to be forked. It is also worthy of notice, that the young
frond is rolled up into a flat spiral, or is cir´cinate (_circino_, to
go round), before it opens.
We will now examine one or two species more minutely.
POLYPODIACEÆ. _Polypódium vulga´re_ (Pl. II. fig. 9, one-third of the
natural size), a member of this family, is very common on old trunks of
trees, on banks, &c. The frond is deeply pinnatifid, the segments being
oblong, blunt (obtuse), scalloped (crenate) at the edges (PI. II. fig.
10), and becoming gradually shorter towards the apex of the frond.
On the back of the fronds are the little orange-coloured groups of
capsules (Pl. II. figs. 9 & 10); these are called _sor´i_ (σορὀς, a
heap). The capsules or _thécæ_ (θἡκη, a case), a magnified group of
which is represented in fig. 11, consist each of an aggregation of
cells, fixed to a stalk (fig. 12); and along the back of the capsule is
a close row of thicker cells, forming an elastic ring, the _an´nulus_.
When the seed-like bodies or spores (σπορἁ, a seed) are ripe, the
annulus becomes straightened from its elastic power, and tears the
capsule open, so that the spores are set free and scattered.
_Aspid´ium filix mas_ (Pl. II. fig. 16, reduced to one third or fourth
of the natural size), is the most common British fern. In this the
fronds are pinnate, _i. e._ the segments (pinnæ) corresponding to those
of _Polypodium_ are cleft entirely to the main stalk, the pinnæ (fig.
17) being pinnatifid; and the segments of the pinnæ or the pinnules
(fig. 18) are oblong, obtuse, and saw-edged (serrate). In all botanical
works the fronds are incorrectly said to be bipinnate.
The sori are brown, kidney-shaped (fig. 18), and differ from those of
_Polypodium_ in being covered by a thin membrane (_indúsium_), which is
fixed to the frond at the notch.
_Scolopen´drium vulgáre_ (Pl. II. fig. 19), the Hart’s-tongue Fern, is
common in hedges and on moist banks. Its fronds are simple or undivided,
strap-shaped, heart-shaped at the base, and narrowed to a point at the
apex. The sori (fig. 20) are brown, narrow, longish (linear), and
transverse or slightly oblique. The indusium is cleft down the middle,
so as to form a longitudinal fissure or suture.
_Reproduction._--The spores of the ferns (Pl. II. fig. 12 _a_) resemble
in appearance the seeds of flowering plants on a small scale. They are
usually brown, covered on the surface with little tubercles or other
markings, and when kept on a slide in a moist atmosphere, as over a
saucer of water covered with a bell-glass, they germinate. When this
takes place, one or more short, brownish, hair-like radicles emerge from
one part of the surface (Pl. II. fig. 13), and a process containing
chlorophyll issues from an adjacent spot. As growth proceeds, the latter
by cell-division gives rise to a flattened two-lobed leafy
cotyledon-like body or _prothal´lium_ (πρὀ, before, θαλλὀς, leaf), with
numerous rootlets springing from the base of the lobes. The prothallium
is of a peculiar dull-green colour, different from that of the young
frond which is subsequently formed. This arises from the absence of
stomata and intercellular passages containing air; for the air in these
passages of leaves and petals contributes greatly to the production of
the brightness of their colours. Moreover, the cells of the prothallium
resemble those of the parenchyma of a leaf, the epidermis with its
wavy-margined cells being absent.
When the prothallium has attained its full development, minute scattered
protrusions from its cells occur on the margin or under surface,
resembling short and blunt hairs; and each of these becomes partitioned
off to form a new cell, within which a number of crowded smaller cells
are produced. These organs are called _antherid´ia_ (anther, and εἶδος,
resemblance); and within each of the crowded smaller cells is contained
a very minute, colourless, coiled fibre, furnished with still finer
filaments, called _cil´ia_ (_cilium_, an eyelash); the ciliated fibres
being termed _spermatozo´a_ (σπἑρμα, seed, ξῶον, animal). At a later
period, other organs are found also on the back of the prothallium.
These are larger than the antheridia, and are composed of several cells,
arranged around a central canal which leads to an embryo-cell situated
at its base (Pl. II. fig. 2). These organs are the _archegónia_ (ἁρχἠ,
beginning, λὁνος, offspring). When the antheridia are ripe, they
discharge the spermatozoa, which are enabled to swim about by means of
their cilia in water (rain), and entering the canal, reach the
embryo-cell, which thus becomes fertilized. When fertilized, the
embryo-cells produce the little fronds which afterwards grow into the
mature plants.
Hence the spores of ferns differ strikingly from the seeds of the higher
plants in not containing the embryo radicle and cotyledons already
formed, these being produced during or after germination; also in the
fertilizing organs, viz. the antheridia or representatives of the
anthers, and the archegonia or the representatives of the pistils, being
produced from the cells of the prothallium.
The more minute of these structures are too difficult of observation and
preparation for any one unaccustomed to microscopic manipulation, so
that they have not been figured in detail; the figures given will,
however, serve to guide the observer in their recognition.
_Preservation._--The ferns may be easily preserved in the entire state,
by laying them flat between sheets of coarse unsized paper, and
subjecting them to moderate pressure in a screw-press; the paper should
be changed, or dried before a fire every two or three days, and the
pressure repeated until the specimens become dry and rigid. They may
then be mounted on sheets of paper, being fastened either with thread
passed round the stalk or portions of the frond with a needle, and tied
in a knot behind, or with strips of paper gummed at the ends.
The minute structures may be preserved either in the dry state or in
glycerine.
CHAPTER VI.
MOSSES, OR MUS´CI (MUSCUS, MOSS).
I need scarcely refer to the figures in Pl. III. to enable the reader to
recognize the Mosses; every one knows them at once by their remarkably
uniform general appearance, their miniature-plantlike form, their
crowded little leaves, concealing the slender wiry stems, their growth
in patches, and their curious urn-shaped fruits raised up on slender
bristle-like stalks.
The leaves of the mosses are simple, _i. e._ not cut into segments, and
consist of one or two layers of cells. The thinness of the leaves
enables these cells to be seen very distinctly, the closely united
cell-walls giving the leaves a netted or reticulated appearance (fig.
48), and the grains of chlorophyll being generally few and readily
distinguished. The veins of the leaves, or the nerves as they are
usually called, scarcely deserve the name; for neither they nor even the
stems contain fibro-vascular tissue, but consist simply of elongate
closely packed cells, and often the leaves have no nerves.
The fruit of the mosses consists of a _capsule_, sometimes called a
_sporan´gium_ (σπορἀ, seed, ἄγγος, vessel), usually placed at the end of
a slender stalk, called the _séta_ (_seta_, a bristle); but sometimes
the stalk is absent or extremely short, when the capsule is said to be
ses´sile (_sessilis_, sitting). The young capsule is covered with a thin
extinguisher-like cap or _calyp´tra_ (καλὑπτρα, a cover), which is
carried up as the capsule and its stalk grow, so as to be either
entirely thrown off, or to remain covering a greater or less portion of
the capsule, when this attains maturity.
PLATE III. [PAGE 54.]
MOSSES.
Fig.
1. _Sphagnum acutifolium_, expanded leaf.
2. _Sphagnum acutifolium_, cells of leaf.
3. Spermatozoa of _Polytrichum piliferum_.
4. _Sphagnum acutifolium._
5. _Sphagnum acutifolium_, capsule.
6. _Gymnostomum truncatulum._
7. _Gymnostomum truncatulum_, leaf.
8. _Gymnostomum truncatulum_, capsule and operculum.
9. _Gymnostomum truncatulum_, spore.
10. _Dicranum heteromallum._
11. _Dicranum heteromallum._
12. _Dicranum heteromallum_, leaf.
13. _Dicranum heteromallum_, operculum.
14. _Dicranum heteromallum_, calyptra.
15. _Dicranum heteromallum_, capsule; 15 _a_, peristome.
16. _Tortula muralis._
17. _Tortula muralis_, leaf.
18. _Tortula muralis_, capsule; _a_, tooth of peristome.
19. _Tortula muralis_, operculum.
20. _Tortula muralis_, calyptra.
21. _Tortula muralis_, archegonia.
22. _Polytrichum piliferum._
23. _Polytrichum piliferum_, leaf.
24. _Polytrichum piliferum_, calyptra.
25. _Polytrichum piliferum_, antheridial stems.
26. _Polytrichum piliferum_, single head.
27. _Polytrichum piliferum_, antheridia and paraphyses.
28. _Funaria hygrometrica._
29. _Funaria hygrometrica._
30. _Funaria hygrometrica_, leaf.
31. _Funaria hygrometrica_, capsule; _a_, operculum.
32. _Funaria hygrometrica_, stalk-like body.
33. _Funaria hygrometrica_, young archegone.
34. _Funaria hygrometrica_, more advanced archegone.
35. _Funaria hygrometrica_, section of young capsule.
36. _Funaria hygrometrica_, calyptra.
37. _Funaria hygrometrica_, antheridia.
38. _Funaria hygrometrica_, spores.
39. _Funaria hygrometrica_, annulus.
40. _Funaria hygrometrica_, archegonia.
41. _Funaria hygrometrica_, antheridial head.
42. _Funaria hygrometrica_, peristome.
43. _Hypnum rutabulum._
44. _Hypnum rutabulum_, leaf.
45. _Hypnum rutabulum_, capsule.
46. _Hypnum rutabulum_, spores.
47. _Hypnum rutabulum_, peristome.
48. _Hypnum rutabulum_, cells of leaf.
49. _Bryum capillare._
[Illustration: Plate III.
W Bagg sculp
_London: John Van Voorst._]
The calyptra is either simply mitre-shaped, or _mítriform_ (Pl. III.
fig. 24), or it is half-cleft, or _dimid´iate_ (figs. 14, 36). When the
capsule is ripe, the upper part usually separates at a circular
horizontal line (fig. 8) as a kind of lid, which is called the
_oper´culum_ (_operculum_, a lid), and thus the spores are enabled to
escape. The rim of the capsule, from which the operculum has separated,
forms its mouth, and this often exhibits a fringe of teeth (figs. 15,
18, 31), arranged in one or more rows; sometimes the teeth are replaced
by a membrane, or, again, both teeth and a membrane may be present. This
mouth-fringe is the _per´istome_ (περἰ, around, στὁμα, mouth). In many
mosses, an elastic row or ring of cells is situated between the mouth of
the capsule and its operculum, called the _annulus_ (figs. 18 & 39);
this, when the capsule is ripe, aids in throwing off the operculum.
It is important to become acquainted with the structure and arrangement
of these parts, as they form characters by which the families and genera
of mosses are distinguished.
The capsules of the mosses form very beautiful microscopic objects,
especially those furnished with a toothed peristome.
Most of the mosses produce their fructification in the winter and
spring.
The class of mosses is divided into two Orders, according to whether the
fruit-stalk is terminal, _i. e._ arises from the end of the stem or its
branches, or whether it is lateral, arising from the side of the stem.
Those with the fruit-stalk terminal, or the end fruited (Pl. III. fig.
22), form the _Ac´rocarpi_ (ἄκρα, summit, καρπὀς, fruit); while those
with the fruit-stalks lateral, or the side-fruited mosses (fig. 43),
constitute the _Pleu´rocarpi_ (πλευρἀ, side). The new shoots or young
branches of the stems of mosses are termed _innovations_.
We will now examine a few common mosses more in detail, beginning with
the ACROCARPI.
_Sphag´num acutifólium_ (Pl. III. fig. 4) is found in pools or bogs,
growing at the margins so as to be partially immersed. In this moss, the
upper branches are grouped into a head. The leaves are crowded, and
overlapping or im´bricate (_imbrex_, a tile) on the elongate stems; they
are egg-shaped (ovate) on the main stems (fig. 1), and narrower or
ovate-lanceolate on the branches; they are nerveless, and finely toothed
at the apex. The capsule (fig. 5) is roundish-ovate, without a
peristome, and the operculum is flattened. The grouped arrangement of
the upper branches renders the species of _Sphagnum_ easily recognized.
