Various

BY HARLAND COULTAS.

The Process of Fertilization.—All organic beings, animals, and plants reproduce themselves by means of fecundated germs, which we call embryos. The embryos of plants form in a particular organ called an ovule, and the matter which fecundates them is termed pollen.

The character of an embryo in organic beings is that it contains, in a rudimentary state, all the organs of which the organic being is composed in its entire developments. Thus, in the animal, the uterine foetus is composed of the head, the trunk, and the extremities; in other words, of all the parts of which the adult animal is composed. In like manner, the embryos of plants, like those of animals, contain all the parts which compose the fabric of the fully developed plant in a rudimentary condition. The embryo of a bean, for example, consists of a plumule or young stem, a pair of leaves or cotyledons, and a radicle or young root, or the entire plant in a rudimentary state; and, by the act of germination, analogous in its effects to the commencement of life in the extra-uterine foetus, all the parts of the plant develop themselves into their wonted figure and hues, in accordance with those peculiar organic laws to which the plant is subjected. But germination does not increase the number of these parts, which existed before its influence was exercised on them.

Now, plants have sexes, or sexual organs, as well as animals. The female sexual organs in plants are named carpels. The pistil, already described, consisting of stigma, style, and germen, is only a fully developed carpel. The male sexual organs are named stamens, the anthers of which contain the pollen or fecundating matter. The stamens and carpels are therefore the essential organs of reproduction in plants, since it is by the mutual action of these bodies that the embryo of the future plant is formed, and the same form of life continued in the earth. Fig. 1 is a representation of a petal, stamen, and the pistil of Berberis vulgaris, or the common barberry. In this plant, the anthers open by two valves to let out the pollen. These valves are seen in the figure, and the pistil is exhibited in section, to show the ovules in the cavity of the germen.

Fig. 1.

The reproductive organs only appear at the epoch when plants attain the full development of all their parts, or arrive at an adult state. The period when this occurs varies greatly in each species, and depends entirely on the peculiarities of its constitution. When this epoch arrives, a visible change takes place in the organic functions; the stem ceases to elongate, and its internodes no longer developing, the leaves remain crowded together in closely approximated whorls, and, after undergoing those peculiar modifications in form and coloring which we have already described, a flower is produced.

The process of fecundation appears to be as follows: As soon as the calyx and corolla are fully expanded, the stamens rapidly develop, their filaments elongate, and the anthers, at first moist and closed, become dry, and, rupturing, discharge the pollen on the stigma of the pistil, which at this time is bedewed with a clammy fluid, which serves to retain the grains of pollen that fall upon its surface. The grains of pollen, after remaining for some time on the humid stigma, absorb its moisture, and are seen to swell so that those which are elliptical assume a spherical form. The thin and highly extensible intine or inner covering of the pollen grain ultimately is pushed, in the form of a tube, through one of the pores or ostioles in the surface of the extine or outer covering, the mode of dehiscence of the pollen grain being always determined by the character of its surface. The pollen tube enters the lax tissue of the stigma, and, by gradual increments of growth, pushes its way down the style into the germen or ovary in which the ovules are found, up to this period, unfertilized. The tube enters one of the unimpregnated ovules through a small hole called the micropyle (from ????? a little, p??? gate), conveying the fecundating fluid matter contained in the cavity of the grain into the young ovule. This fluid matter is called fovilla, and its flow through the pollen tube is easily perceived by the movement of those microscopic corpuscles which it contains.

Fig. 2 is a section through the stigma, and part of the style of Antirrhinum majus, or the common snapdragon. The pollen grains are seen adhering to the surface of the stigma, and the tube is pushing its way down the pistil to the germen.

The ovules having received the impregnating matter, the flower loses its beauty, and nothing remains but the germen, which swells into a fruit abounding with seeds, by which the species is continued. An attentive observer may watch these changes throughout the summer months in any plant that produces flowers and fruit, and may thus satisfy himself of the general correctness of these statements.