A flower of a complete kind consists of the parts of the plant which reproduce or form the seed, enclosed within two particular envelopes. The envelopes of the flower are the beautifully coloured parts called petals, which form the corolla, or inner envelope, and the duller-coloured or green sepals outside the corolla, and which form the calyx. Protected by these coverings, are the central parts or organs. Quite in the middle of the flower is the ovary, made up of one or several portions—the pistils, which contain the future seeds or ovules. The top of the whole, which projects in the middle of the flower, is the stigma, and the prolonged part beneath it is the style, and this surmounts the seed-case or ovary. Outside this central part, and between it and the corolla, are the stamens, each of which—for their number varies—may consist of a stalk or filament, bearing an anther, which is coloured, and contains the pollen, or dust, which fertilizes the ovule, by falling on to the stigma. These central parts are the reproductive organs, and are those which, above all others, are the most important, for without them a plant cannot increase and multiply, and would become extinct. The floral envelopes, beautiful as they are, are not so essential, and are of secondary importance in classification. Now, in some kinds of plants the stigma and the ovary exist in one individual and the stamens in another. The plant which bears the ovary is called the female, and that which has the pollen-making part is termed the male. Hence it is said that plants have sexes. But in a vast number of plants these organs are combined in the same individual. Linnæus considered the stamen to be of primary importance, and established eleven classes of plants distinguished by the numbers of the stamens, and these all relate to plants in which the male and female organs are combined in the same individual. Thus the red valerian has one stamen only, and it was classed in _monandria_, the first part of the word meaning “one” and the last “male.” The lilac has two stamens, and was classed in the _diandria_—“two male;” and other plants were classified up to those which have ten stamens, the pink, for instance, as _decandria_. An eleventh class included all plants which bear flowers containing from twelve to nineteen stamens, such as mignonette. Then two more classes were invented to comprise—1, plants with twenty or more stamens placed on the calyx, as the cherry; 2, others with twenty or more stamens which are placed on stalks rising from below the ovary, as in the buttercup. Other classes were formed according to the relative length of the stamens, as in the foxglove and wallflower, and also from the grouping of the stamens in bundles. Then there were three very important classes in which the sexes are in separate flowers. Finally, the flowerless plants, such as the ferns, lichens, and fungi, were united as cryptogamia, having their organs of reproduction more or less concealed.