Page:Collier's New Encyclopedia v. 02.djvu/149

BOTANY uses of plants and has its application in agriculture, horticulture, forestry, pharmacy, and medicine.

Plants may be divided into (1) those which are reproduced by means of minute one-celled bodies, destitute of an embryo, and called spores; and (2) those which are propagated by multicellular seeds containing each a latent and extremely rudimentary plantlet, the embryo. Plants of the former class have long been known as cryptogams or flowerless plants, the term sporophytes being preferred by many critical writers. The cryptogams include the following groups: Fungi (molds, mildews, rusts, smuts, toadstools and mushrooms); algæ (sea weeds, diatoms, desmids, etc.); lichens, scale mosses, or liverworts, true mosses, ferns, and fern allies (club mosses, horse tails, or scouring rushes, etc.). The fungi, algæ, and lichens are grouped together under the name thallophytes and the scale mosses and true mosses under the name of bryophytes, while the ferns and their allies are often called pteridophytes. Fungi differ from algae in the uniform lack of chlorophyll or green coloring matter. Lichens, the scale-like incrustations, usually of a gray or brown color, found upon rocks, tree trunks, etc., are composite beings, including green cells like those of algae, but surrounded by fine, usually colorless, filaments like a fungus.

Bryophytes.—The most striking feature of the bryophytes (mosses) and pteridophytes (ferns) is a strongly developed 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, but 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 Plants.—The other great division of the vegetable kingdom comprises the phanerogams or flowering plants. From the fact that they produce true seeds, they are technically known as spermatophytes. Flowering plants include 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 developed 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 commonly green. If it is divided to the base, its parts are known as sepals. Its function 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, destructive 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 developed 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 transferred 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 insects.

Flowering plants are primarily divided into the gymnosperms (sago palms, pines, firs, larch, juniper, gingko, etc.), which have no closed ovary, and the angiosperms, in which the seeds are inclosed in a sac-like ovary. The latter group is again divided into two great sections