nal 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 _Coleochaet