The structure of the leaves is also very peculiar and characteristic
(fig. 2). The cells of which they consist are of two kinds, one (fig. 2
_a_) being colourless, elongate, pointed, and containing a spiral fibre;
the other consisting of shorter and narrower obtuse cells, containing
chlorophyll, and situated between the former. In many of the former kind
of cells, little round apertures exist on the under surface, and minute
animals may sometimes be found imprisoned in them.
Another species of _Sphagnum_, _S. obtusifolium_, is common, and greatly
resembles the above, but has shorter and thicker stems, and
rounded-ovate, very concave, and obtuse leaves.
_Gymnos´tomum truncat´ulum_ (Pl. III. fig. 6) is a common little moss,
found on banks and in fields and gardens.
In this there is no peristome, although, in the young condition, a
membrane extends more or less over the interior of the mouth of the
capsule. The stem is slender, rigid, and simple, or but little branched.
The calyptra is dimidiate; the operculum is present (fig. 8), and
terminates above in an oblique beak, or it is obliquely rostrate
(_ros´trum_, a beak) as it is called. The leaves are obovate (fig. 7)
or ovate with the broader part remote from the stem, and narrowed at the
apex, where the nerve protrudes or is ex´current (_excurro_, to run
out). The spores (fig. 9) are reddish brown and smooth.
_Dicránum heteromal´lum_ (Pl. III. figs. 10 & 11) is probably the first
moss the reader will meet with on banks and heaths in the early spring;
and it will be sure to be noticed on account of the bright green colour
of the patches and the beautiful orange-brown capsules.
In this moss the capsule is nodding (cer´nuous) (Pl. III. fig. 15), and
has a single peristome, consisting of sixteen equidistant teeth, each
being deeply cut or cleft longitudinally (fig. 15 _a_), so that there
are thirty-two teeth altogether; and these are marked with internal
cross-bars, or transverse ridges. The calyptra is dimidiate (fig. 14);
and the lid is furnished with a long oblique beak (fig. 15 _b_). The
leaves are crowded, strongly nerved (fig. 12), lanceolate at the base,
and very narrow towards the apex, which is toothed; they are, moreover,
curved, and bent towards one side, or sécund.
_Tor´tula murális_ (Pl. III. fig. 16) may be found on the top of almost
every wall and on waste ground.
In this moss the peristome is single (fig. 18), consisting of thirty-two
spirally twisted teeth, arranged in pairs. They are narrow and slender,
and each is composed of two longitudinal portions (fig. 18 _a_), one of
which is pale yellow, the other reddish brown, like the capsule, and
both are fringed and covered with very minute papillæ. The capsule (fig.
18) is oblong, the ring or annulus remaining for some time. The lid is
conical (fig. 19), with a longish somewhat oblique beak, and the
calyptra is dimidiate (fig. 20). The stems are very short; the leaves
(fig. 17) are oblong, obtuse; the nerve strong, and projecting as a
colourless spirally striated bristle. The bristles often give the
patches of the moss a hoary appearance on wall-tops. The margins of the
leaves are folded back or recurved, giving them a peculiar thickened
appearance.
The largest of our mosses are contained in the next genus, viz.
_Polyt´richum_, some of them having the stems from 2 to 4 inches, or
even more, in height; they are common on heaths and in woods.
_Polyt´richum pilif´erum_ (Pl. III. fig. 22) is very common on open dry
heaths. This moss has simple stems, with the leaves crowded on the lower
part of those which are fertile or fruit-bearing. The fruit-stalk is
terminal (acrocarpous); the capsule ovate, 4-sided or quadrangular, with
a knob or struma (_strúma_, a swelling) at the base, the lid having a
short beak. The calyptra (fig. 24) is half-cleft (dimidiate) and very
hairy. The peristome is single, and consists of sixty-four teeth. The
leaves (fig. 23) are lanceolate, nearly upright, the margins folded
inwards or inflexed; and they end abruptly in a saw-edged or serrated
hair-like point.
_Poly´trichum commúne_, which is also very common, is larger than the
last species, and may easily be distinguished by the curved and serrate
leaves, which have no bristle-point.
In the early spring, patches of both these mosses may be found, in which
the stems are terminated by little rosettes (figs. 25 & 26); these will
be referred to presently.
Keeping still to the end-fruited or Acrocarpous mosses, we have next to
mention _Funa´ria hygromet´rica_ (Pl. III. figs. 28, 29), which is
readily distinguished from most other mosses by the pale apple-green
colour which it possesses before the capsule ripens. It is extremely
common on walls and waste ground.
The capsule of this moss (fig. 31) differs from those of the preceding
mosses in the peristome being double (fig. 42), or composed of an outer
and an inner row of teeth. The outer row consists of sixteen oblique
reddish teeth, which are marked with transverse bars or trabe´culæ
(_trabecula_, a little beam), and their points are connected by a
net-like thin plate. The inner row contains also sixteen teeth, arising
from the division of the membrane lining the capsule; these are
yellowish, thin, and placed opposite the outer teeth. The capsule itself
is pear-shaped or pyriform, orange-red when ripe, curved, and with the
mouth oblique. The calyptra (fig. 36) is half-cleft, and expanded as if
blown out below. The lid (fig. 31 _a_) is convex and obtuse; and the
annulus (fig. 39) is large and easily separable. The fruit-stalks are
curved near the top. The leaves (fig. 30) are ovate, concave, entire,
with a nerve reaching the apex, which is acute and prolonged into a
little point, or apic´ulate. The spores (fig. 38) are small and reddish
brown. The specific name (_hygromet´rica_) of this moss expresses its
hygrometric property; for if either the recent and moist moss be dried
or the dry moss wetted, the fruit-stalk gradually twists in opposite
directions in the two cases.
The last of the Acrocarpous mosses which we shall notice, _Bry´um
capil´lare_ (Pl. III. fig. 49), is tolerably common on trunks of trees,
on the ground, and sometimes on walls.
The capsule of this moss has a double peristome or mouth-fringe; the
outer consisting of sixteen reddish-brown, equidistant, transversely
striped teeth; the inner composed of sixteen thin keeled teeth, more or
less split down the middle, and with two or three intermediate cilia.
The capsule is nodding, smooth, oblong, pear-shaped, slightly narrowed
below the mouth; the lid being somewhat convex, and furnished with a
short slender beak. The calyptra is dimidiate. The leaves (fig. 50) are
obovate, the nerve extending beyond the point, rendering them
bristle-pointed. The seeds (fig. 49 _a_) are small and green.
This moss serves to illustrate a great difficulty, which will often
occur to the student, in determining whether a moss is end-fruited or
side-fruited. For in this, as in many other end-fruited mosses, a little
side-shoot or young branch (innovation) grows from the main stem
immediately below the leaves surrounding the base of the fruit-stalk, so
that the fruit-stalk appears to arise from the side of the stem. The
only method of overcoming the difficulty is to examine carefully the
comparative size and thickness of the stem and the shoot, and to
determine which is the weaker and so the newer. The leaves surrounding
the base of the fruit-stalk, which are mostly somewhat different in
structure from the stem-leaves, are called the _perichæ´tial_ (περἰ,
around, χαἱτη, bristle) leaves.
From among the side-fruited or Pleurocarpous mosses we shall select one
only, _Hyp´num rutab´ulum_ (Pl. III. fig. 43), which is common on the
trunks of trees and on banks.
In this moss, the nodding unequal curved capsule (fig. 45) has a double
peristome, resembling that of _Bryum_ (fig. 47). The calyptra is
half-cleft, and the lid conical and shortly beaked. The stem is
reclining or procumbent, and the pale green imbricated leaves (fig. 41)
are ovate and pointed, faintly saw-edged, the nerve becoming indistinct
at about the middle. It will be noticed that the cells of the leaf (fig.
48) have the prosenchymatous form, or are elongate with pointed ends;
and that the fruit-stalk (fig. 45) is rough with little grains.
_Fructification._--The fruit-producing organs of the mosses are of two
kinds, comparable to those of the flowering plants, but with their names
changed, as in the case of the ferns; the representatives of the anther
being called antheridia, and those of the pistil archegonia. The
antheridia may be best examined in _Polytrichum piliferum_ or _commune_,
the patches of stems with red rosette-like heads (figs. 25, 26) being
readily found in the spring on open heaths. The coloured leaves forming
these heads differ in form from those of the stem, being broader and
very sharp-pointed, and have received the distinctive name of
_perigónial_ (περἰ, around, γνος, offspring) leaves. In the centre of
these leaves, which must be separated with mounted needles in a drop of
water, the antheridia (fig. 27), forming oblong cellular green sacs,
will be seen; and intermingled with them will be found some slender pale
filaments, composed of mostly two rows of cells, which are the
_paraph´yses_ (παρἁφυσις, a side growth). If the antheridia are quite
ripe, they swell somewhat in the water, and from the free or unattached
end a very delicate, colourless, cellular mass gradually escapes. If the
antheridia are not quite ripe, the mass must be liberated by dissection.
On carefully examining this mass under a high power, it will be seen to
consist of very delicate rounded cells (fig. 3 _a_), each containing a
coiled filament, revolving more or less rapidly. After a time, these
filaments (fig. 3 _b_) escape, so that they may be examined more
minutely. They are excessively delicate, and are best seen when dried on
the slide. Each consists of a very slender curved filament, with a still
finer filament, or cilium, arising from it on each side. These are the
spermatozóa or spermatozóids (στἑρμα, seed, ξῶον, animal, εἶδος,
resemblance).
In _Funária_ the antheridia (fig. 37) may also be found, by careful
examination, in the little green heads terminating some of the stems
(fig. 41, of the natural size). In this moss, the paraphyses are
inflated at the summit into little knobs, or they are capitate (fig.
37). The pistil-like organs of mosses, or the _archegónia_, from which
the capsule is formed, must be looked for in the winter or early
spring. They occur in the parts of the stems from which the fruit-stalk
subsequently arises, and are surrounded by perichætial leaves, so as to
resemble in general aspect the antheridial heads. They are readily found
in _Tortula_ and _Funaria_, which are always at hand.
The archegonia (Pl. III. fig. 21) differ in form from the antheridia,
being flask-shaped, with a neck and a dilated base. The neck contains a
slender canal, and within the base is a special embryonal cell, from
which the capsule is subsequently formed. The spermatozoa of the
antheridia pass down the canals of the archegonia, and fertilize the
embryonal cells; but one archegonium only comes to maturity in each
head, the others ceasing to grow, and withering, in which condition they
are found at the base of the fruit-stalk when the capsule is fully
formed. The embryonal cell grows by subdivision, so as to form a
stalk-like body, which as it rises extends the archegonium upwards until
it splits across near the base. Thus the archegonium becomes split
horizontally into two parts, the upper and longer of which forms the
calyptra, whilst the lower remains as a very short tube or sheath
(_vagi´nula_) surrounding the base of the fruit-stalk. The cellular
stalk-like body then swells at the summit, the swollen portion gradually
becoming developed into the capsule, by resolving itself into an outer
wall lined inside with a coat forming the outer row of teeth at the top,
and within this a thinner membrane or spore-sac, the cleft upper margin
of which forms the inner teeth; and within this are contained the
spores. The mass of cells within the spore-sac remains, forming a
central column, called the _columella_.
These stages of growth may be readily traced in _Funaria_. In Plate
III., fig. 40 represents two fertilized archegonia of the natural size,
surrounded by the perichætial leaves; fig. 33 is a still more advanced
archegone. In fig. 34 the calyptra has separated from the vaginule, and
contains the stalk-like body, which is represented alone in fig. 32, the
dark summit indicating the commencing formation of the capsule. Fig. 35
represents the young capsule, in which all the parts are more advanced
in growth.
When the seeds of mosses germinate, they produce at first a green
_Conferva_-like filament, which branches at one end, the cells
containing green endochrome, while brownish little roots are given off
from the other end. The young leafy buds or young stems arise from these
confervoid filaments.
