GREEN ALGÆ--_Continued_.
ORDER III.--POND SCUMS (_Conjugatæ_).
The _Conjugatæ_, while in some respects approaching the _Confervaceæ_
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_, _nʹ_, 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
× 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 _Conjugatæ_ 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° 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 _Œdogonium_ 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 _Œdogonium_ these require
a long period of rest before germinating.
[Illustration: FIG. 19.--Forms of _Zygnemaceæ_. _A_, _Zygnema_. _B_,
_C_, _D_, _Mesocarpus_. All × 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_, _Dʹ_, _Cosmarium_. _D_, and _Dʹ_ 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.--_Siphoneæ_.
The _Siphoneæ_ are algæ occurring both in fresh and salt water, and
are distinguished from other algæ 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._ oögonium. _D_, a ripe oögonium. _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_, × 150. _G_, × 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 oögonia, which
rapidly increase in size, assuming an oval form, and become
separated by walls from the main branch (_C_). Unlike the
antheridium, the oögonia 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_, × 50.]
The oögonia, 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 oögonium 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 zoöspores 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 zoöspore, 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.
ORDER V.--_Characeæ_.
The _Characeæ_, or stone-worts, as some of them are called, are so
very different from the other green algæ that it is highly probable
that they should be separated from them.
The type of the order is the genus _Chara_ (Fig. 23), called
stone-worts from the coating of carbonate of lime found in most of
them, giving them a harsh, stony texture. Several species are common
growing upon the bottom of ponds and slow streams, and range in size
from a few centimetres to a metre or more in height.
The plant (Fig. 23, _A_) consists of a central jointed axis with
circles of leaves at each joint or node. The distance between the
nodes (internodes) may in the larger species reach a length of several
centimetres. The leaves are slender, cylindrical structures, and like
the stem divided into nodes and internodes, and have at the nodes
delicate leaflets.
At each joint of the leaf, in fruiting specimens, attached to the
inner side, are borne two small, roundish bodies, in the commoner
species of a reddish color (Fig. 23, _A_, _r_). The lower of the two
is globular, and bright scarlet in color; the other, more oval and
duller.
Examined with a lens the main axis presents a striated appearance. The
whole plant is harsh to the touch and brittle, owing to the limy
coating. It is fastened to the ground by fine, colorless hairs, or
rootlets.
[Illustration: FIG. 23.--_A_, plant of a stone-wort (_Chara_),
one-half natural size. _r_, reproductive organs. _B_, longitudinal
section through the apex. _S_, apical cell. _x_, nodes. _y_,
internodes. _C_, a young leaf. _D_, cross section of an internode.
_E_, of a node of a somewhat older leaf. _F_, _G_, young sexual organs
seen in optical section. _o_, oögonium. _An._ antheridium. _H_,
superficial view. _G_, _I_, group of filaments containing
spermatozoids. _J_, a small portion of one of these more magnified,
showing a spermatozoid in each cell. _K_, free spermatozoids. _L_, a
piece of a leaf with ripe oögonium (_o_), and antheridium (_An._).
_B_, _H_, × 150. _J_, _K_, × 300. _I_, × 50. _L_, × 25.]
By making a series of longitudinal sections with a sharp razor
through the top of the plant, and magnifying sufficiently, it is
found to end in a single, nearly hemispherical cell (Fig. 23, _B_,
_S_). This from its position is called the "apical cell," and from
it are derived all the tissues of the plant. Segments are cut off
from its base, and these divide again into two by a wall parallel to
the first. Of the two cells thus formed one undergoes no further
division and forms the central cell of an internode (_y_); the other
divides repeatedly, forming a node or joint (_x_).
As the arrangement of these cells is essentially the same in the
leaves and stem, we will examine it in the former, as by cutting
several cross-sections of the whole bunch of young leaves near the
top of the plant, we shall pretty certainly get some sections
through a joint. The arrangement is shown in Figure 23, _E_.
