CLASSIFICATION OF PLANTS.--PROTOPHYTES.
For the sake of convenience it is desirable to collect into groups
such plants as are evidently related; but as our knowledge of many
forms is still very imperfect, any classification we may adopt must be
to a great extent only provisional, and subject to change at any time,
as new forms are discovered or others become better understood.
The following general divisions are usually accepted: I. Sub-kingdom
(or Branch); II. Class; III. Order; IV. Family; V. Genus; VI. Species.
To illustrate: The white pine belongs to the highest great division
(sub-kingdom) of the plant kingdom. The plants of this division all
produce seeds, and hence are called "spermaphytes" ("seed plants").
They may be divided into two groups (classes), distinguished by
certain peculiarities in the flowers and seeds. These are named
respectively "gymnosperms" and "angiosperms," and to the first our
plant belongs. The gymnosperms may be further divided into several
subordinate groups (orders), one of which, the conifers, or
cone-bearing evergreens, includes our plant. This order includes
several families, among them the fir family (_Abietineæ_), including
the pines and firs. Of the sub-divisions (_genera_, sing. _genus_) of
the fir family, one of the most familiar is the genus _Pinus_, which
embraces all the true pines. Comparing different kinds of pines, we
find that they differ in the form of the cones, arrangement of the
leaves, and other minor particulars. The form we have selected differs
from all other native forms in its cones, and also in having the
leaves in fives, instead of twos or threes, as in most other kinds.
Therefore to distinguish the white pine from all other pines, it is
given a "specific" name, _strobus_.
The following table will show more plainly what is meant:
Sub-kingdom,
_Spermaphyta_.
/--------------------^---------------------\
Includes all spermaphytes, or seed plants.
Class,
_Gymnospermæ_.
/------------^------------\
All naked-seeded plants.
Order,
_Coniferæ_.
/--------------^--------------\
All cone-bearing evergreens.
Family,
_Abietineæ_.
/--------^--------\
Firs, Pines, etc.
Genus,
_Pinus_.
/---^---\
Pines.
Species,
_Strobus_.
/-----^-----\
White Pine.
SUB-KINGDOM I.
PROTOPHYTES.
The name Protophytes (_Protophyta_) has been applied to a large number
of simple plants, which differ a good deal among themselves. Some of
them differ strikingly from the higher plants, and resemble so
remarkably certain low forms of animal life as to be quite
indistinguishable from them, at least in certain stages. Indeed, there
are certain forms that are quite as much animal as vegetable in their
attributes, and must be regarded as connecting the two kingdoms. Such
forms are the slime moulds (Fig. 5), _Euglena_ (Fig. 9), _Volvox_
(Fig. 10), and others.
[Illustration: FIG. 5.--_A_, a portion of a slime mould growing on a
bit of rotten wood, × 3. _B_, outline of a part of the same, × 25.
_C_, a small portion showing the densely granular character of the
protoplasm, × 150. _D_, a group of spore cases of a slime mould
(_Trichia_), of about the natural size. _E_, two spore cases, × 5. The
one at the right has begun to open. _F_, a thread (capillitium) and
spores of _Trichia_, × 50. _G_, spores. _H_, end of the thread, × 300.
_I_, zoöspores of _Trichia_, × 300. i, ciliated form; ii, amœboid
forms. _n_, nucleus. _v_, contractile vacuole. _J_, _K_, sporangia of
two common slime moulds. _J_, _Stemonitis_, × 2. _K_, _Arcyria_, × 4.]
Other protophytes, while evidently enough of vegetable nature, are
nevertheless very different in some respects from the higher plants.
The protophytes may be divided into three classes: I. The slime moulds
(_Myxomycetes_); II. The Schizophytes; III. The green monads
(_Volvocineæ_).
CLASS I.--THE SLIME MOULDS.
These curious organisms are among the most puzzling forms with which
the botanist has to do, as they are so much like some of the lowest
forms of animal life as to be scarcely distinguishable from them, and
indeed they are sometimes regarded as animals rather than plants. At
certain stages they consist of naked masses of protoplasm of very
considerable size, not infrequently several centimetres in diameter.
These are met with on decaying logs in damp woods, on rotting leaves,
and other decaying vegetable matter. The commonest ones are bright
yellow or whitish, and form soft, slimy coverings over the substratum
(Fig. 5, _A_), penetrating into its crevices and showing sensitiveness
toward light. The plasmodium, as the mass of protoplasm is called, may
be made to creep upon a slide in the following way: A tumbler is
filled with water and placed in a saucer filled with sand. A strip of
blotting paper about the width of the slide is now placed with one end
in the water, the other hanging over the edge of the glass and against
one side of a slide, which is thus held upright, but must not be
allowed to touch the side of the tumbler. The strip of blotting paper
sucks up the water, which flows slowly down the surface of the slide
in contact with the blotting paper. If now a bit of the substance upon
which the plasmodium is growing is placed against the bottom of the
slide on the side where the stream of water is, the protoplasm will
creep up against the current of water and spread over the slide,
forming delicate threads in which most active streaming movements of
the central granular protoplasm may be seen under the microscope, and
the ends of the branches may be seen to push forward much as we saw in
the amœba. In order that the experiment may be successful, the whole
apparatus should be carefully protected from the light, and allowed to
stand for several hours. This power of movement, as well as the power
to take in solid food, are eminently animal characteristics, though
the former is common to many plants as well.