_Examination._--In the examination of the mosses, the capsules should be
viewed as opake objects while fixed in the forceps; and to discover the
minute structure of the teeth of the peristome, a capsule should be
wetted with spirit, then immersed in water, slit up with fine scissors,
and spread out with the mounted needles, so as to form a transparent
object. In this way, the curious structure of the teeth becomes very
distinct.
It must be noticed that, in the mosses, the antheridia and the
archegonia usually occur in separate flower-like heads; or the mosses
are either monœcious or diœcious (p. 38).
_Preservation._--The mosses may be dried under pressure, and preserved
entire in the same manner as the ferns or the flowering plants. If
simply dried without pressure, their structure can be readily made out
at any future time, by immersing them in water, or by keeping them for a
few hours in a moist atmosphere. The minute structures of mosses may be
mounted in solution of chloride of calcium, or in glycerine; they keep
extremely well without closing the cells.
CHAPTER VII.
ALGÆ (ALGA, SEA-WEED).
The plants belonging to the Class Algæ grow in water, either in that of
the sea or in fresh water; a few of them, however, being found on damp
earth, damp walls, &c. The marine Algæ are commonly known as sea-weeds;
but the fresh-water Algæ generally receive but little popular notice,
forming, as they do, slimy masses or strata, of a green or brownish,
sometimes red, colour.
Algæ are of simple structure, consisting entirely of cells; in some
these are single, in others, united end to end, to form threads or
filaments, or grouped into a leaf-like expansion, or collected few
together into a little spherical group or a flat plate. They possess
none of the fibres, vessels, or ducts of the higher plants, although
some long and slender cells, existing in the stalks of the fronds of the
larger kinds, bear considerable resemblance to woody fibre. They exhibit
no distinction of stem and leaf, but consist of fronds representing the
stem and leaf combined and undistinguishable. And the term frond must be
understood to signify the separate parts arising from the point of
attachment when they are fixed; and in the case of those which are
unattached or free, the entire plant is called a frond.
The Algæ are divided into three Orders, viz. the Fucoid´eæ or
olive-coloured Algæ, the Florid´eæ or red, and the Confervoid´eæ or
green Algæ.
FUCOID´EÆ, Fucoid Algæ, or Melanospor´eæ (μἑλας, black or dark). The
plants composing this order form our largest sea-weeds, and are found
everywhere in the sea and on the sea-shore. They are of an olive-green
or olive-brown colour, and usually become darker on drying.
PLATE IV. [PAGE 64.]
MARINE ALGÆ.
Fig.
1. _Dasya coccinea_, piece of.
2. _Dasya coccinea_, portion with capsule (ceramidium).
3. _Dasya coccinea_, portion of main filament.
4. _Dasya coccinea_, section of filament.
5. _Melobesia polymorpha._
6. _Melobesia polymorpha_, portion with capsules (ceramidia).
7. _Jania rubens._
8. _Jania rubens._
9. _Lithocystis Allmanni._
10. _Ceramium nodosum._
11. _Ceramium nodosum_, filament.
12. _Ceramium rubrum_, filament.
13. _Ceramium rubrum_, tetraspore.
14. _Ceramium rubrum_, end of filament.
15. _Ceramium rubrum_, capsule (favella).
16. _Fucus vesiculosus_, receptacles of.
17. _Fucus vesiculosus_, capsules (conceptacles).
18. _Fucus serratus_, antheridial conceptacles.
19. Spore of _Fucus vesiculosus_.
20. Antheridia of _Fucus serratus_.
21. _Plocamium coccineum_, sporophyll.
22. _Plocamium coccineum_, with capsule (coccidium).
23. _Plocamium coccineum_, portion of frond.
24. _Plocamium coccineum_, tetraspore from sporophyll.
25. _Polysiphonia fastigiata_, portion of.
26. _Polysiphonia fastigiata_, filament with capsules (ceramidia).
27. _Polysiphonia fastigiata_, portion of filament.
28. _Corallina officinalis._
29. _Corallina officinalis_, portion of filament.
30. _Corallina officinalis_, capsule (ceramidium).
31. _Enteromorpha compressa._
32. _Enteromorpha compressa_, cells of frond.
33. _Hypnea purpurascens_, capsule (coccidium).
34. _Hypnea purpurascens_, spores.
35. _Hypnea purpurascens_, filament.
[Illustration: Plate IV.
W Bagg sculp
_London: John Van Voorst._]
_Fúcus vesiculósus_, with its parallel-sided or linear olive-brown
fronds, is known to every one as the seaweed which is hung up to act as
a weather-glass. The fronds have a central stout vein, or midrib, and
scattered air-bladders, mostly in pairs.
The fructification consists of yellowish oval enlargements of the ends
of the fronds, called the _receptacles_ (fig. 16); but these are
somewhat variable in form, being often angular or truncate. On holding
one of the receptacles to the light, it will appear to contain a number
of little grains imbedded in its substance, slightly projecting above
the surface, and in the centre of each is a minute dot or pore. These
grains are the capsules, or _conceptacles_, and contain the spores. The
substance of the receptacles is composed of a beautiful network of
colourless, jointed, cellular fibres (figs. 17 _a_ and 18 _a_), the
meshes of which are filled with a transparent gelatinous substance; but
immediately around the conceptacles the cells are shorter and more
closely packed. The spores (fig. 19) are arranged in the conceptacles in
a radiate manner; they are brown, and surrounded by a colourless sac,
called the _perispore_ (περἰ, around, σπορἀ, seed); and between them are
numerous slender, colourless, jointed filaments, the _paraph´yses_. The
spores are not, however, truly single spores, for they ultimately divide
into eight segments or sporules, each of which is capable of producing a
new plant.
In the conceptacles of some fronds of _Fucus_ no spores will be found,
the conceptacles (fig. 18) being filled with elegantly branched
colourless filaments (fig. 20), the ends of many of them being distended
into little yellowish sacs; these are the _antherid´ia_. The antheridia
contain large numbers of exceedingly minute spermatozoa, furnished with
two cilia, and very similar to those existing in the antheridia of the
mosses; these, escaping through the pore of the conceptacle, fertilize
the spores.
The figure (20) in the plate was drawn from a conceptacle of _Fucus
serrátus_, another common species, differing from _F. vesiculosus_ in
having the margins of the frond serrate; the antheridia of the two
species do not, however, differ in any important respect. To examine the
conceptacles of _Fucus_ and their contents, the receptacles should be
soaked in water, if not fresh, and thin sections made with a sharp
knife. They form very beautiful objects, and may be preserved in
chloride of calcium or glycerine.
FLORID´EÆ, or Rhodosper´meæ (ῥὁδον, rose, σπἑρμα, seed).--The second
Order of Algæ, forming the Florideæ (_flos_, a flower), comprises the
red sea-weeds; a few of them are purple, or greenish-red; so that by the
colour alone they may be readily distinguished from the Fucoids, and
from nearly all those of the next Order, the Confer´voids. A few of them
are leaflike, or possess flat fronds; but most of them consist of finely
divided or feathery fronds. They are often found upon the sea-shore of a
dirty white colour, the colouring matter having been decomposed or
washed out by rain.
We shall consider a few of the genera and species under the heads of the
families to which they belong.
CORALLINA´CEÆ, the Corallines, or calcareous Algæ.--In this family we
have the beautiful _Corallína officinális_ (Pl. IV. fig. 28), the common
Coralline, which is very abundant on the sea-shore, attached to larger
sea-weeds, shells, and rocks. It is hard and chalky, from the presence
of a large proportion of carbonate of lime in its minute cells. The
fronds are composed of jointed and branched filaments. The
fructification (figs. 29 and 30) consists of ovate cellular capsules, or
_ceramid´ia_ (κερἁμιον, earthen vessel), placed mostly at the ends of
pinnate stalks, and containing a tuft of somewhat club-shaped jointed
spores, springing from the base of the capsules (fig. 30). When ripe,
the spores escape from a pore or hole in the end of the capsules. The
spores are 4-jointed, and hence are called _tet´raspores_ (τἑτρα,
_four_).
To observe these spores, the capsules must be soaked in strong vinegar
for some hours, and then washed with water, to dissolve the calcareous
matter.
_Jánia rúbens_ (Pl. IV. figs. 7 and 8) is another common and very
elegant little coralline, and is of a pale red colour. It differs from
the last in the branches being dichot´omous (δἱχα, in two, τομὀς,
cutting) or forked, instead of pinnate. The capsules, or ceramidia, have
also two short horn-like branchlets, placed one on each side, near the
end.
The genus _Melobésia_ has the frond crustaceous, _i. e._ forming a hard
crust or layer. _M. polymor´pha_ (Pl. IV. figs. 5 and 6) is common on
shells, stones, &c. The capsules (ceramidia) here form little blunt
cones, scattered over the crusts, and containing the tufted tetraspores,
as in _Corallina_.
_Lithocys´tis Allman´ni_ (fig. 9) is very minute, and not uncommon upon
sea-weeds, stones, &c. It consists of a single fan-shaped crustaceous
layer of cells, closely investing the body to which it is attached; its
fructification is unknown.
Leaving the family of crustaceous Florideæ, we shall now pass to those
of softer consistence, although all the marine Algæ contain a
considerable quantity of calcareous matter.
RHODOMELA´CEÆ.--In this family we have the large genus _Polysiphónia_,
in which the frond (Pl. IV. figs. 25 and 26) is filamentous, the
filaments being apparently jointed and longitudinally striated. The
filaments are composed of rings of cells (fig. 27), arranged end to end,
and containing dark endochrome. The ends of the colourless cell-walls
separating the endochromes of the cells of adjacent rings produce the
jointed appearance; while the striated appearance is caused by the dark
cells being elongate and the cell-walls thick, so as to form white
interspaces.
The fructification consists of capsules (ceramidia), attached to the
sides of the branches, containing pear-shaped spores, with tetraspores
imbedded in swollen branches of separate plants.
_Polysiphónia fastigiáta_ (fig. 25, a small piece) is common, attached
to the fronds of _Fucus_. Its filaments are rigid, bristle-like, of the
same breadth throughout, forked, and forming globular brown or yellowish
tufts, from 2 to 4 inches long. The joints are broader than long, each
with 16-18 of the dark cells. In the centre of the branches of this
sea-weed is a row of curious objects (fig. 26 _a_), consisting of a
dark-coloured body surrounded with irregular spiny marginal processes,
and with a colourless short process above and below. These require
further investigation.
_P. nigres´cens_ is also common among masses of seaweeds. Its filaments
are brown, pinnate, the branches awl-shaped, and the joints about as
long as broad.
_Dásya coccin´ea_ (Pl. IV. figs. 1 and 2, representing small portions of
a filament) is a very common filamentous red sea-weed of the same
family. The filaments are 6-8 inches long, and bipinnate,--the larger
ones somewhat resembling those of _Polysiphonia_, in being composed of
parallel longitudinal cells, arranged round the centre, but containing
also smaller intermediate cells; while the smallest branches (fig. 2),
which arise in tufts, consist of a single row of cells, little longer
than broad. The fruit consists of ovate capsules (ceramidia), placed at
the base of the branches, and containing a round mass of spores. There
is also another kind of fructification, occurring on distinct plants;
this is formed of one or two rows of tetraspores, immersed in pod-like
capsules, called _stichid´ia_ (στἱχος, row).
DELESSERIA´CEÆ.--In this family, the typical or most highly developed
genus of which, _Delesséria_, has beautiful leaf-like rose-red fronds,
we shall examine the common _Plocámium coccin´eum_ (Pl. IV. figs. 23 and
22). This is of a fine red colour; the fronds are from 2 to 12 inches
long, and consist of numerous branched and bushy filaments. These are
compressed, with the branchlets arranged in alternate rows on the two
margins of the stem. The end branchlets are acute and pectinate
(_pecten_, a comb), or arranged like the teeth of a comb. The cells of
which the filaments consist are small and angular, giving the surface
the appearance of being elegantly netted under a high power. The fruit
(fig. 22) consists of globular capsules, called _coccid´ia_ (κὁκκος, a
berry), placed in the axils or forks at which two branches separate, and
containing a mass of angular spores. There are also tetraspore-pods
(stichidia), as in _Dasya_; and tetraspores (fig. 24) in little
leaf-like altered branches (fig. 21), called _spor´ophylles_ (σπορἁ,
seed, φὑλλον, leaf), and antheridia are present.
