The first great generalization is into—­

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.