After a longer or shorter time the mass of protoplasm contracts and
gathers into little heaps, each of which develops into a structure
that has no resemblance to any animal, but would be at once placed
with plants. In one common form (_Trichia_) these are round or
pear-shaped bodies of a yellow color, and about as big as a pin head
(Fig. 5, _D_), occurring in groups on rotten logs in damp woods.
Others are stalked (_Arcyria_, _Stemonitis_) (Fig. 5, _J_, _K_), and
of various colors,--red, brown, etc. The outer part of the structure
is a more or less firm wall, which breaks when ripe, discharging a
powdery mass, mixed in most forms with very fine fibres.
When strongly magnified the fine dust is found to be made up of
innumerable small cells with thick walls, marked with ridges or
processes which differ much in different species. The fibres also
differ much in different genera. Sometimes they are simple,
hair-like threads; in others they are hollow tubes with spiral
thickenings, often very regularly placed, running around their
walls.
The spores may sometimes be made to germinate by placing them in a
drop of water, and allowing them to remain in a warm place for about
twenty-four hours. If the experiment has been successful, at the end
of this time the spore membrane will have burst, and the contents
escaped in the form of a naked mass of protoplasm (Zoöspore) with a
nucleus, and often showing a vacuole (Fig. 5, _v_), that
alternately becomes much distended, and then disappears entirely. On
first escaping it is usually provided with a long, whip-like
filament of protoplasm, which is in active movement, and by means of
which the cell swims actively through the water (Fig. 5, _I_ i).
Sometimes such a cell will be seen to divide into two, the process
taking but a short time, so that the numbers of these cells under
favorable conditions may become very large. After a time the lash is
withdrawn, and the cell assumes much the form of a small amœba (_I_
ii).
The succeeding stages are difficult to follow. After repeatedly
dividing, a large number of these amœba-like cells run together,
coalescing when they come in contact, and forming a mass of protoplasm
that grows, and finally assumes the form from which it started.
Of the common forms of slime moulds the species of _Trichia_ (Figs.
_D_, _I_) and _Physarum_ are, perhaps, the best for studying the
germination, as the spores are larger than in most other forms, and
germinate more readily. The experiment is apt to be most successful
if the spores are sown in a drop of water in which has been infused
some vegetable matter, such as a bit of rotten wood, boiling
thoroughly to kill all germs. A drop of this fluid should be placed
on a perfectly clean cover glass, which it is well to pass once or
twice through a flame, and the spores transferred to this drop with
a needle previously heated. By these precautions foreign germs will
be avoided, which otherwise may interfere seriously with the growth
of the young slime moulds. After sowing the spores in the drop of
culture fluid, the whole should be inverted over a so-called "moist
chamber." This is simply a square of thick blotting paper, in which
an opening is cut small enough to be entirely covered by the cover
glass, but large enough so that the drop in the centre of the cover
glass will not touch the sides of the chamber, but will hang
suspended clear in it. The blotting paper should be soaked
thoroughly in pure water (distilled water is preferable), and then
placed on a slide, covering carefully with the cover glass with the
suspended drop of fluid containing the spores. The whole should be
kept under cover so as to prevent loss of water by evaporation. By
this method the spores may be examined conveniently without
disturbing them, and the whole may be kept as long as desired, so
long as the blotting paper is kept wet, so as to prevent the
suspended drop from drying up.
CLASS II.--_Schizophytes_.
The Schizophytes are very small plants, though not infrequently
occurring in masses of considerable size. They are among the commonest
of all plants, and are found everywhere. They multiply almost entirely
by simple transverse division, or splitting of the cells, whence their
name. There are two pretty well-marked orders,--the blue-green slimes
(_Cyanophyceæ_) and the bacteria (_Schizomycetes_). They are
distinguished, primarily, by the first (with a very few exceptions)
containing chlorophyll (leaf-green), which is entirely absent from
nearly all of the latter.
The blue-green slimes: These are, with few exceptions, green plants of
simple structure, but possessing, in addition to the ordinary green
pigment (chlorophyll, or leaf-green), another coloring matter, soluble
in water, and usually blue in color, though sometimes yellowish or
red.
