ŒDOGONIUM.
=309.= Œdogonium is also an alga. The plant is sometimes
associated with spirogyra, and occurs in similar situations. Our
attention was called to it in the study of chlorophyll bodies. These we
recollect are, in this plant, small oval disks, and thus differ from
those in spirogyra.
=310. Form of œdogonium.=—Like spirogyra, œdogonium forms simple
threads which are made up of cylindrical cells placed end to end. But
the plant is very different from any member of the group to which
spirogyra belongs. In the first place each cell is not the equivalent
of an individual plant as in spirogyra. Growth is localized or confined
to certain cells of the thread which divide at one end in such a way
as to leave a peculiar overlapping of the cell walls in the form of a
series of shallow caps or vessels (fig. 144), and this is one of the
characteristics of this genus. Other differences we find in the manner
of reproduction.
=311. Fruiting stage of œdogonium.=—Material in the fruiting
stage is quite easily obtainable, and may be preserved for study in
formalin if there is any doubt about obtaining it at the time we need
it for study. This condition of the plant is easily detected because of
the swollen condition of some of the cells, or by the presence of brown
bodies with a thick wall in some of the cells.
=312. Sexual organs of œdogonium. Oogonium and egg.=—The enlarged
cell is the oogonium, the wall of the cell being the wall of the
oogonium. (See fig. 145.) The protoplasm inside, before fertilization,
is the egg-cell. In those cases where the brown body with a thick
wall is present fertilization has taken place, and this body is the
_fertilized egg_, or _oospore_. It contains large quantities of an oily
substance, and, like the fertilized egg of spirogyra and vaucheria, is
able to withstand greater changes in temperature than the vegetative
stage, and can endure drying and freezing for some time without injury.
[Illustration: Fig. 144. Portion of thread of œdogonium, showing
chlorophyll grains, and peculiar cap cell walls.]
[Illustration: Fig. 145. Œdogonium undulatum, with oogonia and dwarf
males; the upper oogonium at the right has a mature oospore.]
In the oogonium wall there can frequently be seen a rift near the
middle of one side, or near the upper end. This is the opening through
which the spermatozoid entered to fecundate the egg.
=313. Dwarf male plants.=—In some species there will also be seen
peculiar club-shaped dwarf plants attached to the side of the oogonium,
or near it, and in many cases the end of this dwarf plant has an open
lid on the end.
=314. Antheridium.=—The end cell of the dwarf male in such
species is the _antheridium_. In other species the spermatozoids are
developed in different cells (antheridia) of the same thread which
bears the oogonium, or on a different thread.
[Illustration: Fig. 146. Zoogonidia of œdogonium escaping. At the right
one is germinating and forming the holdfasts, by means of which these
algæ attach themselves to objects for support. (After Pringsheim.)]
=315. Zoospore stage of œdogonium.=—The egg after a period of
rest starts into active life again. In doing so it does not develop
the thread-like plant directly as in the case of vaucheria and
spirogyra. It first divides into four zoospores which are exactly like
the zoogonidia in form. (See fig. 152.) These germinate and develop
the thread form again. This is a quite remarkable peculiarity of
œdogonium when compared with either vaucheria or spirogyra. It is the
introduction of an intermediate stage between the fertilized egg and
that form of the plant which bears the sexual organs, and should be
kept well in mind.
=316. Asexual reproduction.=—Material for the study of this stage
of œdogonium is not readily obtainable just when we wish it for study.
But fresh plants brought in and placed in a quantity of fresh water may
yield suitable material, and it should be examined at intervals for
several days. This kind of reproduction takes place by the formation
of _zoogonidia_. The entire contents of a cell round off into an oval
body, the wall of the cell breaks, and the zoogonidium escapes. It has
a clear space at the small end, and around this clear space is a row or
crown of cilia as shown in fig. 146. By the vibration of these cilia
the zoogonidium swims around for a time, then settles down on some
object of support, and several slender holdfasts grow out in the form
of short rhizoids which attach the young plant.