RHODYMENIA´CEÆ.--In this family we have _Hyp´nea purpuras´cens_ (Pl. IV.
fig. 35). The filamentous pale purple frond of this sea-weed is from 6
inches to a foot or more in length, the branches being alternate and
spreading. The fructification consists of capsules or coccidia (fig.
32), immersed in the branches, and containing the spores (fig. 34).
Tetraspores also occur in the cells of the surface of the filaments.
CERAMIA´CEÆ.--This is the last family to be noticed. _Cerámium nodósum_
(Pl. IV. figs. 10 and 11), which belongs to it, is a most delicate and
elegant filamentous sea-weed, commonly found attached to other
sea-weeds. The filaments are hair-like or capillary, irregularly
dichot´omous; they consist of colourless cells, 3 or 4 times as long as
broad, and with thick walls. The junctions of the cells are swollen
(fig. 11), and covered with very minute dark red cells, giving them a
knotty and jointed appearance to the naked eye or under a low power. The
globular capsules, or _favel’læ_ (_favus_, a honeycomb), containing the
numerous spores, are situated at the ends of the branchlets, and the
tetraspores (fig. 11) in twos or threes on the outer margins of them.
In _Cerámium rúbrum_, which is also very common, being found attached to
stones, rocks, and the larger Algæ, the filaments (Pl. IV. fig. 12) are
stouter than in _C. nodosum_, branched so as to form tufts from 2 to 10
inches long, and their ends forked, with the tips hooked inwards (fig.
14). The central cells of the filaments are large and rounded, and their
walls are entirely covered with a layer of very small angular red cells.
The globular capsules (fig. 15), or _favellæ_, are situated on the suter
surface of the branches, stalked, and supported by 3 or 4 short
branchlets. The tetraspores (fig. 13) are imbedded in the branches,
towards the ends. The capsules called favellæ differ from the coccidia
in the walls being simply membranous, while the walls of the coccidia,
like those of the ceramidia, are composed of cells.
The tetraspores are usually imbedded, among the cells of the superficial
layer of the filaments, and are not very easily recognized by an
unpractised eye; it will be observed in the figures that they are
sometimes cleft horizontally, at others obliquely.
CONFERVOID´EÆ.--This Order consists principally of the green freshwater
Algæ, although some of them are yellowish brown, purple, or red, and
some are marine. Their general structure may be best illustrated by
selecting certain common examples from the families composing the order.
The families are 13 in number. The species which are figured in the
plates are found in fresh water, except when otherwise stated.
PLATE V. [PAGE 70.]
FRESHWATER ALGÆ.
Fig.
1. _Conferva floccosa_, single filament.
2. _Lyngbya muralis_, single filament.
3. _Ulothrix mucosa_ (?), filament.
4. _Synedra radians_, prepared frustules.
5. _Synedra radians_, tuft of natural frustules.
6. _Cladophora crispata_, with zoospores (_a_).
7. _Batrachospermum moniliforme_, portion of filament.
8. _Batrachospermum moniliforme_, filament.
9. _Closterium acerosum._
10. _Draparnaldia glomerata._
11. _Spirogyra quinina._
12. _Spirogyra nitida_, filaments conjugating.
13. _Zygnema cruciata._
14. _Coleochæte scutata._
15. _Xanthidia_ in flint.
16. _Micrasterias rotata._
17. _Gomphonema acuminatum._
18. _Gomphonema acuminatum_, prepared frustules.
19. _Ankistrodesmus falcatus._
20. _Pediastrum Boryanum._
21. _Hyalotheca dissiliens._
22. _Pinnularia viridis._
23. _Fragilaria capucina._
24. _Fragilaria capucina_, prepared frustules; _s_* side view of _F.
virescens_.
25. _Scenedesmus quadricauda._
26. _Schizogonium_, probably a form of _Lyngbya_.
27. _Campylodiscus costatus._
28. _Nitzschia minutissima_, front view.
29. _Nitzschia minutissima_, valves.
30. _Epithemia turgida._
31. _Diatoma elongatum_, natural frustules.
32. _Diatoma elongatum_, prepared frustules.
33. _Cocconeis placentula._
[Illustration: Plate V.
_W Bagg sculp_
_London: John Van Voorst._]
CONFERVA´CEÆ.--On removing some of the soft green matter found adhering
to the stems of water-plants in any pool or pond, one of the species of
_Conferva_, _C. flocculosa_, is almost sure to be met with. On close
inspection with the naked eye, the green filaments of which it consists
are just visible, as extremely fine, soft, silky threads; and, under a
high power of the microscope, the filaments are seen to be unbranched,
and composed of a single row of cells (Pl. V. fig. 1), or joints, as
they are called in technical works; these are 2 or 3 times as long as
broad. In some specimens the joints are swollen, so as to present a
rounded outline. In another common species, _C. bombyc´ina_, the
filaments are somewhat more slender, and the joints are from 3 to 5
times as long as broad.
_Cladoph´ora crispáta._--This Confervoid forms large, entangled,
dull-green masses, composed of branched, tufted, somewhat rigid and
coarse filaments. It is often a troublesome overrunner of the
fresh-water vivarium. The filaments are composed of thick-walled cells
(Pl. V. fig. 6), from 4 to 6 times as long as broad, and often
containing minute starch-granules.
The Confervaceæ have two modes of reproduction. The first of these
consists in the division of the endochrome of the joints into a number
of distinct segments, each of which becomes furnished at one end with
two very slender cilia (Pl. V. fig. 6 _a_). After a time, these ciliated
bodies, which are called _zo´ospores_ (ξῶον, animal, σπορἀ, seed) or
_gonid´ia_ (λονἠ, seed, εἶδος, resemblance), escape from the cells
either by their rupture or through a papillary orifice, and swim about
in the water, ultimately losing their cilia and growing into cells
resembling those of the parent plant. In the second method, which
occurs, for instance, in _Conferva bombycina_, certain of the joints
enlarge so as to become rounded or inflated; their endochrome then
becomes coated with a new cell-wall, and so forms a spore, which
subsequently escapes from the cell and germinates.
CHÆTOPHORA´CEÆ.--_Draparnal´dia glomeráta_ forms small green jelly-like
masses, adhering to sticks and stones in water. These consist of
branched filaments (Pl. V. fig. 10), prolonged at the ends into
colourless hair-like points, and composed of single rows of cells, the
green endochrome forming a band across the middle of each cell, the ends
being colourless.
In _Coleochæ´te scutáta_ (Pl. V. fig. 14) the cells are closely united,
so as to form a minute flat green disk. In the natural state, this
beautiful little object adheres to the submerged leaves and stems of
water-plants, and is therefore difficult to be found. But if a few
healthy water-plants be kept for some time in a glass jar, the little
_Coleochæte_, which is about as large as a pin’s head, will often be
found adhering to the side of the glass.
BAT´RACHOSPER´MEÆ.--The members of this family resemble to the naked eye
the little masses of _Draparnaldia_, and they are found in the same
localities. They are of various colours, being green, brown, purple, or
red. They consist, as in _Bat´rachosper´mum monilifor´me_ (Pl. V. fig.
8), of branched filaments, which have a knotty appearance under a low
power. The larger filaments are composed of cells arranged end to end,
the knots consisting of numerous smaller whorled filaments, _i. e._
filaments arising from around them at the same level (fig. 7). The cells
composing the whorled filaments are beaded or moniliform, and are
prolonged into colourless hair-like points. The globules seen among the
branches (fig. 7) consist of groups of spores.
ZYGNEMA´CEÆ.--The members of this family resemble the Confervaceæ in
consisting of simple cellular filaments (Pl. V. figs. 11, 13), but
differ from them in the elegant arrangement of the endochrome: this
forms beautiful spiral bands, as in _Spirog´yra quini´na_ (fig. 11), or
star-shaped masses, as in _Zygne´ma crucia´ta_ (fig. 13). A remarkably
curious phenomenon met with in them is the manner in which the spores
are formed, and which is known as _conjugation_. In this process the
opposite cells of two distinct filaments, lying near together, push out
protrusions of the cell-walls, which meet and open into each other,
forming cross tubes, as in _Spirog´yra nit´ida_ (Pl. V. fig. 12). The
contents of the opposite cells of the filaments then unite, forming
large spores, which remain either in the cells of one of the filaments
or in the cross tubes.
The three species figured are common in clear pools.
DESMIDIA´CEÆ.--The Desmidiaceæ are truly microscopic, few of them being
even perceptible to the naked eye without the very closest examination.
They are very beautiful, on account of their bright green colour and
often elegant forms. Many of them are very common, existing in every
pond or ditch; but they abound most in clear open boggy pools on heaths.
On placing some water containing them in a glass jar and exposing it to
the light, they will often be found adhering to the glass, or forming a
layer on the surface of the muddy sediment.
The Desmidiaceæ consist mostly of single cells (Pl. V. figs. 9, 16); and
these consist of two equal halves or segments, as indicated either by a
paleness of the endochrome or a deep constriction at the line of
junction, which is called the _suture_. The cells are often elegantly
lobed and cut, or spiny; and in many the surface exhibits minute
markings, consisting of little protrusions of the cell-wall outwards, or
inflations, as they are called.
Their reproduction is effected by division and conjugation. In the
process of division the cells gradually separate at the suture, and a
new half-cell is formed upon each old half, which grows until it attains
the size and form of the original half of the parent-cell. The
conjugation is effected by two cells approximating so that their sutures
are near together, the cells then open at the sutures, and the effused
contents become united to form a spore or sporange, from which one or
more individuals are formed. These spores are often elegantly spinous on
the surface.
Among the species selected for illustration is _Clostérium acerósum_
(Pl. V. fig. 9), in which the cells are single, elongate, very slightly
curved or lunate; the endochrome forms long bands, often containing
numerous globules or transparent vesicles. At each end of the cells is a
round transparent vesicle, containing exceedingly minute granules, which
exhibit a trembling kind of motion. Between the cell-wall and the
cell-contents very fine currents may also be detected, forming a
circulation resembling that in the hairs of _Tradescantia_.
In _Micrastérias rotáta_ (Pl. V. fig. 16) the cells, which are single,
are deeply cleft into two segments at the suture, the segments being
again regularly cut into five lobes, which are toothed or dentate.
In _Hyalothéca dissil´iens_ (Pl. V. fig. 21) the cells are united into a
cylindrical filament, and are surrounded by a very delicate gelatinous
sheath. In _Ankistrodes´mus falcátus_ (Pl. V. fig. 19) the cells
resemble those of _Closterium_ in shape, but are aggregated into
faggot-like bundles, and are very much smaller. In the beautiful little
_Pedias´trum boryánum_ (Pl. V. fig. 20) the cells are aggregated into a
disk, the marginal cells being bidentate or having each two points, so
that the whole resembles a star. The species of _Pediastrum_ are
reproduced by the contents of each cell subdividing into numerous
ciliated segments or zoospores, which subsequently escape in a mass from
the cell, ultimately losing their cilia, and reuniting to form a new
individual.
In _Scenedes´mus quadricau´da_ (Pl. V. fig. 25) the oblong cells are
united, side by side, the outermost cells being furnished with a bristle
at each end. The division of these cells takes place obliquely, so that
in the divided groups the cells are situated in two alternate rows.
The spores of many of the Desmidiaceæ are spinous, and they are often
found fossil in flint (Pl. V. fig. 15). To detect them in this
substance, thin slips of flint may be examined under a half-inch power;
or the chips of flint may be cemented to a slide with balsam, and ground
down on a hone.
The Desmidiaceæ must be mounted in the moist state: the smaller ones
will keep well in chloride of calcium; but the larger ones are injured
both by that liquid and by glycerine. The remarks made upon mounting, at
page 15, are especially applicable to these delicate organisms.
