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.