[Illustration: FIG. 6.--Blue-green slime (_Oscillaria_). _A_, mass of
filaments of the natural size. _B_, single filament, × 300. _C_, a
piece of a filament that has become separated. _s_, sheath, × 300.]
As a representative of the group, we will select one of the commonest
forms (_Oscillaria_), known sometimes as green slime, from forming a
dark blue-green or blackish slimy coat over the mud at the bottom of
stagnant or sluggish water, in watering troughs, on damp rocks, or
even on moist earth. A search in the places mentioned can hardly fail
to secure plenty of specimens for study. If a bit of the slimy mass is
transferred to a china dish, or placed with considerable water on a
piece of stiff paper, after a short time the edge of the mass will
show numerous extremely fine filaments of a dark blue-green color,
radiating in all directions from the mass (Fig. 6, _a_). The filaments
are the individual plants, and possess considerable power of motion,
as is shown by letting the mass remain undisturbed for a day or two,
at the end of which time they will have formed a thin film over the
surface of the vessel in which they are kept; and the radiating
arrangement of the filaments can then be plainly seen.
If the mass is allowed to dry on the paper, it often leaves a bright
blue stain, due to the blue pigment in the cells of the filament. This
blue color can also be extracted by pulverizing a quantity of the
dried plants, and pouring water over them, the water soon becoming
tinged with a decided blue. If now the water containing the blue
pigment is filtered, and the residue treated with alcohol, the latter
will extract the chlorophyll, becoming colored of a yellow-green.
The microscope shows that the filaments of which the mass is
composed (Fig. 6, _B_) are single rows of short cylindrical cells of
uniform diameter, except at the end of the filament, where they
usually become somewhat smaller, so that the tip is more or less
distinctly pointed. The protoplasm of the cells has a few small
granules scattered through it, and is colored uniformly of a pale
blue-green. No nucleus can be seen.
If the filament is broken, there may generally be detected a
delicate, colorless sheath that surrounds it, and extends beyond the
end cells (Fig. 6, _c_). The filament increases in length by the
individual cells undergoing division, this always taking place at
right angles to the axis of the filament. New filaments are produced
simply by the older ones breaking into a number of pieces, each of
which rapidly grows to full size.
The name "oscillaria" arises from the peculiar oscillating or swinging
movements that the plant exhibits. The most marked movement is a
swaying from side to side, combined with a rotary motion of the free
ends of the filaments, which are often twisted together like the
strands of a rope. If the filaments are entirely free, they may often
be observed to move forward with a slow, creeping movement. Just how
these movements are caused is still a matter of controversy.
The lowest of the _Cyanophyceæ_ are strictly single-celled, separating
as soon as formed, but cohering usually in masses or colonies by means
of a thick mucilaginous substance that surrounds them (Fig. 7, _D_).
The higher ones are filaments, in which there may be considerable
differentiation. These often occur in masses of considerable size,
forming jelly-like lumps, which may be soft or quite firm (Fig. 7,
_A_, _B_). They are sometimes found on damp ground, but more commonly
attached to plants, stones, etc., in water. The masses vary in color
from light brown to deep blackish green, and in size from that of a
pin head to several centimetres in diameter.
[Illustration: FIG. 7.--Forms of _Cyanophyceæ_. _A_, _Nostoc_. _B_,
_Glœotrichia_, × 1. _C_, individual of _Glœotrichia_. _D_,
Chroöcoccus. _E_, _Nostoc_. _F_, Oscillaria. _G_, _H_, _Tolypothrix_.
All × 300. _y_, heterocyst. _sp._ spore.]
In the higher forms special cells called heterocysts are found. They
are colorless, or light yellowish, regularly disposed; but their
function is not known. Besides these, certain cells become
thick-walled, and form resting cells (spores) for the propagation of
the plant (Fig. 7, C. _sp._). In species where the sheath of the
filament is well marked (Fig. 7, _H_), groups of cells slip out of the
sheath, and develop a new one, thus giving rise to a new plant.
The bacteria (_Schizomycetes_), although among the commonest of
organisms, owing to their excessive minuteness, are difficult to
study, especially for the beginner. They resemble, in their general
structure and methods of reproduction, the blue-green slimes, but are,
with very few exceptions, destitute of chlorophyll, although often
possessing bright pigments,--blue, violet, red, etc. It is one of
these that sometimes forms blood-red spots in flour paste or bits of
bread that have been kept very moist and warm. They are universally
present where decomposition is going on, and are themselves the
principal agents of decay, which is the result of their feeding upon
the substance, as, like all plants without chlorophyll, they require
organic matter for food. Most of the species are very tenacious of
life, and may be completely dried up for a long time without dying,
and on being placed in water will quickly revive. Being so extremely
small, they are readily carried about in the air in their dried-up
condition, and thus fall upon exposed bodies, setting up decomposition
if the conditions are favorable.
