The next portion of my subject is the study of the methods by which
these special cells reproduce the plant.
1st. Asexual methods.
1. Rejuvenescence gives rise to a swarm-spore or zoospore. The whole of
the protoplasm of a cell contracts, becomes rounded and rearranged, and
escapes into the water, in which the plant floats as a mass of
protoplasm, clear at one end and provided with cilia by which it is
enabled to move, until after a time it comes to rest, and after
secreting a wall forms a new plant by ordinary cell-division. Example:
Oedogonium.
2. Free-cell formation forms swarm-spores which behave as above.
Example: Achlya.
3. Free-cell formation forms the typical motionless spore of algæ and
fungi. For instance, in the asci of lichens there are formed from a
portion of the protoplasm four or more small ascospores, which secrete a
cell-wall and lie loose in the ascus. Occasionally these spores may
consist of two or more cells. They are set free by the rupture of the
ascus, and germinate by putting out through their walls one or more
filaments which branch and form the thallus of a new individual. Various
other spores formed in the same way are known as _tetraspores_, etc.
4. Cell-division with rejuvenescence forms the spores of mosses and
higher cryptogams.
To take the example of moss spores:
Certain cells in the sporogonium of a moss are called mother-cells. The
protoplasm of each one of these becomes divided into four parts. Each of
these parts then secretes a cell-wall and becomes free as a spore by the
rupture or absorption of the wall of the mother-cell. The germination of
the spores I shall describe later.
5. A process of budding which in the yeast plant and in mosses is merely
vegetatively reproductive, in fungi becomes truly reproductive, namely,
the buds are special cells arising from other special cells of the
hyphæ.
For example, the so-called "gills" of the common mushroom have their
surface composed of the ends of the threads of cells constituting the
hyphæ. Some of these terminal cells push out a little finger of
protoplasm, which swells, thickens its wall, and becomes detached from
the mother-cell as a spore, here called specially a _basidiospore_.
Also in the common gray mould of infusions and preserves, Penicillium,
by a process which is perhaps intermediate between budding and
cell-division, a cell at the end of a hypha constricts itself in several
places, and the constricted portions become separate as _conidiospores_.
_Teleutospores, uredospores_, etc., are other names for spores similarly
formed.
These conidiospores sometimes at once develop hyphæ, and sometimes, as
in the case of the potato fungus, they turn out their contents as a
swarm-spore, which actively moves about and penetrates the potato leaves
through the stomata before they come to rest and elongate into the
hyphal form.
So far for asexual methods of reproduction.