SUB-KINGDOM II.
ALGÆ.[3]
[3] Algæ (sing. _alga_).
In the second sub-kingdom of plants is embraced an enormous assemblage
of plants, differing widely in size and complexity, and yet showing a
sufficiently complete gradation from the lowest to the highest as to
make it impracticable to make more than one sub-kingdom to include
them. They are nearly all aquatic forms, although many of them will
survive long periods of drying, such forms occurring on moist earth,
rocks, or the trunks of trees, but only growing when there is a
plentiful supply of water.
All of them possess chlorophyll, which, however, in many forms, is
hidden by the presence of a brown or red pigment. They are ordinarily
divided into three classes--I. The Green Algæ (_Chlorophyceæ_);
II. Brown Algæ (_Phæophyceæ_); III. Red Algæ (_Rhodophyceæ_).
CLASS I.--GREEN ALGÆ.
The green algæ are to be found almost everywhere where there is
moisture, but are especially abundant in sluggish or stagnant fresh
water, being much less common in salt water. They are for the most
part plants of simple structure, many being unicellular, and very few
of them plants of large size.
We may recognize five well-marked orders of the green algæ--I. Green
slimes (_Protococcaceæ_); II. _Confervaceæ_; III. Pond scums
(_Conjugatæ_); IV. _Siphoneæ_; V. Stone-worts (_Characeæ_).
ORDER I.--_Protococcaceæ_.
The members of this order are minute unicellular plants, growing
either in water or on the damp surfaces of stones, tree trunks, etc.
The plants sometimes grow isolated, but usually the cells are united
more or less regularly into colonies.
A common representative of the order is the common green slime,
_Protococcus_ (Fig. 11, _A_, _C_), which forms a dark green slimy
coating over stones, tree trunks, flower pots, etc. Owing to their
minute size the structure can only be made out with the microscope.
[Illustration: FIG. 11.--_Protococcaceæ._ _A_, _C_, Protococcus. _A_,
single cells. _B_, cells dividing by fission. _C_, successive steps in
the process of internal cell division. In _C_ iv, the young cells have
mostly become free. _D_, a full-grown colony of _Pediastrum_. _E_, a
young colony still surrounded by the membrane of the mother cell. _F_,
_Scenedesmus_. All, × 300. _G_, small portion of a young colony of the
water net (_Hydrodictyon_), × 150.]
Scraping off a little of the material mentioned into a drop of water
upon a slide, and carefully separating it with needles, a cover
glass may be placed over the preparation, and it is ready for
examination. When magnified, the green film is found to be composed
of minute globular cells of varying size, which may in places be
found to be united into groups. With a higher power, each cell
(Fig. 11, _A_) is seen to have a distinct cell wall, within which is
colorless protoplasm. Careful examination shows that the chlorophyll
is confined to several roundish bodies that are not usually in
immediate contact with the wall of the cell. These green masses are
called chlorophyll bodies (chloroplasts). Toward the centre of the
cell, especially if it has first been treated with iodine, the
nucleus may be found. The size of the cells, as well as the number
of chloroplasts, varies a good deal.
With a little hunting, specimens in various stages of division may
be found. The division takes place in two ways. In the first
(Fig. 11, _B_), known as fission, a wall is formed across the cell,
dividing it into two cells, which may separate immediately or may
remain united until they have undergone further division. In this
case the original cell wall remains as part of the wall of the
daughter cells. Fission is the commonest form of cell multiplication
throughout the vegetable kingdom.
The second form of cell division or internal cell division is shown
at _C_. Here the protoplasm and nucleus repeatedly divide until a
number of small cells are formed within the old one. These develop
cell walls, and escape by the breaking of the old cell wall, which
is left behind, and takes no part in the process. The cells thus
formed are sometimes provided with two cilia, and are capable of
active movement.
Internal cell division, as we shall see, is found in most plants,
but only at special times.
Closely resembling _Protococcus_, and answering quite as well for
study, are numerous aquatic forms, such as _Chlorococcum_ (Fig. 12).
These are for the most part destitute of a firm cell wall, but are
imbedded in masses of gelatinous substance like many _Cyanophyceæ_.
The chloroplasts are smaller and less distinct than in
_Protococcus_. The cells are here oval rather than round, and often
show a clear space at one end.
[Illustration: FIG. 12.--_Chlorococcum_, a plant related to
_Protococcus_, but the naked cells are surrounded by a colorless
gelatinous envelope. _A_, motionless cells. _B_, a cell that has
escaped from its envelope and is ciliated, × 300.]
Owing to the absence of a definite membrane, a distinction between
fission and internal cell division can scarcely be made here. Often
the cells escape from the gelatinous envelope, and swim actively by
means of two cilia at the colorless end (Fig. 12, _B_). In this
stage they closely resemble the individuals of a _Volvox_ colony, or
other green _Flagellata_, to which there is little doubt that they
are related.