A simple experiment to show this may be performed by taking two test
tubes and partly filling them with an infusion of almost any organic
substance (dried leaves or hay, or a bit of meat will answer). The
fluid should now be boiled so as to kill any germs that may be in it;
and while hot, one of the vessels should be securely stopped up with a
plug of cotton wool, and the other left open. The cotton prevents
access of all solid particles, but allows the air to enter. If proper
care has been taken, the infusion in the closed vessel will remain
unchanged indefinitely; but the other will soon become turbid, and a
disagreeable odor will be given off. Microscopic examination shows the
first to be free from germs of any kind, while the second is swarming
with various forms of bacteria.
[Illustration: FIG. 8.--Bacteria.]
These little organisms have of late years attracted the attention of
very many scientists, from the fact that to them is due many, if not
all, contagious diseases. The germs of many such diseases have been
isolated, and experiments prove beyond doubt that these are alone the
causes of the diseases in question.
If a drop of water containing bacteria is examined, we find them to
be excessively small, many of them barely visible with the strongest
lenses. The larger ones (Fig. 8) recall quite strongly the smaller
species of oscillaria, and exhibit similar movements. Others are so
small as to appear as mere lines and dots, even with the strongest
lenses. Among the common forms are small, nearly globular cells;
oblong, rod-shaped or thread-shaped filaments, either straight or
curved, or even spirally twisted. Frequently they show a quick
movement which is probably in all cases due to cilia, which are,
however, too small to be seen in most cases.
[Illustration: FIG. 9.--_Euglena_. _A_, individual in the active
condition. _E_, the red "eye-spot." _c_, flagellum. _n_, nucleus. _B_,
resting stage. _C_, individual dividing, × 300.]
Reproduction is for the most part by simple transverse division, as in
oscillaria; but occasionally spores are produced also.
CLASS III.--GREEN MONADS (_Volvocineæ_).
This group of the protophytes is unquestionably closely related to
certain low animals (_Monads_ or _Flagellata_), with which they are
sometimes united. They are characterized by being actively motile, and
are either strictly unicellular, or the cells are united by a
gelatinous envelope into a colony of definite form.
Of the first group, _Euglena_ (Fig. 9), may be selected as a type.
This organism is found frequently among other algæ, and occasionally
forms a green film on stagnant water. It is sometimes regarded as a
plant, sometimes as an animal, and is an elongated, somewhat
worm-like cell without a definite cell wall, so that it can change
its form to some extent. The protoplasm contains oval masses, which
are bright green in color; but the forward pointed end of the cell
is colorless, and has a little depression. At this end there is a
long vibratile protoplasmic filament (_c_), by means of which the
cell moves. There is also to be seen near this end a red speck (_e_)
which is probably sensitive to light. A nucleus can usually be seen
if the cell is first killed with an iodine solution, which often
will render the flagellum (_c_) more evident, this being invisible
while the cell is in motion. The cells multiply by division.
Previous to this the flagellum is withdrawn, and a firm cell wall is
formed about the cell (Fig. 9, _B_). The contents then divide into
two or more parts, which afterwards escape as new individuals.
Of the forms that are united in colonies[2] one of the best known is
_Volvox_ (Fig. 10). This plant is sometimes found in quiet water,
where it floats on or near the surface as a dark green ball, just
large enough to be seen with the naked eye. They may be kept for some
time in aquaria, and will sometimes multiply rapidly, but are very
susceptible to extremes of temperature, especially of heat.
[2] The term "colony" is, perhaps, inappropriate, as the whole mass of
cells arises from a single one, and may properly be looked upon as an
individual plant.
[Illustration: FIG. 10.--_Volvox._ _A_, mature colony, containing
several smaller ones (_x_), × 50. _B_, Two cells showing the cilia,
× 300.]
The colony (Fig. 10, _A_) is a hollow sphere, the numerous green
cells of which it is composed forming a single layer on the outside.
By killing with iodine, and using a strong lens, each cell is seen
to be somewhat pear-shaped (Fig. _B_), with the pointed end out.
Attached to this end are two vibratile filaments (cilia or
_flagella_), and the united movements of these cause the rolling
motion of the whole colony. Usually a number of young colonies
(Fig. _x_) are found within the mother colony. These arise by the
repeated bipartition of a single cell, and escape finally, forming
independent colonies.
Another (sexual) form of reproduction occurs, similar to that found
in many higher plants; but as it only occurs at certain seasons, it
is not likely to be met with by the student.
Other forms related to _Volvox_, and sometimes met with, are
_Gonium_, in which there are sixteen cells, forming a flat square;
_Pandorina_ and _Eudorina_, with sixteen cells, forming an oval or
globular colony like _Volvox_, but much smaller. In all of these the
structure of the cells is essentially as in _Volvox_.