[Illustration: Fig. 147. Portion of thread of œdogonium showing
antheridia.]
[Illustration: Fig. 148. Portion of thread of œdogonium showing upper
half of egg open, and a spermatozoid ready to enter. (After Klebahn).]
=317. Sexual reproduction. Antheridia.=—The antheridia are short
cells which are formed by one of the ordinary cells dividing into a
number of disk-shaped ones as shown in fig. 147. The protoplasm in each
antheridium forms two spermatozoids (sometimes only one) which are of
the same form as the zoogonidia but smaller, and yellowish instead of
green. In some species a motile body intermediate in size and color
between the spermatozoids and zoogonidia is first formed, which after
swimming around comes to rest on the oogonium, or near it, and develops
what is called a “dwarf male plant” from which the real spermatozoid is
produced.
[Illustration: Fig. 149. Male nucleus just entering egg at left side.]
[Illustration: Fig. 150. Male nucleus fusing with female nucleus.]
[Illustration: Fig. 151. The two nuclei fused, and fertilization
complete.
Figs. 149-151.—Fertilization in œdogonium. (After Klebahn).]
=318. Oogonia.=—The oogonia are formed directly from one of the
vegetative cells. In most species this cell first enlarges in diameter,
so that it is easily detected. The protoplasm inside is the egg-cell.
The oogonium wall opens, a bit of the protoplasm is emitted, and the
spermatozoid then enters and fertilizes it (fig. 148). Now a hard brown
wall is formed around it, and, just as in spirogyra and vaucheria, it
passes through a resting period. At the time of germination it does
not produce the thread-like plant again directly, but first forms four
zoospores exactly like the zoogonidia (fig. 152). These zoospores then
germinate and form the plant.
=319. Œdogonium compared with spirogyra.=—Now if we compare
œdogonium with spirogyra, as we did in the case of vaucheria, we find
here also that there is an advance upon the simple condition which
exists in spirogyra. Growth and division of the thread is limited to
certain portions. The sexual organs are differentiated. They usually
differ in form and size from the vegetative cells, though the oogonium
is simply a changed vegetative cell. The sexual organs are
differentiated among themselves, the antheridium is small, and the
oogonium large. The gametes are also differentiated in size, and the
male gamete is motile, and carries in its body the nucleus which fuses
with the nucleus of the egg-cell.
[Illustration: Fig. 152. Fertilized egg of œdogonium after a period of
rest escaping from the wall of the oogonium, and dividing into the four
zoospores. (After Juranyi.)]
But a more striking advance is the fact that the fertilized egg does
not produce the vegetative thread of œdogonium directly, but first
forms four zoospores, each of which is then capable of developing into
the thread. On the other hand we found that in spirogyra the zygospore
develops directly into the thread form of the plant.
[Illustration: Fig. 153. Tuft of chætophora, natural size.]
[Illustration: Fig. 154. Portion of chætophora showing branching.]
=320. Position of œdogonium.=—Œdogonium is one of the true
thread-like algæ, green in color, and the threads are divided into
distinct cells. It, along with many relatives, was once placed
in the old genus conferva. These are all now placed in the group
_Confervoideæ_, that is, the _conferva-like algæ_.
=321. Relatives of œdogonium.=—Many other genera are related
to œdogonium. Some consist of simple threads, and others of branched
threads. An example of the branched forms is found in chætophora,
represented in figures 153, 154. This plant grows in quiet pools or
in slow-running water. It is attached to sticks, rocks, or to larger
aquatic plants. Many threads spring from the same point of attachment
and radiate in all directions. This, together with the branching of the
threads, makes a small, compact, greenish, rounded mass, which is held
firmly together by a gelatinous substance. The masses in this species
are about the size of a small pea, or smaller. Growth takes place in
chætophora at the ends of the threads and branches. That is, growth is
apical. This, together with the branched threads and the tendency to
form cell masses, is a great advance of the vegetative condition of the
plant upon that which we find in the simple threads of œdogonium.