DIATOMA´CEÆ, or Siliceous Algæ.--The members of this family are singly
very minute; but when existing in large numbers, as they are often found
at the bottom of ditches and ponds, on the submerged stems of
water-plants, or upon damp ground, they form yellowish-brown evident
masses or strata. They occur both in sea-and in fresh water. They
usually consist, like the Desmidiaceæ, of single cells, which are called
frustules. But they are especially characterized by the cell-walls being
imbued with silica or flint, so that if the frustules be heated to
redness upon the point of a knife or a slip of platinum-foil, which
destroys the organic part of the cells, the coat of silica remains,
exhibiting the perfect form of the original cells or frustules. The form
of the frustules is very different in the various genera and species, as
represented in Pl. V. figs. 22, 23, 27, 30, 31, and Pl. VI. figs. 16,
17, 23; and it will be noticed that, in the figures, two views are given
of each frustule, _f_ indicating the front view, and _s_ the side view.
In all the front views, as in Pl. V. fig. 22, one or more lines will be
observed running longitudinally down the middle of the frustules, and
corresponding to the indications of division existing in the cells of
the Desmidiaceæ. Each half of a frustule is called a _valve_, and the
line at which these valves meet is called the _suture_. That side or
aspect of the frustule in which the suture lies (fig. 22 _f_) is the
_front view_; and the other aspect of the frustule (fig. 22 _s_) is the
_side view_. The frustules are mostly four-sided--the main breadths of
the two opposite valves forming two sides, and the bent margins of the
valves, with the back and front of the hoop, forming the two other
sides; so that the view presented by the side of a frustule is the same
as that of a single valve. The suture is the line at which the division
of the frustules takes place in the formation of new individuals. In
this process the cell-contents divide into two parts, as in ordinary
endogenous cell-formation,--the two new surfaces thus produced becoming
coated with a new portion of cell-wall or valve, so that two frustules
now occupy the place of the original one. At the same time a siliceous
band, encircling the frustules at the line of suture, is formed to fill
up the interval between the edges of the parent valves; this is the
_hoop_ (Pl. V. fig. 22 _f_; Pl. VI. fig. 10 _f_), and beneath it lie the
two newly formed valves. In many cases I believe that each half-frustule
becomes coated with a new entire cell-wall, with its siliceous valves.
The frustules of the Diatomaceæ are constantly undergoing division when
in vigorous growth. After the frustules have divided, the new ones
either separate entirely, as is perhaps most commonly the case; or they
remain united, sometimes completely, so as to constitute a filament (Pl.
V. fig. 23), while at others the frustules cohere only at the angles (Pl
VI. fig. 23), so as to form a zigzag chain.
In some species, the frustules are attached to foreign bodies by means
of a gelatinous cushion (Pl. V. fig. 5; Pl. VI. fig. 7); while in others
they are situated upon a simple or branched gelatinous stalk (Pl. V.
fig. 17) or stipes (_stipes_, a stem).
When the frustules are examined in the living state, the cell-contents
resemble those of ordinary vegetable cells, excepting in regard to the
colour, and exhibit granules and globules, and sometimes a nucleus is
visible. It will also be noticed that many of the free frustules move
slowly across the field of the microscope; but the cause of the motion
is unknown.
When the frustules have been properly prepared, the surface of the
valves exhibits a number of coarser or finer markings, consisting of
dots, lines (striæ), flutings, or networks, &c., arranged with great
regularity and symmetry, often of extreme minuteness, and rendering them
exquisite objects under the microscope. The exhibition of these markings
requires not only that the valves shall be properly prepared and
mounted, but that the object-glasses be of good quality, and that the
management of the light be thoroughly understood; so that to a beginner,
their examination is often a matter of great difficulty; for only the
very coarsest or largest of these markings can be perceived in the
natural frustules.
The appearance of these markings, and even their apparent absence or
presence, frequently depends upon the kind of illumination used: thus,
under one kind of illumination the valves may appear simply white or
coloured, while under another they appear covered with lines, and under
a third with dots. It will often be observed, also, that the colour of
the valves varies according to the illumination and the power used--the
same valve appearing white, yellow, brown, blue, &c.; and the wet or dry
state of the frustules often cause a decided difference in their
appearance as regards colour.
To illustrate the forms and markings of the frustules and valves, we may
select the following species taking first those which occur in fresh
water.
In _Epithémia tur´gida_ (Pl. V. fig. 30), the side-view or valve (_s_)
exhibits transverse or slightly radiating lines, with intermediate rows
of dots--these markings being continued over the margins of the valves
so as to appear also in the front view (fig. 30 _f_), ceasing at the
hoop. The frustules are curved or arcuate (_ar´cus_, a bow) in the side
view, oblong and narrowed at the ends in the front view.
In _Fragilária capucína_ (Pl. V. fig. 23), which is extremely common in
fresh-water pools, &c., the frustules are united side by side into long
filaments, which are often twisted. In the separate and prepared
frustule, the front view (Pl. V. fig. 24 _f_) is rectangular, the valves
(_s_) being narrowly lance-shaped or lanceolate. The valves under
ordinary illumination appear colourless and without markings, but, by
proper management of the light, very fine transverse striæ are seen upon
them, consisting of rows of very minute dots. Fig. 24 _s*_ represents
the valve of _Fragilaria vires´cens_, a nearly allied species.
_Diat´oma elongátum_ (Pl. V. fig. 31) is often found with the above. Its
frustules are coherent at the angles. The front view (fig. 32 _f_) is
rectangular, often slightly narrowed in the middle; and the valves are
narrowly linear, and capitate at the ends; they are also transversely
striated.
In _Synédra splen´dens_ (Pl. V. fig. 5) the frustules radiate from a
soft gelatinous cushion. They are linear in the front view (fig. 4 _f_),
the valves (fig. 4 _s_) being gradually narrowed or attenuated from the
middle to the ends, and exhibit transverse striæ interrupted opposite a
middle longitudinal line.
In _Campylodis´cus costátus_ (Pl. V. fig. 27) the frustules are
disk-shaped and curved, so as somewhat to resemble a saddle. The
markings consist of central dots, with radiating coarse flutings.
_Nitzsch´ia minutis´sima_ (Pl. V. fig. 28) has oblique valves, _i. e._
the front half of the suture is not opposite the back portion; the
valves (fig. 29) are constricted in the middle, and the ends narrowed
and prolonged. The markings consist of a row of oblong dots or puncta
(_punctum_, a point). This species often forms yellowish layers upon
damp paths, &c.
In the next group, the valves have a longitudinal line running down the
middle of the valves, with a little knob or nodule in its centre (Pl. V.
fig. 22 _s_), both consisting of internal thickened portions of the
valves.
In _Cocconéis placen´tula_ (Pl. V. fig. 33) the valves are oval, and the
markings consist of longitudinal rows of minute dots, with a marginal
row of puncta; these markings are invisible under ordinary illumination.
In _Gomphonéma acuminátum_ (Pl. V. fig. 17) the frustules are attached
to a branched stalk (stipes); they are wedge-shaped or cúneate
(_cúneus_, a wedge) and transversely striate (fig. 18), the striæ
consisting of dots.
In _Pinnulária vir´idis_ (Pl. V. fig. 22), which is very often seen
slowly traversing the field of the microscope when a drop of pond-water
is examined, the frustules in the front view are linear, the valves
being elliptic oblong, and transversely striated, the striæ consisting
of furrows.
In _Gyrosig´ma (Pleurosig´ma) attenuátum_ (Pl. VI. fig. 16) the valves
are sigmoid, or somewhat resemble a Greek ς (sigma) in outline, and the
markings consist of rectangularly crossed rows of very fine dots; in the
front view, the frustules are linear-oblong with truncate ends.
_Tabellária flocculósa_ (Pl. VI. fig. 23) has the frustules adherent
only at the angles, as in _Diátoma_. They are rectangular, and in the
front view exhibit a row of longitudinal dark lines interrupted in the
middle; these have been compared to the vittæ of the fruit of the
Umbelliferæ, and have received the same name.
Among the marine species may be mentioned _Melosíra nummuloídes_ (Pl.
VI. fig. 9), in which the frustules are united into a chain or
cylindrical filament. This is very common among sea-weeds, &c.; and it
illustrates well the process of division of the frustules (fig. 10 _f_).
The valves are covered with fine dots, and near each end of the
frustules is a projecting rim encircling it, and appearing as a curved
line extending beyond the margin of the frustule in the front view. In
_Actinocy´clus undulátus_ (Pl. VI. fig. 5) the frustules are separate,
disk-shaped, and the valves are divided into six equal parts by six
rays, each alternate portion of the surface of the valves being situated
on a lower level than those adjacent, so that an alteration in the focus
is required to bring into view the dots on any two adjacent divisions of
the valve. The surface of the valves is covered with easily recognized
dots. The form of the surface is best seen in the front view (fig. 5
_f_) when the frustule is placed on its edge.
_Rhabdonéma arcuátum_ (Pl. VI. fig. 7), which is very commonly found
attached to sea-weeds, resembles _Tabellária_ in the frustules having
the vittæ (Pl. VI. fig. 8 _f_). The frustules form short filaments,
attached by a little gelatinous cushion. The valves have transverse
striæ, interrupted in the middle (fig. 8 _s_).
_Gyrosig´ma (Pleurosig´ma) angulátum_ (Pl. VI. fig. 17) resembles _G.
attenuátum_ in the sigmoid form; but the markings consist of lines
crossing each other obliquely; and these are resolvable into rows of
dots (fig. 17 _a_) under suitable illumination.
In _Coscinodis´cus radiátus_ (Pl. VI. fig. 3) the frustules are
disk-shaped, the valves being elegantly sculptured with easily
recognized cell-like markings or dots, so as to resemble a piece of
vegetable cellular tissue. But in some other species the dots are very
minute, and difficult to be shown satisfactorily. These markings consist
of depressions or pits in the surface of the valves. That this is the
case may easily be seen by examining a fragment of the valve, when the
shadings of the broken ends of the netted thicker portions, which
project like teeth, strongly contrast with the difficultly
distinguishable portions of the thin interspaces. The fossil forms from
the Bermuda deposit are best for the investigation of this structure;
many of these are extremely beautiful microscopic objects, their
markings resembling those on the engine-turned back of a watch.
The detection of the finer markings of the Diatomaceæ, which, according
to my view, consist of depressions like those upon the valves of
_Coscinodiscus_, is a matter of great difficulty to those who are
unaccustomed to the use of the microscope, and who have not a complete
set of apparatus. The main point to be attended to in bringing them into
view, is to use one-sided oblique light, _i. e._ to turn the mirror by
its stem as much as possible to one side, and then to incline it so as
to throw the light upon the object. In this way the valves of the
species of _Gyrosigma_, for instance, appear covered with lines (Pl. VI.
figs. 16 and 17); but the lines are spurious, _i. e._ they are the
optical expression of rows of minute dots (figs. 16 _a_, 17 _a_); and
when oblique light is thrown upon the valves from all sides, by means of
a special achromatic condenser, in which the central rays are excluded,
the dots become distinct, and the markings resemble those on the valves
of _Coscinodiscus_. To show the finer dots clearly, a valve should be
crushed, so as to obtain a fragment as flat as possible; for the surface
of the valves is curved more or less in all the species. The valves of
_G. angulatum_ are generally used to test the quality of the
object-glasses of the microscope, and also for practice in “making out”
the lines and dots; there are, however, many Diatomaceæ more difficult.
As the nature of these markings is a disputed point, the discussion of
which is not adapted for an elementary work, I must refer for further
details to the ‘Micrographic Dictionary;’ it may be remarked, however,
that some observers have regarded them as cells, and others as
elevations or tubercles on the surface of the valves.
The preparation of the valves for showing the markings should be
effected by burning the frustules, or the mass containing them, on a
strip of platinum-foil over a spirit-lamp. The incinerated mass should
then be transferred to a slide, and the valves separated with the
greatest care by a bristle mounted in a hair-pencil stick under a low
power of the microscope.
