1. The vegetative type of reproduction, in which one or more ordinary
cells separate from the parent plant and become an independent plant;
and--
2. The special-cell type of reproduction, in which either one special
cell reproduces the plant, or two special cells by their union form the
origin of the new plant; these two modifications of the process are
known respectively as asexual and sexual.
The third modification is a combination of the two others, namely, the
asexual special cell does not directly reproduce its parent form, but
gives rise to a structure in which sexual special cells are developed,
from whose coalescence springs again the likeness of the original plant.
This is termed alternation of generations.
The sexual special cell is termed the _spore_.
The sexual special cells are of one kind or of two kinds.
Those which are of one kind may be termed, from their habit of yoking
themselves together, _zygoblasts_, or conjugating cells.
Those which are of two kinds are, first, a generally aggressive and
motile fertilizing or so-called "male cell," called in its typical form
an _antherozoid_; and, second, a passive and motionless receptive or
so-called "female cell," called an _oosphere_.
The product of the union of two zygoblasts is termed a _zygospore_.
The product of the union of an antherozoid and an oosphere is termed an
_oospore_.
In many cases the differentiation of the sexual cells does not proceed
so far as the formation of antherozoids or of distinct oospheres; these
cases I shall investigate with the others in detail presently.
First, then, I will point out some of the modes of vegetative
reproduction.
The commonest of these is cell division, as seen in unicellular plants,
such as protococcus, where the one cell which composes the plant simply
divides into two, and each newly formed cell is then a complete plant.
The particular kind of cell division termed "budding" here deserves
mention. It is well seen in the yeast-plant, where the cell bulges at
one side, and this bulge becomes larger until it is nipped off from the
parent by contraction at the point of junction, and is then an
independent plant.
Next, there is the process by which one plant becomes two by the dying
off of some connecting portion between two growing parts.
Take, for instance, the case of the liverworts. In these there is a
thallus which starts from a central point and continually divides in a
forked or dichotomous manner. Now, if the central portion dies away, it
is obvious that there will be as many plants as there were forkings, and
the further the dying of the old end proceeds, the more young plants
will there be.
Take again, among higher plants, the cases of suckers, runners, stolons,
offsets, etc. Here, by a process of growth but little removed from the
normal, portions of stems develop adventitious roots, and by the dying
away of the connecting links may become independent plants.
Still another vegetative method of reproduction is that by bulbils or
gemmæ.
A bulbil is a bud which becomes an independent plant before it commences
to elongate; it is generally fleshy, somewhat after the manner of a
bulb, hence its name. Examples occur in the axillary buds of _Lilium
bulbiferum_, in some _Alliums_, etc.
The gemma is found most frequently in the liverworts and mosses, and is
highly characteristic of these plants, in which indeed vegetative
reproduction maybe said to reach its fullest and most varied extent.
Gemmæ are here formed in a sort of flat cup, by division of superficial
cells of the thallus or of the stem, and they consist when mature of
flattened masses of cells, which lie loose in the cup, so that wind or
wet will carry them away on to soil or rock, when, either by direct
growth from apical cells, as with those of the liverworts, or with
previous emission of thread-like cells forming a "protonema," in the
case of the mosses, the young plant is produced from them.
The lichens have a very peculiar method of gemmation. The lichen-thallus
is composed of chains or groups of round chlorophyl-containing cells,
called "gonidia," and masses of interwoven rows of elongated cells which
constitute the hyphæ. Under certain conditions single cells of the
gonidia become surrounded with a dense felt of hyphæ, these accumulate
in numbers below the surface of the thallus, until at last they break
out, are blown or washed away, and start germination by ordinary cell
division, and thus at once reproduce a fresh lichen-thallus. These
masses of cells are called soredia.
Artificial budding and grafting do not enter into the scope of this
paper.
As in the general growth and the vegetative reproduction of plants
cell-division is the chief method of cell formation, so in the
reproduction of plants by special cells the great feature is the part
played by cells which are produced not by the ordinary method of cell
division, but by one or the other processes of cell formation, namely,
free-cell formation or rejuvenescence.
If we broaden somewhat the definition of rejuvenescence and free-cell
formation, and do not call the mother-cells of spores of mosses, higher
cryptogams, and also the mother-cells of pollen-grains, reproductive
cells, which strictly speaking they are not, but only producers of the
spores or pollen-grains, then we may say that _cell-division is confined
to vegetative processes, rejuvenescence and free-cell formation are
confined to reproductive processes_.
Rejuvenescence may be defined as the rearrangement of the whole of the
protoplasm of a cell into a new cell, which becomes free from the
mother-cell, and may or may not secrete a cell-wall around it.
If instead of the whole protoplasm of the cell arranging itself into one
mass, it divides into several, or if portions only of the protoplasm
become marked out into new cells, in each case accompanied by rounding
off and contraction, the new cells remaining free from one another, and
usually each secreting a cell wall, then this process, whose relation to
rejuvenescence is apparent, is called free-cell formation.
The only case of purely vegetative cell-formation which takes place by
either of these processes is that of the formation of endosperm in
Selaginella and phanerogams, which is a process of free-cell formation.
On the other hand, the universal contraction and rounding off of the
protoplasm, and the formation by either rejuvenescence or free-cell
formation, distinctly mark out the special or true reproductive cell.
Examples of reproductive cells formed by rejuvenescence are:
1. The swarm spores of many algæ, as Stigeoclonium (figured in Sachs'
"Botany"). Here the contents of the cell contract, rearrange themselves,
and burst the side of the containing wall, becoming free as a
reproductive cell.
2. The zygoblasts of conjugating algæ, as in Spirogyra. Here the
contents of a cell contract and rearrange themselves only after contact
of the cell with one of another filament of the plant. This zygoblast
only becomes free after the process of conjugation, as described below.
3. The oosphere of characeæ, mosses and liverworts, and vascular
cryptogams, where in special structures produced by cell-divisions there
arise single primordial cells, which divide into two portions, of which
the upper portion dissolves or becomes mucilaginous, while the lower
contracts and rearranges itself to form the oosphere.
4. Spores of mosses and liverworts, of vascular cryptogams, and pollen
cells of phanerogams, which are the analogue of the spores.
The type in all these cases is this: A mother-cell produces by
cell-division four daughter-cells. This is so far vegetative. Each
daughter-cell contracts and becomes more or less rounded, secretes a
wall of its own, and by the bursting or absorption of the wall of its
mother-cell becomes free. This is evidently a rejuvenescence.
Examples of reproductive cells formed by free-cell formation are: