Not all flowers are perfect or complete. Some lack petals, as the buckwheat, where the colored calyx replaces petals. Others have neither calyx nor corolla, as in the sycamore or plane tree. Most plants, however, have both calyx and corolla. In some very few plants certain of the flowers have no stamens, when they are said to be _pistillate_ or female flowers, and certain others have no pistils, when they are called _staminate_ or male flowers. In other words, the sexes are in different flowers in the same cluster or plant, as is true of the walnut and hickories, when they are said to be _monœcious_. In still others the sexes are on entirely different plants, in which case they are _diœcious_, as in practically all willows. In the latter case there are _pistillate_ or female plants and _staminate_ or male plants. While it is a commonplace that peas do not look like daisies, nor a carnation like a rose, this simple observation does not begin to tell us of the wonderfully different flower shapes and colors that are to be found along any roadside. The perfect and complete flower that we have been studying is quite regular, composed as often as not of four or five petals, as many sepals, with five or ten stamens and perhaps a single pistil. Yet there is literally no limit to the variations from this scheme, and some of these must be understood here in order that the life-histories and behavior of plants discussed in later chapters may tell their full story. The figures on page 44 show a regular flower, with five separate petals and sepals (Fig. 39). Such flowers are said to be _polypetalous_, i.e., separate petals. Sometimes three of the petals are larger, two smaller, in which case the flower is lopsided or, as it is said, _inequilateral_. Again all the petals are united to form a regular and equilateral tube, as in lily of the valley, when they are _gamopetalous_, _i.e._, united petals (Figure 42). As we shall see in the chapter on Plant Families, this is a distinction between two great groups of plants, as important in their classification as negro and white man are in classifying humans. In peas, beans, the locust tree, and related plants the petals are much changed to form an irregular flower, with a keellike or prow-shaped part made from the uniting of two petals. Two more unite to form the wings, and the remaining and larger petal forms the standard. Figure 40 and the explanation under it illustrate this unusual form of flowers.