[Footnote: Read at a meeting of the Chemists' Assistants' Association. December 16, 1885.]
By C.E. STUART, B.Sc.
In two previous papers read before this Association I have tried to condense into as small a space as I could the processes of the nutrition and of the growth of plants; in the present paper I want to set before you the broad lines of the methods by which plants are reproduced.
Although in the great trees of the conifers and the dicotyledons we have apparently provision for growth for any number of years, or even centuries, yet accident or decay, or one of the many ills that plants are heirs to, will sooner or later put an end to the life of every individual plant.
Hence the most important act of a plant--not for itself perhaps, but for its race--is the act by which it, as we say, "reproduces itself," that is, the act which results in the giving of life to a second individual of the same form, structure, and nature as the original plant.
The methods by which it is secured that the second generation of the plant shall be as well or even better fitted for the struggle of life than the parent generation are so numerous and complicated that I cannot in this paper do more than allude to them; they are most completely seen in cross fertilization, and the adaptation of plant structures to that end.
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.
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 gemmae.
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.
Gemmae 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.