Elements of Structural and Systematic Botany

D >> Douglas Houghton Campbell >> Elements of Structural and Systematic Botany

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[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, x 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 algae, 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.--_Confervaceae_.

Under this head are included a number of forms of which the simplest
ones approach closely, especially in their younger stages, the
_Protococcaceae_. Indeed, some of the so-called _Protococcaceae_ 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,
x 50. _B_, a single cell treated with chromic acid, and stained with
alum cochineal. _n_, nucleus. _py._ pyrenoid, x 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., x 150. _D_, a zooespore x 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 deg.-25 deg. 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 (zooespores) (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 zooespores, from their animal-like movements. They are
provided with two cilia, closely resembling the motile cells of the
_Protococcaceae_ and _Volvocineae_.

There are very many examples of these simple _Confervaceae_, some like
_Conferva_ being simple rows of cells, others like _Stigeoclonium_
(Fig. 14, _A_), _Chaetophora_ 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.--_Confervaceae_. _A_, _Stigeoclonium_. _B_,
_Draparnaldia_, x 50. _C_, a piece of _Draparnaldia_, x 2. _D_, part
of a filament of _Conferva_, x 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
_OEdogonium_ and its relatives. There are numerous species of
_OEdogonium_ not uncommon in stagnant water growing in company with
other algae, 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
zooespores (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 zooespore 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 zooespores are formed, smaller than the ordinary
ones, that attach themselves to the filaments bearing the female
reproductive organ (ooegonium), and grow into small plants bearing
the male organ (antheridium), (Fig. 16, _B_).

[Illustration: FIG. 16.--_A_, portion of a filament of _OEdogonium_,
with two ooegonia (_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 zooespore of _OEdogonium_. _D_, a similar
spore germinating. _E_, base of a filament showing the disc (_d_) by
which it is attached. _F_, another species of _OEdogonium_ with a ripe
spore (_sp._). _G_, part of a plant of _Bulbochaete_. _C_, _D_, x 300;
the others x 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 "ooegonium." When the
egg cell is ripe, the ooegonium 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 zooespores 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 ooegonium. By careful
observation the student may possibly be able to follow the
spermatozoid into the ooegonium, 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 _Coleochaete_, x 50. _B_, a few
cells from the margin, with one of the hairs.]

Much like _OEdogonium_, but differing in being branched, is the genus
_Bulbochaete_, characterized also by hairs swollen at the base, and
prolonged into a delicate filament (Fig. 16, _G_).

The highest members of the _Confervaceae_ are those of the genus
_Coleochaete_ (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 algae 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
_OEdogonium_, but the reproductive organs are more specialized.

CHAPTER V.

GREEN ALGAE--_Continued_.

ORDER III.--POND SCUMS (_Conjugatae_).

The _Conjugatae_, while in some respects approaching the _Confervaceae_
in structure, yet differ from them to such an extent in some respects
that their close relationship is doubtful. They are very common and
familiar plants, some of them forming great floating masses upon the
surface of every stagnant pond and ditch, being commonly known as
"pond scum." The commonest of these pond scums belong to the genus
_Spirogyra_, and one of these will illustrate the characteristics of
the order. When in active growth these masses are of a vivid green,
and owing to the presence of a gelatinous coating feel slimy, slipping
through the hands when one attempts to lift them from the water.
Spread out in water, the masses are seen to be composed of slender
threads, often many centimetres in length, and showing no sign of
branching.

[Illustration: FIG. 18.--_A_, a filament of a common pond scum
(_Spirogyra_) separating into two parts. _B_, a cell undergoing
division. The cell is seen in optical section, and the chlorophyll
bands are omitted, _n_, _n'_, the two nuclei. _C_, a complete cell.
_n_, nucleus. _py._ pyrenoid. _D_, _E_, successive stages in the
process of conjugation. _G_, a ripe spore. _H_, a form in which
conjugation takes place between the cells of the same filament. All
x 150.]

For microscopical examination the larger species are preferable.
When one of these is magnified (Fig. 18, _A_, _C_), the unbranched
filament is shown to be made up of perfectly cylindrical cells, with
rather delicate walls. The protoplasm is confined to a thin layer
lining the walls, except for numerous fine filaments that radiate
from the centrally placed nucleus (_n_), which thus appears
suspended in the middle of the cell. The nucleus is large and
distinct in the larger species, and has a noticeably large and
conspicuous nucleolus. The most noticeable thing about the cell is
the green spiral bands running around it. These are the
chloroplasts, which in all the _Conjugatae_ are of very peculiar
forms. The number of these bands varies much in different species of
_Spirogyra_, but is commonly two or three. These chloroplasts, like
those of other plants, are not noticeably different in structure
from the ordinary protoplasm, as is shown by extracting the
chlorophyll, which may be done by placing the plants in alcohol for
a short time. This extracts the chlorophyll, but a microscopic
examination of the decolored cells shows that the bands remain
unchanged, except for the absence of color. These bands are
flattened, with irregularly scalloped margins, and at intervals have
rounded bodies (pyrenoids) imbedded in them (Fig. 18, _C_, _py._).
The pyrenoids, especially when the plant has been exposed to the
light for some time, are surrounded by a circle of small granules,
which become bluish when iodine is applied, showing them to be
starch. (To show the effect of iodine on starch on a large scale,
mix a little flour, which is nearly all starch, with water, and add
a little iodine. The starch will immediately become colored blue,
varying in intensity with the amount of iodine.) The cells divide
much as in _Cladophora_, but the nucleus here takes part in the
process. The division naturally occurs only at night, but by
reducing the temperature at night to near the freezing point (4 deg. C.,
or a little lower), the process may be checked. The experiment is
most conveniently made when the temperature out of doors approaches
the freezing point. Then it is only necessary to keep the plants in
a warm room until about 10 P.M., when they may be put out of doors
for the night. On bringing them in in the morning, the division will
begin almost at once, and may be easily studied. The nucleus divides
into two parts, which remain for a time connected by delicate
threads (Fig. 18, _B_), that finally disappear. At first no nucleoli
are present in the daughter nuclei, but they appear before the
division is complete.

New filaments are formed by the breaking up of the old ones, this
sometimes being very rapid. As the cells break apart, the free ends
bulge strongly, showing the pressure exerted upon the cell wall by
the contents (Fig. 18, _A_).

Spores like those of _OEdogonium_ are formed, but the process is
somewhat different. It occurs in most species late in the spring, but
may sometimes be met with at other times. The masses of fruiting
plants usually appear brownish colored. If spores have been formed
they can, in the larger species at least, be seen with a hand lens,
appearing as rows of dark-colored specks.

Two filaments lying side by side send out protuberances of the cell
wall that grow toward each other until they touch (Fig. 18, _D_). At
the point of contact, the wall is absorbed, forming a continuous
channel from one cell to the other. This process usually takes place
in all the cells of the two filaments, so that the two filaments,
connected by tubes at regular intervals, have the form of a ladder.

In some species adjoining cells of the same filament become
connected, the tubes being formed at the end of the cells (Fig. 18,
_H_), and the cell in which the spore is formed enlarges.

Soon after the channel is completed, the contents of one cell flow
slowly through it into the neighboring cell, and the protoplasm of
the two fuses into one mass. (The union of the nuclei has also been
observed.) The young spore thus formed contracts somewhat, becoming
oval in form, and soon secretes a thick wall, colorless at first,
but afterwards becoming brown and more or less opaque. The
chlorophyll bands, although much crowded, are at first
distinguishable, but later lose the chlorophyll, and become
unrecognizable. Like the resting spores of _OEdogonium_ these require
a long period of rest before germinating.

[Illustration: FIG. 19.--Forms of _Zygnemaceae_. _A_, _Zygnema_. _B_,
_C_, _D_, _Mesocarpus_. All x 150.]

There are various genera of the pond scums, differing in the form of
the chloroplasts and also in the position of the spores. Of these may
be mentioned _Zygnema_ (Fig. 19, _A_), with two star-shaped
chloroplasts in each cell, and _Mesocarpus_ (Fig. 19, _B_, _D_), in
which the single chloroplast has the form of a thin median plate. (B
shows the appearance from in front, _C_ from the side, showing the
thickness of the plate.) _Mesocarpus_ and the allied genera have the
spore formed between the filaments, the contents of both the uniting
cells leaving them.

[Illustration: FIG. 20.--Forms of Desmids. _A_, _B_, _Closterium_.
_C_, _D_, _D'_, _Cosmarium_. _D_, and _D'_ show the process of
division. _E_, _F_, _Staurastrum_; _E_ seen from the side, _F_ from
the end.]

Evidently related to the pond scums, but differing in being for the
most part strictly unicellular, are the desmids (Fig. 20). They are
confined to fresh water, and seldom occur in masses of sufficient size
to be seen with the naked eye, usually being found associated with
pond scums or other filamentous forms. Many of the most beautiful
forms may be obtained by examining the matter adhering to the leaves
and stems of many floating water plants, especially the bladder weed
(_Utricularia_) and other fine-leaved aquatics.

The desmids include the most beautiful examples of unicellular
plants to be met with, the cells having extremely elegant outlines.
The cell shows a division into two parts, and is often constricted
in the middle, each division having a single large chloroplast of
peculiar form. The central part of the cell in which the nucleus
lies is colorless.

Among the commonest forms, often growing with _Spirogyra_, are
various species of _Closterium_ (Fig. 20, _A_, _B_), recognizable at
once by their crescent shape. The cell appears bright green, except
at the ends and in the middle. The large chloroplast in each half is
composed of six longitudinal plates, united at the axis of the cell.
Several large pyrenoids are always found, often forming a regular
line through the central axis. At each end of the cell is a vacuole
containing small granules that show an active dancing movement.

The desmids often have the power of movement, swimming or creeping
slowly over the slide as we examine them, but the mechanism of these
movements is still doubtful.

In their reproduction they closely resemble the pond scums.

ORDER IV.--_Siphoneae_.

