most striking fea ture of the bryophytes (mosses) and pteridophytes (ferns) is a strongly de veloped alternation of generations. Thus, in a fern, the spores, after falling to the ground, do not produce directly another plant like the one which bore them, hut give rise to a minute plantlet, often heart shaped, known as a prothallium. Upon this are borne the antheridia, or male organs, and archegonia, or female organs. Fertilization is accomplished by motile antherozoids, developed in the antheridia.
Flowering other great division of the vegetable kingdom com prises the phanerogams or flowering plants. From the fact that they produce true seeds, they are technically known as spermatophytes. Flowering plants in clude all our ordinary trees and shrubs of temperate climates as well as most of the herbaceous vegetation growing upon the land. In a complete or highly de veloped flower there are four series of parts. The outermost, which is also the lowest on the stem, is the calyx. It is usually more or less cup-shaped and com monly green. If it is divided to the base, its parts are known as sepals. Its func tion is, in general, protective. It shields the innermore delicate parts of the flower. especially in the early and tender stages, against injury from the weather, de structive insects, etc. The next series of floral members is the corolla, which is usually showy and of a color other than green. Its function is, in part, protective, but its bright coloration, as well as its peculiar forms, has undoubtedly been de veloped to attract and facilitate the visits of insects for the fertilizing of the flower. The corolla may consist of a cup or tube or may be made up of separate parts, the petals.
Interior to the petals are the stamens. These consist of a thread-like stalk portion, the filament, and a usually two celled sac, the anther. In the cells of the anther is the dustlike pollen. At the center of the flower stands the pistil. This may consist of a simple, highly modified leaf, or may be composed of several such members, the carpels, more or less completely fused together. When fully developed, the pistil has three parts, a basal sac, the ovary, surmounted by a short or long columnar portion, the style, which in its turn, bears at or near its usually enlarged summit, a soft, often viscid area, the stigma, for the reception of the pollen. In the ovary are one or
more globose or oval bodies, the ovules, which, after fertilization by the pollen, become seeds. These ovules are borne upon the incurved edges of the carpels, although this fact is often very obscure.
The pistil and stamens are the essential parts of the flower, while the calyx and corolla, one or both of which may be wholly lacking, are accessory parts. When stamens and pistil are found in the same flower, it is said to be perfect. When they occur in different flowers upon the same individual, the plant is said to be monoecious, while a species in which stamens are borne in the flowers of one individual and the pistils in the flowers of another is dioecious.
In order that a flower may perfect seeds, it is (with certain rare exceptions) necessary that the pollen grains be trans ferred from the anthers to the stigma. This transfer, pollenation, is sometimes effected by a contrivance or movement within the flower itself. External agents, however, are often necessary. These are chiefly wind, currents of water, or in sects.
Flowering plants are primarily divided into the gymnosperms (sago palms, pines, firs, larch, juniper, gingko, etc.), which have no closed ovary, and the angio sperms, in which the seeds are inclosed in a sac-like ovary. The latter group is again divided into two great sections according to the number of rudimentary leaves in the embryo. The monocotyle dons, which have only one seed leaf or cotyledon, include the grasses, sedges, rushes, cat tails, palms, lilies, orchids, etc. These plants usually have leaves with parallel veins and flowers built upon the plan of three. They are also called endogens from the fact that the growth of their stems in thickness is effected not by the addition of external layers, but by the expansion and increased complex ity of their internal tissues. The angio sperms, with two seed leaves, are the dicotyledons or exogens, having usually net-veined leaves, and flowers more often upon the plan a five. Exogens include the willows, oaks, elms, pinks, butter cups, mustards, roses, beans, violets, asters, etc.