As the stem grows, a covering is formed over the large internodal
cell (_y_) by the growth of cells from the nodes. These grow both
from above and below, meeting in the middle of the internode and
completely hiding the long axial cell. A section across the
internode shows the large axial cell (_y_) surrounded by the
regularly arranged cells of the covering or cortex (Fig. 23, _D_).
All the cells contain a layer of protoplasm next the wall with
numerous oval chloroplasts. If the cells are uninjured, they often
show a very marked movement of the protoplasm. These movements are
best seen, however, in forms like _Nitella_, where the long
internodal cells are not covered with a cortex. In _Chara_ they are
most evident in the root hairs that fasten the plant to the ground.
The growth of the leaves is almost identical with that of the stem,
but the apical growth is limited, and the apical cell becomes
finally very long and pointed (Fig. 23, _C_). In some species the
chloroplasts are reddish in the young cells, assuming their green
color as the cells approach maturity.
The plant multiplies non-sexually by means of special branches that
may become detached, but there are no non-sexual spores formed.
The sexual organs have already been noticed arising in pairs at the
joints of the leaves. The oögonium is formed above, the antheridium
below.
The young oögonium (_F_, _O_) consists of a central cell, below
which is a smaller one surrounded by a circle of five others, which
do not at first project above the central cell, but later completely
envelop it (_G_). Each of these five cells early becomes divided
into an upper and a lower one, the latter becoming twisted as it
elongates, and the central cell later has a small cell cut off from
its base by an oblique wall. The central cell forms the egg cell,
which in the ripe oögonium (_L_, _O_) is surrounded by five,
spirally twisted cells, and crowned by a circle of five smaller
ones, which become of a yellowish color when full grown. They
separate at the time of fertilization to allow the spermatozoids to
enter the oögonium.
The antheridium consists at first of a basal cell and a terminal
one. The latter, which is nearly globular, divides into eight nearly
similar cells by walls passing through the centre. In each of these
eight cells two walls are next formed parallel to the outer surface,
so that the antheridium (apart from the basal cell) contains
twenty-four cells arranged in three concentric series (_G_, _an._).
These cells, especially the outer ones, develop a great amount of a
red pigment, giving the antheridium its characteristic color.
The diameter of the antheridium now increases rapidly, and the
central cells separate, leaving a large space within. Of the inner
cells, the second series, while not increasing in diameter,
elongate, assuming an oblong form, and from the innermost are
developed long filaments (_I_, _J_) composed of a single row of
cells, in each of which is formed a spermatozoid.
The eight outer cells are nearly triangular in outline, fitting
together by deeply indented margins, and having the oblong cells
with the attached filaments upon their inner faces.
If a ripe antheridium is crushed in a drop of water, after lying a
few minutes the spermatozoids will escape through small openings in
the side of the cells. They are much larger than any we have met
with. Each is a colorless, spiral thread with about three coils, one
end being somewhat dilated with a few granules; the other more
pointed, and bearing two extremely long and delicate cilia (_K_). To
see the cilia it is necessary to kill the spermatozoids with iodine
or some other reagent.
After fertilization the outer cells of the oögonium become very
hard, and the whole falls off, germinating after a sufficient period
of rest.
According to the accounts of Pringsheim and others, the young plant
consists at first of a row of elongated cells, upon which a bud is
formed that develops into the perfect plant.
There are two families of the _Characeæ_, the _Chareæ_, of which
_Chara_ is the type, and the _Nitelleæ_, represented by various
species of _Nitella_ and _Tolypella_. The second family have the
internodes without any cortex--that is, consisting of a single long
cell; and the crown at the top of the oögonium is composed of ten
cells instead of five. They are also destitute of the limy coating of
the _Chareæ_.
Both as regards the structure of the plant itself, as well as the
reproductive organs, especially the very complex antheridium, the
_Characeæ_ are very widely separated from any other group of plants,
either above or below them.