The _Siphoneae_ are algae occurring both in fresh and salt water, and
are distinguished from other algae by having the form of a tube,
undivided by partition walls, except when reproduction occurs. The
only common representatives of the order in fresh water are those
belonging to the genus _Vaucheria_, but these are to be had almost
everywhere. They usually occur in shallow ditches and ponds, growing
on the bottom, or not infrequently becoming free, and floating where
the water is deeper. They form large, dark green, felted masses, and
are sometimes known as "green felts." Some species grow also on the
wet ground about springs. An examination of one of the masses shows it
to be made up of closely matted, hair-like threads, each of which is
an individual plant.

In transferring the plants to the slide for microscopic examination,
they must be handled very carefully, as they are very easily
injured. Each thread is a long tube, branching sometimes, but not
divided into cells as in _Spirogyra_ or _Cladophora_. If we follow
it to the tip, the contents here will be found to be denser, this
being the growing point. By careful focusing it is easy to show that
the protoplasm is confined to a thin layer lining the wall, the
central cavity of the tube being filled with cell sap. In the
protoplasm are numerous elongated chloroplasts (_cl._). and a larger
or smaller number of small, shining, globular bodies (_ol._). These
latter are drops of oil, and, when the filaments are injured,
sometimes run together, and form drops of large size. No nucleus can
be seen in the living plant, but by treatment with chromic acid and
staining, numerous very small nuclei may be demonstrated.

[Illustration: FIG. 21.--_A_, _C_, successive stages in the
development of the sexual organs of a green felt (_Vaucheria_). _an._
antheridium. _og._ ooegonium. _D_, a ripe ooegonium. _E_, the same after
it has opened. _o_, the egg cell. _F_, a ripe spore. _G_, a species in
which the sexual organs are borne separately on the main filament.
_A_, _F_, x 150. _G_, x 50. _cl._ chloroplasts. _ol._ oil.]

When the filaments are growing upon the ground, or at the bottom of
shallow water, the lower end is colorless, and forms a more or less
branching root-like structure, fastening it to the earth. These
rootlets, like the rest of the filament, are undivided by walls.

One of the commonest and at the same time most characteristic
species is _Vaucheria racemosa_ (Fig. 21, _A_, _F_). The plant
multiplies non-sexually by branches pinched off by a constriction at
the point where they join the main filament, or by the filament
itself becoming constricted and separating into several parts, each
one constituting a new individual.

The sexual organs are formed on special branches, and their
arrangement is such as to make the species instantly recognizable.

The first sign of their development is the formation of a short
branch (Fig. 21, _A_) growing out at right angles to the main
filament. This branch becomes club-shaped, and the end somewhat
pointed and more slender, and curves over. This slender, curved
portion is almost colorless, and is soon shut off from the rest of
the branch. It is called an "antheridium," and within are produced,
by internal division, numerous excessively small spermatozoids.

As the branch grows, its contents become very dense, the oil drops
especially increasing in number and size. About the time that the
antheridium becomes shut off, a circle of buds appears about its
base (Fig. 21, _B_, _og._). These are the young ooegonia, which
rapidly increase in size, assuming an oval form, and become
separated by walls from the main branch (_C_). Unlike the
antheridium, the ooegonia contain a great deal of chlorophyll,
appearing deep green.

When ripe, the antheridium opens at the end and discharges the
spermatozoids, which are, however, so very small as scarcely to be
visible except with the strongest lenses. They are little oval
bodies with two cilia, which may sometimes be rendered visible by
staining with iodine.

[Illustration: FIG. 22.--_A_, non-sexual reproduction in _Vaucheria
sessilis_. _B_, non-sexual spore of _V. geminata_, x 50.]

The ooegonia, which at first are uniformly colored, just before
maturity show a colorless space at the top, from which the
chloroplasts and oil drops have disappeared (_D_), and at the same
time this portion pushes out in the form of a short beak. Soon after
the wall is absorbed at this point, and a portion of the contents is
forced out, leaving an opening, and at the same time the remaining
contents contract to form a round mass, the germ or egg cell
(Fig. 21, _E_, _o_). Almost as soon as the ooegonium opens, the
spermatozoids collect about it and enter; but, on account of their
minuteness, it is almost impossible to follow them into the egg
cell, or to determine whether several or only one enter. The
fertilized egg cell becomes almost at once surrounded by a wall,
which rapidly thickens, and forms a resting spore. As the spore
ripens, it loses its green color, becoming colorless, with a few
reddish brown specks scattered through it (_F_).

In some species the sexual organs are borne directly on the filament
(Fig. 21, _G_).

Large zooespores are formed in some of the green felts (Fig. 22,
_A_), and are produced singly in the ends of branches that become
swollen, dark green, and filled with very dense protoplasm. This end
becomes separated by a wall from the rest of the branch, the end
opens, and the contents escape as a very large zooespore, covered
with numerous short cilia (_A_ ii). After a short period of
activity, this loses its cilia, develops a wall, and begins to grow
(III, IV). Other species (_B_) produce similar spores, which,
however, are not motile, and remain within the mother cell until
they are set free by the decay of its wall.

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