For example, the so-called "gills" of the common mushroom have their surface composed of the ends of the threads of cells constituting the hyphae. 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 hyphae, 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.
I shall now consider the sexual methods.
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 algae 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 Florideae 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 Coleochaeteae is considered together with the Florideae, we find a transition between the ordinary case of Coleochaete and that of Dudresnaya. In Coleochaete, 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 Coleochaete. 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.