A flower, in its most complex form, consists of parts arranged in four whorls arranged concentrically. The first and second whorls, commencing from the outside, usually consist of leaf-like bodies, united or distinct, and are called respectively the calyx and the corolla. The third whorl consists of stamens, which are the male reproductive organs of the plant, and each stamen consists essentially of a case—the anther—in which are formed a number of little pollen cells. When the anther is ripe it opens, thus liberating the pollen, so that it may be dispersed by insects, by the wind, or by other mechanical means. The remaining whorl constitutes the pistil, which is generally made up of parts (carpels) arranged round a common centre, and each surmounted by a stigma adapted for the reception of the pollen cells. This portion of the flower contains the ovules, enclosed in a case called the ovary, and is, therefore, the female organ of the plant. When the ovules have been fertilised by the pollen, they develop into seeds, each one of which contains an embryo plant; and the ovary itself, ripening at the same time, develops into the fruit.
Such is the general description of a flower in its most complex form, but it must be remembered that one or more of the whorls named above may often be absent. Thus, calyx or corolla, or both, may not exist; and the male and female organs may be developed on separate flowers of the same plant, or even, as is frequently the case, on different plants of the same species. In the latter instance the flowers are spoken of as unisexual, those bearing the stamens being the staminate or male flowers, and those bearing the pistil the pistillate or female flowers.
The Phanerogams are divided into two main groups, the Gymnosperms and the Angiosperms. In the former the ovules are naked, no ovary or seed-case being developed. The pollen, carried by the wind, falls directly on the ovule, and then develops a tube which penetrates to the nucleus of the ovule, thus fertilising it. In the Angiosperms the ovules are always enclosed in an ovary, and the pollen grains, alighting on the stigma, are held by a gummy secretion. The tubes they produce then penetrate through the underlying tissues, and thus come into contact with the ovules.
The Gymnosperms include a group of small palm-like trees and shrubs (the Cycadeæ), of which the so-called Sago Palm is a representative; and the Coniferæ or cone-bearing shrubs and trees, which may be spoken of collectively as the Pines. In the latter the leaves are either stiff, linear, and needle-like, or short and scale-like, or are divided into narrow lobes; and the plants are noted for their resinous secretions. The flowers are always unisexual, and are generally arranged in cylindrical or short catkins, where they are protected by closely packed scales; but the female flowers may be solitary. There is no calyx or corolla, but the naked ovules and seeds are sometimes more or less enclosed in the scales (bracts) or in a fleshy disc.
The Angiosperms form the highest division of the flowering plants; and are subdivided into two extensive groups—the Monocotyledons and the Dicotyledons. The chief distinguishing feature of these is that implied in the above names, the embryo of the former containing but one rudimentary leaf (cotyledon), while that of the latter contains two. The Monocotyledons are also characterised by having the bundles of vessels (vascular bundles) of the stems dispersed; the veins of the leaves are also usually parallel, and the parts of the flower are arranged in whorls of three or six. In the Dicotyledons the vascular bundles of the stem are united into a ring which surrounds a central pith; the veins of the leaves form a network, and the parts of the flower are arranged in whorls of four or five.
We are now enabled to understand the relative positions of the principal groups of plants in the scale of vegetable life, and to locate approximately the forms with which we have to deal; and to aid the reader in this portion of his work we present a brief summary of the classification of plants in the form of a table for reference:—
THE CLASSIFICATION OF PLANTS
I. CRYPTOGAMIA—Flowerless plants.
(a) THALLOPHYTES—Leafless, cellular plants.
1. Protophyta—Unicellular plants.