What I want to point out at present are the principles and not so much the details of reproduction, and I wish you to notice, as I proceed, what is true not only of reproduction in plants but also of all processes in nature, namely, the paucity of typical methods of attaining the given end, and the multiplicity of special variation from those typical methods. When we see the wonderfully varied forms of plant life, and yet learn that, so to speak, each edifice is built with the same kind of brick, called a cell, modified in form and function; when we see the smallest and simplest equally with the largest and most complicated plant increasing in size subject to the laws of growth by intussusception and cell division, which are universal in the organic world; we should not be surprised if all the methods by which plants are reproduced can be reduced to a very small number of types. 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.