The _tetrapetalous_ (or four-petalled) corolla is usually arranged like a cross, and is much more frequent than the _dipetalous_ or _tripetalous_; for examples, we need only refer to the large and important family of the Cruciferæ. The number five (_pentapetalous_) is still more common; but we meet with it in other organs besides petals, and it seems particularly characteristic of the vegetable kingdom. Thus, all the Umbelliferæ have five sepals, five petals, and five stamens; in all the Crassulaceæ, the number five applies, not only to the sepals and the petals, as well as to the stamens, but also to the carpels which compose the ovary. Corollas with six, eight, nine, ten, or twelve petals are relatively rarer; and when the petals become so numerous that we cannot count them, we have to deal with transformations of stamens into petals, with those monstrosities of cultivation which we call double flowers, _flores pleni_, where all the male organs have disappeared,--flowers wholly unfit for fructification. The petals of the corolla are not always free. Like those of the calyx, they may be attached to one another by their edges, but this union invariably takes place, as in the former, from bottom to top. Therefore, we never see any petals united at the top, and disengaged at the bottom (see Fig. 47, _a_.) But the reader must take careful note that this invariable characteristic is not peculiar only to the floral envelopes; it is not met with in the stamens, for these may be united, either by their anthers, as in the whole family of the Synantheræ, or by their filaments, as in the Leguminosæ. And what we have said of the stamens applies also to the parts constituting the pistil. This radical difference between the perianth and the true reproductive organs ought, from the beginning, to have fixed the attention of botanists on the centripetal and centrifugal metamorphosis of the leaf, which we have spoken of in "The Circle of the Year." [Illustration: FIG. 47, _a_.--Natural junction of Petal.] [Illustration: FIG. 47, _b_.--Unnatural (and impossible) junction.] * * * * * In many plants we are permitted to follow step by step, as it were, the union of the petals, and their definitive transformation into what is called the _monopetalous_ corolla. The term _monopetaloid_ ought then to be rejected, if we are to believe that the _monopetalous_ corolla is the result of the metamorphosis of a single leaf. The word _gamopetalous_, or, rather, _gamophyllous_, is preferable. As we have remarked in reference to the calyx, we shall here repeat that the expressions _bilobed_, _trilobed_, _quadrilobed_, or _bipartite_, _tripartite_, _quadripartite_ corollas, are radically vicious, because they are the consequence of a false point of view, according to which the _monopetalous_ corolla will be simply a single metamorphosed leaf, susceptible of being more or less deeply divided from top to bottom.