This is, however, a substitute for the proper method, which is dangerous
in the hands of one unpractised in chemical manipulation. It is
this:--The mass of Diatomaceæ (the water containing it having been
carefully poured off as far as possible) is put into a Florence
oil-flask, and strong nitric acid (aquafortis) gently added, more than
sufficient to cover it. The mixture is then carefully boiled over a
spirit-lamp for some time. When it is cold, distilled water is added,
the whole shaken, and allowed to settle. The watery part is then gently
poured off, more water added, and this poured off after settling, and
the process repeated until a drop of the water evaporated to dryness on
a slide leaves no residue. The Diatomaceæ then form a white sediment at
the bottom of the water, and can be transferred to a slide with a
dipping-tube. The drop is then dried with a gentle heat, and the valves
mounted as dry transparent objects (p. 12).
If the valves have coarse markings, they may be mounted in balsam; but
if the markings are fine, balsam makes them much more difficult of
detection.
Many of the most beautiful Diatomaceæ are found in the fossil state; and
specimens of these are sold already mounted. I would advise those
unacquainted with them to purchase a slide of the “Bermuda” or
“Richmond” earth, which abounds in the species of _Coscinodiscus_; and
of the “San Fiore deposit,” which contains many species of _Epithemia_,
_Navicula_, _Pinnularia_, &c. These may be procured from Mr. Norman, 178
City Road, or from the microscope-makers.
VOLVOCIN´EÆ.--The Volvocineæ are inhabitants of clear fresh-water pools,
on heaths and bogs. They are very minute, of a rounded or plate-like
(tabular) form, of a green colour, and are pretty readily distinguished
from most of the other Algæ by their free motion; for they swim about in
the water like animals, as which they were formerly considered. They
consist usually of groups of thick-walled soft cells, each being
furnished with one or two cilia, by means of which the movement of the
compound bodies is produced.
In the beautiful _Volvox globátor_ (which is not uncommon) the cells
form a hollow sphere (Pl. VI. fig. 18), studded with exceedingly minute
green spots or zoospore-like bodies, representing the green endochrome
of the component cells, and from each of which very fine radiating lines
extend, so as to give the surface a netted appearance; the lines
consisting of delicate processes of the endochrome, which may be
compared with those existing in the cells of the hairs of
_Tradescantia_. In the interior of the parent globes are often seen
several young organisms, usually eight, of a deep green colour; these
escape by the rupture of the parent, so as to form independent beings.
Sometimes they are found of a yellow colour, and furnished with a thick
transparent coat; these are called “resting spores,” as they remain for
some time before undergoing their full development.
The cilia of _Volvox_, of which there are two to each of the component
cells, are difficult to detect; they are best seen when the organism is
dried without a cover, or after moistening them with a little solution
of iodine, which dyes them brown.
_Synúra volvox_ (Pl. VI. fig. 13) is a still more minute member of this
family, and is often found rolling along among _Confervæ_. The greenish
zoospore-like bodies of this Alga have one cilium only, and arise from a
common centre by a narrowing of the base (fig. 14).
In _Gónium pectorále_ (fig. 11 _a_) the green bodies, which are sixteen
in number, and furnished each with two cilia, are grouped into a flat
square plate; and in the very minute _Gonium tranquil´lum_ (fig. 11 _c_)
these bodies are also sixteen in number, and arranged in a tabular form,
but are without cilia.
SIPHONA´CEÆ.--The structure of this family may be illustrated by the
genus _Vauchéria_, of which two or three species are common on damp
ground or in freshwater pools, forming a green layer. At first sight,
the filaments of which the little plants consist appear like those of a
stout _Conferva_; but on close examination they are found to be
branched, and not jointed, consisting of a single cell from end to end
(Pl. VI. fig. 26). The reproduction is effected by the agency of two
kinds of organs, antheridia and capsules (sporangia), situated near each
other (fig. 26 _a_) on the walls of the filaments, of which they are
protrusions or outgrowths--their cavities being separated from that of
the filament by a partition or septum. The antheridia produce
spermatozoa, and the sporangia each a spore, the one fertilizing the
other in the ordinary manner. In addition to this method of
fructification, zoospores are also produced--the ends of the filaments
becoming swollen, the contents cut off by a septum, and forming single
large zoospores covered with cilia, the further development of which
resembles that occurring in the Confervaceæ.
OSCILLATORIA´CEÆ.--The members of this family are commonly found in
stagnant water or on shaded damp ground, especially in the cold seasons
of the year, forming green strata or masses.
PLATE VI. [PAGE 84.]
FRESHWATER ALGÆ.
Fig.
1. _Oscillatoria autumnalis._
2. _Oscillatoria nigra._
3. _Coscinodiscus radiatus._
4. _Nostoc minutissimum._
5. _Actinocyclus undulatus._
6. _Bacterium._
7. _Rhabdonema arcuatum._
8. _Rhabdonema arcuatum_, prepared frustules.
9. _Melosira nummuloides._
10. _Melosira nummuloides_, prepared frustules.
11. _a_, _b_, _Gonium pectorale_: 11 _c_. _Gonium tranquillum._
12. _Spirulina oscillarioides._
13. _Synura volvox._
14. _Synura volvox._
15. _Gyrosigma attenuatum_, front view.
16. _Gyrosigma attenuatum_, side view; 16 _a_, portion of a valve.
17. _Gyrosigma angulatum_; 17 _a_, portion of a valve.
18. _Volvox globator._
19. _Glœocapsa._
20. _Chara vulgaris_, globule.
21. _Chara vulgaris_, portion of filament.
22. _Chara vulgaris_, branch with nucule and globule.
23. _Tabellaria flocculosa._
24. _Tabellaria flocculosa_, prepared frustules.
25. _Palmella cruenta._
26. _Vaucheria Ungeri_ (_sessilis_).
27. _Vaucheria Ungeri_, capsule.
[Illustration: Plate VI.
_W Bagg sculp_
_London: John Van Voorst._]
_Oscillatoria autumnális_ (Pl. VI. fig. 1) occurs everywhere upon damp
shaded banks of ditches, especially when newly made, forming a
greenish-black closely adherent stratum. Under the microscope it is seen
to consist of innumerable palish-green filaments; these are jointed or
transversely striated, some being straight, others curved, the ends
often exhibiting a writhing or worm-like movement. The appearance of
these fibres is peculiar, seeming as if they were solid throughout, and
so differing from that of the Confervaceæ, in which the cell-walls are
readily distinguishable from the cell-contents. The fibres easily break
across at the joints; and the last few segments are often narrowed and
rounded, so as to form a blunt point. When they have been left in water,
they exhibit colourless tubular sheaths surrounding and extending beyond
them. These sheaths consist of the consolidated outer portions of the
cell-walls; for when the cells undergo transverse division, and expand
by growth in the direction of the length of the filament, the original
septa or inner walls are broken through, and their remains may often be
seen on the inner surface of the sheath, appearing as little teeth.
_Oscillatoria nigra_ (Pl. VI. fig. 2) is another very similar species,
forming blackish-green masses, and is common in ditches. It has longer
filaments than the last, with narrowed and slightly curved ends; and the
endochrome is distinctly granular.
In two other genera of this family, _Vib´rio_ and _Spirulína_, the
filaments are spiral. _Vib´rio spiril´lum_ is excessively minute,
colourless, and found in decomposing vegetable mixtures. The short
filaments move rapidly through the water, with a corkscrew-like motion.
In _Spirulína oscillarioídes_ (Pl. VI. fig. 12), which is more rarely
found in clear pond-waters among _Confervæ_, the filaments are greenish,
and form a beautiful simple spiral, resembling that of a very slender
spiral vessel.
_Lyng´bya murális_ (Pl. V. fig. 2) is very common on damp walls, gravel
walks, &c. It forms a bright grass-green layer, consisting of somewhat
rigid curled filaments. The endochrome is usually broader than long; and
the cells of the filaments are often found empty, the endochrome having
escaped in the form of gonidia.
Pl. VI. fig. 6 represents a species of _Bactérium_ which is not uncommon
in decomposing vegetable liquids; the filaments are short, curved,
pointed at the ends, and have four joints.
Fig. 26 represents a _Schizogónium_, found upon damp paths. The
filaments resemble those of _Lyngbya_, but are united in pairs.
Fig. 3 represents a filament of a _U´lothrix_, which is common in
freshwater pools, showing the curious manner in which the endochrome is
arranged in the cells, forming bands partially lining the cell-walls.
NOSTOCHA´CEÆ.--Two species of the typical genus _Nos´toc_ will serve to
represent this family. _Nos´toc commúne_ is found on damp ground or in
ponds, and forms to the naked eye firmish, olive-green, skin-like,
plaited masses, an inch or more in diameter. Under the microscope it is
seen to consist of numerous beaded fibres, imbedded in worm-like
gelatinous sheaths; these are curved and interwoven to form the compound
mass. In the middle of many of the filaments is an enlarged colourless
cell, called the vesicular cell, which is related to the reproduction,
but in a manner not yet determined.
_Nostoc minutis´simum_ (Pl. VI. fig. 4) forms solid gelatinous
bluish-green masses, varying in size from a pin’s head to a pea; it is
found upon unhealthy water-plants kept in glass vessels. The component
filaments are very slender, wavy, and the sheaths often have a brownish
tinge.
ULVACEÆ.--These Algæ are mostly marine--some, however, being found in
brackish or fresh water, or on damp ground, thatch, moss, &c. They are
generally of considerable size, forming flat or tubular fronds, often
several inches long, a few being filamentous. They consist of one or
more sheets or layers of cells, containing mostly green endochrome. This
at first fills the cells, but subsequently becomes converted into single
spores, or subdivided into numerous ciliated zoospores.
_Ul´va latis´sima_ is very common on the sea-coast, being found attached
to stones, shells, &c. It forms a broad, flat, green, rounded or oblong,
thin frond, wavy and crumpled at the margins, and from 6 to 18 inches in
length. The minute cells form two layers, adherent to each other. The
zoospores formed are numerous in each cell.
_Enteromor´pha compres´sa_ (Pl. IV. fig. 31) is also common in the sea
and in brackish ditches; it is often found floating. The frond is green,
tubular, flattened or compressed, and branched, the branches being
usually simple and narrowed at the base. The frond consists of two
layers of minute cells, separated by a space rendering it hollow. The
zoospores are numerous in the cells (fig. 32).
PALMELLA´CEÆ.--These Algæ are found in fresh or salt water, or on damp
earth, wood, &c. They are green or red, forming round or irregular
masses or strata. They consist of loosely connected cells, imbedded in a
gelatinous mass or matrix, thus forming a frond.
_Chlorococ´cum vulgáre_ (Pl. II. fig. 1) is very common upon the bark of
elm-trees, palings, &c., forming a green granular crust. It consists of
minute rounded or oval cells, mostly undergoing division into twos,
fours, or eights. These cells are attached to the sides or ends of very
fine colourless filaments. It is most probable that this organism, which
is usually placed among the Algæ, consists of the gonidia of a Lichen.
_Chlorococcum muror´um_ forms a somewhat similar but soft and thin green
layer, upon damp walls or other porous bodies. It consists of very
minute oval green cells, with thick walls, and imbedded in the ends of
prolongations of a gelatinous matrix.
_Palmel´la cruen´ta_ (Pl. VI. fig. 25) forms a portwine-red layer at the
bottom of damp walls or on the ground. It is composed of pale red cells,
imbedded in no definite order in a colourless gelatinous matrix. The
cells are filled with red granules, and are often found undergoing
division.
Pl. VI. fig. 19 represents a species of _Glœocap´sa_, in which the
cell-envelopes do not soften and unite to form a gelatinous matrix, as
in _Palmella_ and other members of the family, but are persistent. This
species occurs in fresh water containing Confervæ.
CHARA´CEÆ.--This family consists of the single genus _Chára_, the
systematic position of which is not agreed upon by authors; as however
its structure will be better understood after what has been gone over,
it may be conveniently considered here.