There are a number of curious forms common in fresh water that are
probably related to _Protococcus_, but differ in having the cells
united in colonies of definite form. Among the most striking are
the different species of _Pediastrum_ (Fig. 11, _D_, _E_), often met
with in company with other algæ, and growing readily in aquaria when
once established. They are of very elegant shapes, and the number of
cells some multiple of four, usually sixteen.
The cells form a flat disc, the outer ones being generally provided
with a pair of spines.
New individuals arise by internal division of the cells, the
contents of each forming as many parts as there are cells in the
whole colony. The young cells now escape through a cleft in the wall
of the mother cell, but are still surrounded by a delicate membrane
(Fig. 11, _E_). Within this membrane the young cells arrange
themselves in the form of the original colony, and grow together,
forming a new colony.
A much larger but rarer form is the water net (Fig. 11, _G_), in
which the colony has the form of a hollow net, the spaces being
surrounded by long cylindrical cells placed end to end. Other common
forms belong to the genus _Scenedesmus_ (Fig. 11, _F_), of which
there are many species.
ORDER II.--_Confervaceæ_.
Under this head are included a number of forms of which the simplest
ones approach closely, especially in their younger stages, the
_Protococcaceæ_. Indeed, some of the so-called _Protococcaceæ_ are
known to be only the early stages of these plants.
A common member of this order is _Cladophora_, a coarse-branching
alga, growing commonly in running water, where it forms tufts,
sometimes a metre or more in length. By floating out a little of it in
a saucer, it is easy to see that it is made up of branching filaments.
The microscope shows (Fig. 13, _A_) that these filaments are rows of
cylindrical cells with thick walls showing evident stratification.
At intervals branches are given off, which may in turn branch,
giving rise to a complicated branching system. These branches begin
as little protuberances of the cell wall at the top of the cell.
They increase rapidly in length, and becoming slightly contracted at
the base, a wall is formed across at this point, shutting it off
from the mother cell.
The protoplasm lines the wall of the cell, and extends in the form
of thin plates across the cavity of the cell, dividing it up into a
number of irregular chambers. Imbedded in the protoplasm are
numerous flattened chloroplasts, which are so close together as to
make the protoplasm appear almost uniformly green. Within the
chloroplasts are globular, glistening bodies, called "pyrenoids."
The cell has several nuclei, but they are scarcely evident in the
living cell. By placing the cells for a few hours in a one per cent
watery solution of chromic acid, then washing thoroughly and
staining with borax carmine, the nuclei will be made very evident
(Fig. 13, _B_). Such preparations may be kept permanently in dilute
glycerine.
[Illustration: FIG. 13.--_Cladophora._ _A_, a fragment of a plant,
× 50. _B_, a single cell treated with chromic acid, and stained with
alum cochineal. _n_, nucleus. _py._ pyrenoid, × 150. _C_, three stages
in the division of a cell. i, 1.45 p.m.; ii, 2.55 p.m.; iii,
4.15 p.m., × 150. _D_, a zoöspore × 350.]
If a mass of actively growing filaments is examined, some of the
cells will probably be found in process of fission. The process is
very simple, and may be easily followed (Fig. 13, _C_). A ridge of
cellulose is formed around the cell wall, projecting inward, and
pushing in the protoplasm as it grows. The process is continued
until the ring closes in the middle, cutting the protoplasmic body
completely in two, and forms a firm membrane across the middle of
the cell. The protoplasm at this stage (_C_ iii.) is somewhat
contracted, but soon becomes closely applied to the new wall. The
whole process lasts, at ordinary temperatures (20°-25° C.), from
three to four hours.
At certain times, but unfortunately not often to be met with, the
contents of some of the cells form, by internal division, a large
number of small, naked cells (zoöspores) (Fig. 13, _D_), which
escape and swim about actively for a time, and afterwards become
invested with a cell wall, and grow into a new filament. These cells
are called zoöspores, from their animal-like movements. They are
provided with two cilia, closely resembling the motile cells of the
_Protococcaceæ_ and _Volvocineæ_.
There are very many examples of these simple _Confervaceæ_, some like
_Conferva_ being simple rows of cells, others like _Stigeoclonium_
(Fig. 14, _A_), _Chætophora_ and _Draparnaldia_ (Fig. 14, _B_, _C_),
very much branched. The two latter forms are surrounded by masses of
transparent jelly, which sometimes reach a length of several
centimetres.
[Illustration: FIG. 14.--_Confervaceæ_. _A_, _Stigeoclonium_. _B_,
_Draparnaldia_, × 50. _C_, a piece of _Draparnaldia_, × 2. _D_, part
of a filament of _Conferva_, × 300.]
