The distinctive character of these methods is that the cell from which
the new individual is derived is incapable of producing by division or
otherwise that new individual without the aid of the protoplasm of
another cell.
Why this should be we do not know; all that we can do is to guess that
there is some physical or chemical want which is only supplied through
the union of the two protoplasmic masses. The process is of benefit to
the species to which the individuals belong, since it gives it a greater
vigor and adaptability to varying conditions, for the separate
peculiarities of two individuals due to climatic or other conditions are
in the new generation combined in one individual.
The simplest of the sexual processes is conjugation. Here the two
combining cells are apparently of precisely similar nature and
structure. I say apparently, because if they are really alike it is
difficult to see what is gained by the union.
Conjugation occurs in algæ and fungi. A typical case is that of
Spirogyra. This is an alga with its cells in long filaments. Two
contiguous cells of two parallel filaments push each a little projection
from its cell-wall toward the other. When these meet, the protoplasm of
each of the two cells contracts, and assumes an elliptical form--it
undergoes rejuvenescence. Next an opening forms where the two cells are
in contact, and the contents of one cell pass over into the other, where
the two protoplasmic bodies coalesce, contract, and develop a cell-wall.
The zygospore thus formed germinates after a long period and forms a new
filament of cells.
Another example of conjugation is that of Pandorina, an alga allied to
the well-known volvox. Here the conjugating cells swim free in water;
they have no cell-wall, and move actively by cilia. Two out of a number
approach, coalesce, contract, and secrete a cell-wall. After a long
period of rest, this zygospore allows the whole of its contents to
escape as a swarm-spore, which after a time secretes a gelatinous wall,
and by division reproduces the sixteen-celled family.
We now come to fertilization, where the uniting cells are of two kinds.
The simplest case is that of Vaucheria, an alga. Here the vegetative
filament puts out two protuberances, which become shut off from the body
of the filament by partitions. The protoplasm in one of these
protuberances arranges itself into a round mass--the oosphere or female
cell. The protoplasm of the other protuberance divides into many small
masses, furnished with cilia, the spermatozoids or male cells. Each
protuberance bursts, and some of the spermatozoids come in contact with
and are absorbed by the oosphere, which then secretes a cell-wall, and
after a time germinates.
The most advanced type of fertilization is that of angiosperms.
In them there are these differences from the above process: the contents
of the male cell, represented by the pollen, are not differentiated into
spermatozoids, and there is no actual contact between the contents of
the pollen tube and the germinal vesicle, but according to Strashurger,
there is a transference of the substance of the nucleus of the pollen
cell to that of the germinal vesicle by osmose. The coalescence of the
two nuclei within the substance of the germinal vesicle causes the
latter to secrete a wall, and to form a new plant by division, being
nourished the while by the mother plant, from whose tissues the young
embryo plant contained in the seed only becomes free when it is in an
advanced stage of differentiation.
Perhaps the most remarkable cases of fertilization occur in the Florideæ
or red seaweeds, to which class the well-known Irish moss belongs.
Here, instead of the cell which is fertilized by the rounded
spermatozoid producing a new plant through the medium of spores, some
other cell which is quite distinct from the primarily fertilized cell
carries on the reproductive process.
If the allied group of the Coleochæteæ is considered together with the
Florideæ, we find a transition between the ordinary case of Coleochæte
and that of Dudresnaya. In Coleochæte, the male cell is a round
spermatozoid, and the female cell an oosphere contained in the base of a
cell which is elongated into an open and hair-like tube called the
trichogyne. The spermatozoid coalesces with the oosphere, which secretes
a wall, becomes surrounded with a covering of cells called a cystocarp,
which springs from cells below the trichogyne, and after the whole
structure falls from the parent plant, spores are developed from the
oospore, and from them arises a new generation.
In Dudresnaya, on the other hand, the spermatozoid coalesces indeed with
the trichogyne, but this does not develop further. From below the
trichogyne, however, spring several branches, which run to the ends of
adjacent branches, with the apical cells of which they conjugate, and
the result of this conjugation is the development of a cystocarp similar
to that of Coleochæte. The remarkable point here is the way in which the
effect of the fertilizing process is carried from one cell to another
entirely distinct from it.
Thus I have endeavored to sum up the processes of asexual and of sexual
reproduction. But it is a peculiar characteristic of most classes of
plants that the cycle of their existence is not complete until both
methods of reproduction have been called into play, and that the
structure produced by one method is entirely different from that
produced by the other method.
Indeed, it is only in some algæ and fungi that the reproductive cells of
one generation produce a generation similar to the parent; in all other
plants a generation A produces are unlike generation B, which may either
go on to produce another generation, C, and then back to A, or it may go
on producing B's until one of these reproduces A, or again it may
directly reproduce; A. Thus we have the three types:
1. A-B-C.--A-B-C.--A..................... etc.
2. A-B-B.--B-B...................B--A ... etc.
3. A B A B A............................. etc.
The first case is not common, the usual number of generations being two
only; but a typical example of the occurrence of three generations is in
such fungi as _Puccinia Graminis_. Here the first generation grows on
barberry leaves, and produces a kind of spore called an _æcidium spore_.
These æcidium spores germinate only on a grass stem or leaf, and a
distinct generation is produced, having a particular kind of spore
called an _uredospore_. The uredospore forms fresh generations of the
same kind until the close of the summer, when the third generation with
another kind of spore, called a _teleutospore_, is produced.
The teleutospores only germinate on barberry leaves, and there reproduce
the original æcidium generation.