There are several species of _Chara_, the one illustrated, _Chara
vulgáris_ (Pl. VI. fig. 21), being commonly found in ditches and pools.
It consists of long main stems, often a foot in length, which are
branched, and surrounded at tolerably regular intervals by whorls of
branchlets. In some species, the stems and branches consist simply of
elongated cells, arranged end to end; while in others, of which _Chara
vulgaris_ is one, the central cells are surrounded by a number of
narrower spirally arranged cells, forming an outer coating.
The Charæ have long formed interesting microscopic objects, on account
of the circulation of the protoplasm being visible in the cells, as in
the hairs of _Tradescantia_. This is best seen in those species in which
the outer layer of cells is absent from the stems, and which were
formerly arranged in a separate genus (_Nitella_). But it may also be
seen in the stems and especially the young branchlets of any of the
other species; and as the granules of the protoplasm are large, the
phenomenon is more easily witnessed than in _Tradescantia_.
The fructification consists of two kinds of organs, viz. red _globules_
(Pl. VI. fig. 22) representing the anther-organ, and green capsules
(fig. 22), or _nucules_, corresponding to the ovaries. The structure of
the globules is very curious. Their transparent walls (fig. 20) consist
of eight somewhat triangular plates, each of which is composed of cells
radiating from a centre; and from the inside of each of these centres
arises a tubular cell extending to the middle of the globule, the
unattached ends giving origin to numerous colourless coiled filaments,
consisting of minute cells arranged end to end, each containing a very
minute coiled spiral fibre, to which are attached two exceedingly
slender cilia. These ciliated fibres are the spermatozoa. The capsules
or nucules (fig. 22), which are situated near the globules, are
urn-shaped, coated with spiral cells, and crowned with five shorter
cells. When the globules are ripe, they become ruptured by the
separation of the valves; and the spermatozoa, escaping from the cells
of the coiled filaments, swim about and enter a canal in the capsules to
fertilize the ovule contained within.
The _Charæ_ grow readily in a glass jar of fresh water, with a few
pebbles at the bottom; and if the plants be not overgrown with
Confervoids, the fructification will continue to be produced almost
throughout the year.
The circulation is best seen in the whorled branchlets, a portion of the
growing ends being placed in a live-box, or simply laid upon a slide in
water and covered with thin glass.
_Preservation._--The Algæ are best preserved in two ways,--the entire
fronds being dried upon paper under pressure, as directed for the Ferns;
and small portions, showing the minuter structures and fructification,
being mounted in chloride of calcium or glycerine. If it is required to
preserve the marine Algæ according to the first method, they should
first be immersed for a time in fresh water, to dissolve out the saline
matters derived from the sea-water, which would keep them damp and
ultimately spoil them. After these matters have been removed, the fresh
water should be changed, and pieces of paper placed beneath them while
suspended in the water; on withdrawing the paper carefully, keeping the
Algæ at the same time spread out, they may be made to retain the
required position; and when the water has drained away, and the
remaining moisture has mostly evaporated, they may be submitted to
pressure in a press.
The Confervoid Algæ may be conveniently spread out upon paper and
preserved in the same manner, as some of the distinguishing characters
are founded upon their appearance in the dry state, their adhesion to
the paper, &c. Moreover they can then at any time be minutely examined,
by the immersion of a small portion in water.
CHAPTER VIII.
LICHENS.
The Lichens are found growing upon the bark of trees, old palings, &c.
Those most easily seen with the naked eye form grey or coloured dryish
patches or pendulous tufts; while the smaller ones are singly easily
overlooked, from their minute size and close adhesion to the _mátrix_ or
body upon which they grow, forming, by their aggregation, the grey or
otherwise-coloured dry and brittle coatings of almost every tree or
decaying branch.
The Lichens derive their nourishment from the air, and not from the
matrix--in this respect differing from the Fungi, with some of which, as
we shall presently see, they agree in the structure of the fruit.
The structure of the Lichens is simple, no distinction of root, stem,
and leaves existing in them, although certain dry root-like fibres exist
in many of them, by which the plant is fixed to the matrix. The whole
consists mainly of a frond or _thal´lus_ (θαλλὀς, a leaf). This is
either raised above the surface of the matrix in a shrubby form, or
spread upon the surface as a flexible lobed layer (Pl. II. fig. 2), or
it is dry and brittle (crustaceous) and closely adherent (Pl. II. fig.
26).
The fructification consists of little saucers, disks, or streak-like
furrows, often of a different colour from the thallus, the structure of
which will be best illustrated by reference to a few common species.
PARMELIA´CEÆ.--_Parmélia parietína_ (Pl. II. fig. 2) is a very common
Lichen, found on the bark of trees, on old palings, &c. It is of an
orange-yellow colour, the thallus being flat, lobed, and scalloped
(crenate) at the margins. The structure of the thallus may serve to
represent that of most of the Lichens. It consists of three layers,--an
upper cortical or epidermic layer, which is continued over the margins
to the under surface of the thallus, to form the under layer; and
between these is the middle or medullary layer. The medullary layer
consists of interwoven fibres, which are more closely packed towards the
upper and under surfaces, so as to give them a cellular appearance. Near
the upper part of the medullary layer, a number of minute rounded green
cells exist, lying loosely in its meshes (Pl. II. fig. 4 _a_). These
green cells (gonidia) appear to correspond to the buds of the higher
plants, and, when detached from the plants, they are capable of growing
into new individuals.
On the upper surface of the thallus of _Parmelia_ (fig. 2) the
fructification may be observed. This consists of saucers or shields
(_apothécia_, ἁποθἡκη, a repository), formed of a raised and expanded
portion of the thallus (fig. 3), and containing the spores. The spores
are enclosed in closely set upright cells, or spore-sacs (figs. 4 _b_
and 5 _b_), called _as´ci_ (ἁσκὀς, a bottle) or thecæ; and intermingled
with the asci are filaments, enlarged and coloured at the ends
(_paraphyses_), which are probably abortive asci.
In systematic works upon the Lichens, the saucers and their contents are
included in the term apothecia, the saucer alone being called the
_excip´ulum_ (_excipulum_, a receiver); the mass of asci and paraphyses
forming the _nucleus_ or _thalámium_ (θἁλαμος, a bed).
The yellow spores are very minute, each ascus containing eight of them,
and they are divided by a transverse partition or septum.
Near the margins of the lobes of the thallus are small dark points.
These are the pouting mouths of little capsules (_spermogonia_) sunk in
the substance of the thallus, and containing numerous filaments,
terminated by very minute stick-shaped bodies (_spermatia_), which break
off and escape through the orifices of the capsules. These are probably
the representatives of the anthers of flowering plants and of the
antheridia of the ferns. They are, however, so difficult to find and
examine, that I must refer to the Dictionary for a further description
and figures of them.
LECIDIN´EÆ.--This family contains the genus _Cladónia_, three or four
species of which are common on boggy heaths, banks, &c., viz. _Cladonia
coccif´era_, the Scarlet Cup-moss (Pl. II. fig. 21); _C. pyxidáta_, the
Common Cup-moss (Pl. II. fig. 23); _C. vermiculáris_ (Pl. II. fig. 22);
and _C. rangiferína_, the Reindeer Moss (Pl. II. fig. 24). The thallus
of these Lichens forms little rounded irregularly overlapping scales,
with scalloped edges, overgrowing the surface upon which the Lichens are
found. The fruit-stalks, or _podétia_ (ποῦς, a foot), are hollow
(fistulose), and either simple and dilated into cups (Pl. II. fig. 23),
or branched with the corners or angles between the adjacent branches
perforated. The apothecia in the young state resemble those of
_Parmelia_ on a small scale; but as they approach maturity, the centre
becomes pushed up, so that the spore-layer is extended over the ends of
the stalks. In _C. coccifera_ and _pyxidata_ the cups are proliferous at
the margin; _i. e._, branches upon which the apothecia are placed arise
from it. The asci and paraphyses are very minute, but do not differ
essentially in structure from those of _Parmelia_. In _C. vermicularis_
the podetia are pointed and more solid than in the other species, the
apothecia forming very minute spots at their apices.
GRAPHID´EÆ.--To this family belong _Gráphis scripta_ (Pl. II. fig. 26)
and _Opeg´rapha betulína_ (Pl. II. fig. 30). These little Lichens are
easily overlooked, from the thin and but slightly raised thallus being
only visible to the naked eye as a discoloration of the bark of the
trees upon which they grow; while the fructification is very minute,
forming little black streaks or _lirel´læ_ (_lira_, a furrow),
irregularly arranged, and resembling somewhat the letters of some of the
Oriental alphabets.
In _Graphis scripta_ (fig. 26) the thallus is thin, somewhat membranous,
smoothish, shining, greyish white, and faintly bordered with black. The
lirellæ (fig. 27) are partly sunk in the bark, winding and narrow, some
being simple, others branched; and they are surrounded by a raised
border, formed by the thallus. The lirellæ are lined at the sides with a
black (carbonaceous) layer or _excip´ulum_, within which are situated
the asci and paraphyses. The spores (Pl. II. fig. 29) are 8-cleft, the
segments being again divided longitudinally into little spores or
sporidia.
In _Opegrapha betulina_ (Pl. II. fig. 30), which is found on the bark of
the birch-tree, the thallus is thin, dirty yellowish white, bordered
with black. The lirellæ (figs. 31, 32) are mostly simple, without a
raised border of the thallus, and the excipulum forms a complete lining
to them. The spores (fig. 33 _a_) are 3-cleft, and taper at the ends.
CALICI´EÆ.--_Calic´ium clavel´lum_ (Pl. II. fig. 6) is a pretty little
Lichen, growing upon old boards and farm-buildings. The thallus is
granular and greyish white. The apothecia (fig. 7) are stalked and
black, but of a lighter colour than the mass of spores forming the
nucleus. The spores are very minute, black, oblong, and divided by a
transverse septum.
The Lichens are divided into two Orders, according to whether the
apothecia are open before the spores are ripe, as in the species noticed
above, or whether the apothecia only open to discharge the ripe spores.
The first Order forms the _Gymnocar´pi_ (γυμνὀς, naked, καρπὀς, fruit);
the second forms the _Angiocar´pi_ (ἁγγεῖον, vessel, capsule).
_Preservation._--The Lichens are readily preserved, on account of their
dry nature; they need simply be kept in a dry place, and glued to pieces
of card. If room is an object, they may be dried under pressure, as in
the case of the flowering plants. When re-moistened, the minute
structures may be easily made out by sections. The smaller ones may be
mounted dry, in cells made of the wax cement (p. 16). The minute
structures keep well in chloride of calcium or glycerine.
CHAPTER IX.
FUNGI.
The Fungi form the lowest class of plants: as examples of them, may be
mentioned mushrooms, toadstools, puff-balls, the mould of paste, the
blue mould of cheese, &c. The more minute Fungi are very common, forming
beautiful microscopic objects, although they are rarely studied by the
microscopic observer.
Fungi live usually upon rotting or decaying vegetable substances, as
rotten wood, the dead leaves and stems of plants, &c.; but sometimes
they are found upon living plants, and some of them exist upon decaying
animal matters, and even in living animals.
Fungi exhibit no separation of root, stem, or leaves, as exists in the
higher plants; nor do they contain chlorophyll, the presence of which is
so generally associated with the idea of a plant. But they consist of
aggregations of mostly elongate cells, forming branched and interlacing
colourless fibres, buried like roots in the substance (matrix) upon
which they grow, and from which they derive their nourishment; this
portion of the Fungus is called the _mycélium_ (μὑκης, a fungus). The
portion of the Fungus projecting beyond the surface of the matrix is the
fructification; and this is the part usually called the fungus, the
mycelium being overlooked by a casual observer. So that here we have a
character distinguishing the Fungi from the Lichens, which derive their
nourishment from the air, and not from the matrix. The absence of the
green cells, or gonidia, forms another character by which the nearly
allied members of this class of plants can be distinguished.