Among the marine forms related to these may be mentioned the sea
lettuce (_Ulva_), shown in Figure 15. The thin, bright-green,
leaf-like fronds of this plant are familiar to every seaside student.
[Illustration: FIG. 15.--A plant of sea lettuce (_Ulva_). One-half
natural size.]
Somewhat higher than _Cladophora_ and its allies, especially in the
differentiation of the reproductive parts, are the various species of
_Œdogonium_ and its relatives. There are numerous species of
_Œdogonium_ not uncommon in stagnant water growing in company with
other algæ, but seldom forming masses by themselves of sufficient size
to be recognizable to the naked eye.
The plant is in structure much like _Cladophora_, except that it is
unbranched, and the cells have but a single nucleus (Fig. 16, _E_).
Even when not fruiting the filaments may usually be recognized by
peculiar cap-shaped structures at the top of some of the cells.
These arise as the result of certain peculiarities in the process of
cell division, which are too complicated to be explained here.
There are two forms of reproduction, non-sexual and sexual. In the
first the contents of certain cells escape in the form of large
zoöspores (Fig. 16, _C_), of oval form, having the smaller end
colorless and surrounded by a crown of cilia. After a short period
of active motion, the zoöspore comes to rest, secretes a cell wall
about itself, and the transparent end becomes flattened out into a
disc (_E_, _d_), by which it fastens itself to some object in the
water. The upper part now rapidly elongates, and dividing repeatedly
by cross walls, develops into a filament like the original one. In
many species special zoöspores are formed, smaller than the ordinary
ones, that attach themselves to the filaments bearing the female
reproductive organ (oögonium), and grow into small plants bearing
the male organ (antheridium), (Fig. 16, _B_).
[Illustration: FIG. 16.--_A_, portion of a filament of _Œdogonium_,
with two oögonia (_og._). The lower one shows the opening. _B_, a
similar filament, to which is attached a small male plant with an
antheridium (_an._). _C_, a zoöspore of _Œdogonium_. _D_, a similar
spore germinating. _E_, base of a filament showing the disc (_d_) by
which it is attached. _F_, another species of _Œdogonium_ with a ripe
spore (_sp._). _G_, part of a plant of _Bulbochæte_. _C_, _D_, × 300;
the others × 150.]
The sexual reproduction takes place as follows: Certain cells of a
filament become distinguished by their denser contents and by an
increase in size, becoming oval or nearly globular in form (Fig. 16,
_A_, _B_). When fully grown, the contents contract and form a naked
cell, which sometimes shows a clear area at one point on the
surface. This globular mass of protoplasm is the egg cell, or female
cell, and the cell containing it is called the "oögonium." When the
egg cell is ripe, the oögonium opens by means of a little pore at
one side (Fig. 16, _A_).
In other cells, either of the same filament or else of the small
male plants already mentioned, small motile cells, called
spermatozoids, are formed. These are much smaller than the egg cell,
and resemble the zoöspores in form, but are much smaller, and
without chlorophyll. When ripe they are discharged from the cells in
which they were formed, and enter the oögonium. By careful
observation the student may possibly be able to follow the
spermatozoid into the oögonium, where it enters the egg cell at the
clear spot on its surface. As a result of the entrance of the
spermatozoid (fertilization), the egg cell becomes surrounded by a
thick brown wall, and becomes a resting spore. The spore loses its
green color, and the wall becomes dark colored and differentiated
into several layers, the outer one often provided with spines
(Fig. 16, _F_). As these spores do not germinate for a long time,
the process is only known in a comparatively small number of
species, and can hardly be followed by the ordinary student.
[Illustration: FIG. 17.--_A_, plant of _Coleochæte_, × 50. _B_, a few
cells from the margin, with one of the hairs.]
Much like _Œdogonium_, but differing in being branched, is the genus
_Bulbochæte_, characterized also by hairs swollen at the base, and
prolonged into a delicate filament (Fig. 16, _G_).
The highest members of the _Confervaceæ_ are those of the genus
_Coleochæte_ (Fig. 17), of which there are several species found in
the United States. These show some striking resemblances to the red
seaweeds, and possibly form a transition from the green algæ to the
red. The commonest species form bright-green discs, adhering firmly
to the stems and floating leaves of water lilies and other aquatics.
In aquaria they sometimes attach themselves in large numbers to the
glass sides of the vessel.
Growing from the upper surface are numerous hairs, consisting of a
short, sheath-like base, including a very long and delicate filament
(Fig. 17, _B_). In their methods of reproduction they resemble
_Œdogonium_, but the reproductive organs are more specialized.