VAUCHERIA.
=299.= The plant vaucheria we remember from our study in an
earlier chapter. It usually occurs in dense mats floating on the water
or lying on damp soil. The texture and feeling of these mats remind one
of “felt,” and the species are sometimes called the “green felts.” The
branched threads are continuous, that is there are no cross walls in
the vegetative threads. This plant multiplies itself in several ways
which would be too tedious to detail here. But when fresh bright green
mats can be obtained they should be placed in a large vessel of water
and set in a cool place. Only a small amount of the alga should be
placed in a vessel, since decay will set in more rapidly with a large
quantity. For several days one should look for small green bodies which
may be floating at the side of the vessel next the lighted window.
[Illustration: Fig. 138. Portion of branched thread of vaucheria.]
=300. Zoogonidia of vaucheria.=—If these minute floating green
bodies are found, a small drop of water containing them should be
mounted for examination. If they are rounded, with slender hair-like
appendages over the surface, which vibrate and cause motion, they very
likely are one of the kinds of reproductive bodies of vaucheria. The
hair-like appendages are _cilia_, and they occur in pairs, several of
them distributed over the surface. These rounded bodies are _gonidia_,
and because they are motile they are called _zoogonidia_.
By examining some of the threads in the vessel where they occurred we
may have perhaps an opportunity to see how they are produced. Short
branches are formed on the threads, and the contents are separated from
those of the main thread by a septum. The protoplasm and other contents
of this branch separate from the wall, round up into a mass, and escape
through an opening which is formed in the end. Here they swim around in
the water for a time, then come to rest, and germinate by growing out
into a tube which forms another vaucheria plant. It will be observed
that this kind of reproduction is not the result of the union of two
different parts of the plant. It thus differs from that which is termed
sexual reproduction. A small part of the plant simply becomes separated
from it as a special body, and then grows into a new plant, a sort of
multiplication. This kind of reproduction has been termed _asexual
reproduction_.
[Illustration: Fig. 139. Young antheridium and oogonium of Vaucheria
sessilis, before separation from contents of thread by a septum.]
=301. Sexual reproduction in vaucheria.=—The organs which
are concerned in sexual reproduction in vaucheria are very readily
obtained for study if one collects the material at the right season.
They are found quite readily during the spring and autumn, and may be
preserved in formalin for study at any season, if the material cannot
be collected fresh at the time it is desired for study. Fine material
for study often occurs on the soil of pots in greenhouses during the
winter. While the zoogonidia are more apt to be found in material which
is quite green and freshly growing, the sexual organs are usually more
abundant when the threads appear somewhat yellowish, or yellow green.
=302. Vaucheria sessilis; the sessile vaucheria.=—In this plant
the sexual organs are sessile, that is they are not borne on a stalk
as in some other species. The sexual organs usually occur several in a
group. Fig. 139 represents a portion of a fruiting plant.
=303. Sexual organs of vaucheria. Antheridium.=—The antheridia
are short, slender, curved branches from a main thread. A septum is
formed which separates an end portion from the stalk. This end cell
is the _antheridium_. Frequently it is collapsed or empty as shown in
fig. 140. The protoplasm in the antheridium forms numerous small oval
bodies each with two slender lashes, the cilia. When these are formed
the antheridium opens at the end and they escape. It is after the
escape of these spermatozoids that the antheridium is collapsed. Each
spermatozoid is a male gamete.
[Illustration: Fig. 140. Vaucheria sessilis, one antheridium between
two oogonia.]
[Illustration: Fig. 141. Vaucheria sessilis; oogonium opening and
emitting a bit of protoplasm; spermatozoids; spermatozoids entering
oogonium. (After Pringsheim and Goebel.)]
=304. Oogonium.=—The oogonia are short branches also, but
they become large and somewhat oval. The septum which separates the
protoplasm from that of the main thread is as we see near the junction
of the branch with the main thread. The oogonium, as shown in the
figure, is usually turned somewhat to one side. When mature the pointed
end opens and a bit of the protoplasm escapes. The remaining protoplasm
forms the large rounded egg-cell which fills the wall of the oogonium.
In some of the oogonia which we examine this egg is surrounded by a
thick brown wall, with starchy and oily contents. This is the
fertilized egg (sometimes called here the oospore). It is freed from
the oogonium by the disintegration of the latter, sinks into the mud,
and remains here until the following autumn or spring, when it grows
directly into a new plant.
[Illustration: Fig. 142. Fertilization in vaucheria, _mn_, male
nucleus; _fn_, female nucleus. Male nucleus entering the egg and
approaching the female nucleus. (After Oltmans.)]
=305. Fertilization.=—Fertilization is accomplished by the
spermatozoids swimming in at the open end of the oogonium, when one of
them makes its way down into the egg and fuses with the nucleus of the
egg.
[Illustration: Fig. 143. Fertilization of vaucheria. _fn_, female
nucleus; _mn_, male nucleus. The different figures show various stages
in the fusion of the nuclei.]
=306. The twin vaucheria (V. geminata).=—Another species of
vaucheria is the twin vaucheria. This is also a common one, and may be
used for study instead of the sessile vaucheria if the latter cannot
be obtained. The sexual organs are borne at the end of a club-shaped
branch. There are usually two oogonia, and one antheridium between them
which terminates the branch. In a closely related species, instead of
the two oogonia there is a whorl of them with the antheridium in the
center.
=307. Vaucheria compared with spirogyra.=—In vaucheria we have a
plant which is very interesting to compare with spirogyra in several
respects. Growth takes place, not in all parts of the thread, but is
localized at the ends of the thread and its branches. This represents a
distinct advance on such a plant as spirogyra. Again, only specialized
parts of the plant in vaucheria form the sexual organs. These are
short branches. Farther there is a great difference in the size of the
two organs, and especially in the size of the gametes, the supplying
gametes (spermatozoids) being very minute, while the receptive gamete
is large and contains all the nutriment for the fertilized egg. In
spirogyra, on the other hand, there is usually no difference in size
of the gametes, as we have seen, and each contributes equally in the
matter of nutriment for the fertilized egg. Vaucheria, therefore,
represents a distinct advance, not only in the vegetative condition of
the plant, but in the specialization of the sexual organs. Vaucheria,
with other related algæ, belongs to a group known as the _Siphoneæ_, so
called because the plants are tube-like or _siphon_-like.
[Illustration: Fig. 143_a_. Botrydium granulatum. _A_, the whole plant;
_B_, swarm spore; _C_, planogametes; _a_, a single gamete; _b_-_e_, two
gametes in process of fusion; _f_, zygote.]
=308. Botrydium granulatum.=—An example of one of the simpler
members of the Siphoneæ is Botrydium granulatum. It is found sometimes
in abundance on wet ground which is colored green or red by its
presence, according to the stage of development. The plant body is long
pear-shaped, the smaller end attached to the ground by slender branched
rhizoids (Fig. 143). The protoplasm contains many nuclei and lines the
inside of the wall. When multiplication takes place large numbers of
small zoospores with one cilium each are formed in the protoplasm, and
escape at free end. Reproduction takes place by two-ciliated gametes,
which fuse in pairs to form zygospores. In dry seasons the protoplasm
in the pear-shaped plant passes down into the rhizoids and forms
small rounded _planospores_. All the stages of development are too
complicated to describe here.