PLATE VII. [PAGE 96.]
FUNGI.
Fig.
1. _Agaricus micaceus_: _a_, gills.
2. _Agaricus campestris_: _a_, spores; _b_, basidia.
3. _Physarum album_, on a piece of stick.
4. _Physarum album_, spores.
5. _Uredo segetum_, spores.
6. _Uredo caries_, spores.
7. _Uredo candida_, on leaf of Shepherd’s Purse (_Capsella_): _s_,
spores.
8. _Æcidium grossulariæ_, sorus.
9. _Æcidium grossulariæ_: _p_, spore-capsules (peridia); _s_,
anther-capsules (spermogonia).
10. _Nemaspora crocea_: _a_, spores.
11. _Torula casei._
12. _Torula herbarum_, on a piece of stick.
13. _Torula herbarum_, spores.
14. _Phragmidium bulbosum_, on bramble-leaf.
15. _Phragmidium bulbosum_, stylo-spores and paraphyses.
16. _Puccinia graminis_, on a piece of straw.
17. _Puccinia graminis_, spores.
18. _Sporocybe alternata_, filament and spores.
19. _Botrytis parasitica_, on Shepherd’s Purse.
20. _Botrytis parasitica_, spores and filaments.
21. _Rhinotrichum_, species of.
22. _Rhinotrichum_, heads of spores.
23. _Rhinotrichum_, spores detached.
24. _Rhinotrichum_, spores.
25. _Penicillium glaucum._
26. _Penicillium glaucum_, head of spores.
27. _Coremium leucopus._
28. _Tubercularia vulgaris._
29. _Tubercularia vulgaris_, divided receptacle.
30. _Tubercularia vulgaris_, filaments.
31. _Tubercularia vulgaris_, spores.
32. _Sphæria fragiformis._
33. _Trichothecium roseum_, on a piece of stick.
34. _Trichothecium roseum._
35. _Trichothecium roseum_, filaments and spores.
[Illustration: Plate VII.
W Bagg sculp
_London: John Van Voorst._]
The fructification of the Fungi occurs in two distinct forms, in one of
which the seeds or spores are naked, and situated at the ends of slender
cells or filaments, whilst in the other the spores are contained in
usually flask-like cells, called asci, similar to those occurring in the
Lichens. In a few Fungi, antheridial organs, called spermogonia, as in
the case of the Lichens, have also been detected. The Fungi are divided
into six Orders, from each of which a few species may be selected to
illustrate their structure more in detail.
HYMENOMYCE´TES (ὑμἠν, membrane, μὑκης, fungus). This is the highest
Order of Fungi, containing a large number of genera and species; as
examples of which may be mentioned the common Mushroom, Toadstools, &c.
Their general structure may be illustrated by the examination of the
common Mushroom (_Agar´icus campes´tris_); the species figured (Pl. VII.
fig. 1), however, being _Agaricus micáceus_, which is common at the root
of trees, the bottom of decaying posts, &c.
The vegetative part of the fungus consists of a cotton-like mycelium,
which is composed of slender, colourless, interwoven filaments,
popularly known as the spawn. The portion commonly called the mushroom
corresponds to the fructification, and consists of certain parts visible
to the naked eye. These are an expanded portion at the top, forming a
hemispherical cap, receptacle, or _píleus_ (_pileus_, a cap), and a
stalk, or _stípes_, upon which the cap is supported. On the under
surface of the cap are a number of nearly parallel, radiating,
dark-coloured plates or _gills_, somewhat resembling the gill-plates of
a fish. The dark colour of the gills arises from the presence of the
spores, which are coloured, although in some species they are white. The
surface of the gills, upon which the spores are situated, is called the
_hyménium_ (ὑμἠν, membrane). The spores (Pl. VII. fig. 2 _a_) are
microscopic and very minute, and are situated at the ends of little
stalks or points (Pl. VII. fig. 2 _b_), called _sterig´mata_ (στἡρυγμα,
a prop), which are four in number, and consist of prolongations of the
colourless cells of the hymenium; and these cells are the _basid´ia_
(βασἱδιον, a little base). The detection of the basidia requires great
care, as they are very minute and transparent; the best way to observe
them is to cut a very small portion from the uninjured edge of one of
the gills with a fine pair of scissors, and to examine it in water. If
the gills have been bruised, the spores are easily rubbed off, and their
connexion with the basidia destroyed.
In the young state of the Mushroom, the fructification appears as little
knobs upon the spawn or mycelium. Upon cutting these through
perpendicularly, the cap and stalk are found to be enclosed in a skin or
wrapper, called the _vol´va_ (_volva_, a wrapper), and the margin of the
cap is continuous with the surface of the stalk, the connecting membrane
forming the _veil_, or _vélum_ (_velum_, a veil). As the Fungus grows,
the cap rises and bursts the volva, which withers and disappears; and
the veil is torn through, the portion remaining in connexion with the
stalk encircling it as a collar or ring (_an´nulus_).
_Merúlius lach´rymans_, the Dry-rot Fungus, which belongs to the same
family (Agaricíni) as the Mushroom, deserves notice from its very
destructive action upon decaying timber. The filaments of the mycelium
may readily be detected in the rotting wood by examining a thin section
in water. The receptacle forms a yellowish-orange or brownish flattened
mass, some inches in breadth, with white downy margins; and the surface
exhibits folds arranged so as to form large irregular pores, instead of
gills as in the Mushroom.
In other families the cap and stalk appear fused together and
undistinguishable; or the fructification assumes the form of the cap or
the stalk only, and the hymenium does not form gills. Thus in the family
Polyporei (the members of which are common on the trunks of trees and on
rotten posts, and some of which are very large) the basidia are situated
upon the inner surface of tubes immersed in the under part of the mass,
their orifices forming minute pores, yet visible to the naked eye. In
other families the basidia are placed upon the outer surface of a
club-shaped or branched receptacle (Clavarini), or upon external
prickle-like points (Hydnei), &c.
In the family Tremellini the receptacle forms a gelatinous mass, and the
basidia are situated upon its surface, terminating the filaments of
which it is composed. One species, _Dacrymy´ces stillátus_, is very
common on fir posts in the winter and spring, forming little roundish,
yellowish-orange, cushion-like masses.
GASTEROMYCE´TES (γαστἠρ, belly, hollow, μὑκης, fungus).--This order of
Fungi, which contains the Puff-balls and many others not readily
procured, must be very briefly noticed. The spores are contained in a
capsule or perid´ium (πηρἱδιον, a little bag) which is often of large
size, and are situated upon a hymenium forming folds, partitions
(septa), or a lattice-work.
A small species, _Physárum album_ (Pl. VII. fig. 3), is often found
growing upon rotten stems of plants and decaying sticks. The capsules
are minute, grey, brittle, and black within, from the presence of the
spores (fig. 4), which are lens-shaped, and arise from the ends of short
filaments.
CONIOMYCE´TES (κὁνις, dust, μὑκης, fungus).--This Order contains some
beautiful microscopic Fungi, several of which are very common. Many of
them grow upon living plants, while others are found upon decaying
stems, sticks, &c. The mycelium consists of inconspicuous, fine
filaments, which run beneath the epidermis and bark of leaves and stems,
or exist in the intercellular passages, the fruit bursting through the
surface. The spores are short-stalked, forming _sty´lospores_ (στῦλος,
stalk, σπὁρος, seed) or _conid´ia_ (κονἱδιον, little dust). But there is
great confusion in the descriptions of the spores of the same Fungus by
different botanical authors, some describing the fruit (in Pl. VII. fig.
15, for instance) as composed of rows of spores, while others regard it
as forming a single septate (_septum_, a partition) or partitioned
spore.
_Tor´ula herbárum_ (Pl. VII. fig. 12) is very common on the decaying
stems of plants, especially those belonging to the Parsley order
(Umbelliferæ), forming greenish-black streaks or patches. The spores
(fig. 13) are grouped into chains or beaded (moniliform) rows, with very
short stalks, and these are crowded to form the black patches visible to
the naked eye. Under the microscope the spores appear of a brown colour.
_Torula cásei_ (Pl. VII. fig. 11) forms reddish or white patches upon
decaying cheese. It consists of branched, interwoven, tufted filaments
(_flocci_), with comparatively large spherical spores arranged in rows
at their ends.
_Nemas´pora crócea_ (Pl. VII. fig. 10) is a very curious member of this
Order, and is found upon decaying beech-sticks. It appears as an
orange-coloured tendril-like gelatinous mass of spores, bursting through
a little pore on the surface of the bark. The spores (fig. 10 _a_) are
very minute, slender, and curved, and under a high power appear jointed.
_Æcid´ium grossuláriæ_ (Pl. VII. fig. 8) is found very commonly on the
leaves of the gooseberry-bush. It forms to the naked eye oval or rounded
spots (_sori_), of a red colour; and on close examination, the spots
appear dotted with yellow points. Each point is the orifice of an open
capsule (_peridium_), which has burst through the epidermis of the leaf
(Pl. VII. fig. 9 _p_). The capsules are split or lacerated at the
margins, and form little cups containing the spores. The spores are very
minute, yellow, and are arranged in closely packed moniliform rows. The
red colour depends upon the altered chlorophyll of the leaf. On the
leaves containing the spore-capsules or peridia will be found smaller,
brownish-yellow capsules (_spermogonia_) partly imbedded in their
substance (Pl. VII. fig. 9 _s_). These contain minute filaments
(_sterigmata_), terminated by short rows of rounded cells (_spermatia_),
which are supposed to exert an antheridial function. The species of
_Æcidium_ are very numerous, and many of them are extremely common--as
those upon the nettle, the barberry, the dandelion, the wood-anemone,
the violet, and buttercups. The groups of capsules form exquisite opake
objects under a low power of the microscope.
_Phragmid´ium bulbósum_ (Pl. VII. fig. 14) is another very beautiful
coniomycetous Fungus. It forms little reddish, afterwards sooty dots
upon the under surface of the leaves of various species of Bramble
(_Rúbus_). The oblong spores (fig. 15) are from 2-to 4-septate, and
stalked, the stalks being swollen or bulbous at the base. The spores,
which appear brown when magnified, are covered with little knobs
(tuberculate) on the surface; and the uppermost little spore or
sporidium is terminated by a minute point (apiculate). Among the spores
are numerous barren filaments or paraphyses.
_Puccin´ia gram´inis_ (Pl. VII. fig. 16) is to be found everywhere upon
damp rotting straw, and upon grasses. It forms sooty irregular streaks,
consisting of densely crowded, one-partitioned (uniseptate) spores (fig.
17), which appear brown under the microscope. This Fungus is sometimes
called “mildew.” There are numerous other species of _Puccinia_ which
occur upon common plants.
_Urédo seg´etum_ is the “smut” of wheat, barley, and oats--a fungus too
well known to the farmer. It forms sooty masses, bursting through the
epidermis of the stalk and ears of the corn, and soiling the fingers
when handled. The spores (Pl. VII. fig. 5) are exceedingly minute, and
the stalks are so slender and loosely connected with them that they are
not readily detected. Under the microscope the spores appear brown and
faintly dotted, this appearance arising from a reticulated structure of
the surface, similar to that of the poppy-seed on a very small scale.
_Urédo cáries_ is the “bunt” of corn. It grows within the grain, filling
it with a sooty, fœtid mass. The spores (Pl. VII. fig. 6) are
considerably larger than those of the last species, and their surface is
distinctly reticulated. They are attached to the filaments of the
mycelium, as in _Uredo segetum_.
The spores of both these species of _Uredo_ may be found in most kinds
of flour and bread, especially in those of inferior quality.
_Urédo can´dida_ (Pl. VII. fig. 7) is another species, forming white
dots upon the leaves of the common Shepherd’s Purse (_Capsel´la bur´sa
pastor´is_)--which is easily recognized by the form and arrangement of
the pods (fig. 19). The spores (_s_) are rather large and white.
Other species of _Uredo_ are very common upon numerous species of weeds
o
Showing first 200,000 characters. Full text: gutenberg.org/ebooks/48016