=758. Divided, or compound leaves.=—The rose, sumac, elder, hickory, walnut, locust, pea, clover, American creeper, etc., are examples of divided or compound leaves. The former are pinnately compound, and the latter are palmately compound. The leaf of the honey-locust is twice pinnately compound or bipinnate, and some are three times pinnately compound.[44] It is evident that compound leaves are only extreme forms of lobed or cut leaves and that the form of all bears a definite relation to the primary venation. There has been a reduction of mesophyll and of the area of smaller venation. _Unifoliate_ (for a single leaflet, as in orange and lemon where the compound leaf is greatly reduced and consists of one pinna attached to the petiole by a joint). _Bifoliate_ for one with two leaflets; _trifoliate_ for one with three leaflets, as in the clover; _plurifoliate_ for many leaflets. _Odd pinnate_ for a pinnate leaf with one or more pairs of leaflets and one odd leaflet at the end. So leaves are _palmately bifoliate_, etc., _pinnately bifoliate_, etc. _Decompound_ leaves are those where they are more than twice compound, as _ternately decompound_ in the common meadow rue (Thalictrum). _Perfoliate_ leaves are seen in the bellwort (Uvularia), _connate perfoliate_, as in some of the honeysuckles where the bases of opposite leaves are joined together around the stem. _Equitant_ leaves are found in the iris, where the leaves fit over one another at the base like a saddle. =759.= These forms of leaves probably have some definite significance. It is not quite clear why they should have developed as they have; though it is possible to explain several important relations of these forms to their environment. (1) The reduction of the surface of the leaf, with the retention of the firmer portions, allows freer movement of the air and affords the leaf greater protection from injury during violent winds, just as the finely dissected leaves of some water plants are less liable to injury from movement of the more dense medium in which they live. It is possible that here we may have an explanation of one of the factors involved in this reduction of leaf surface. (2) In trees with compound leaves, like the hickory, walnut, locust, ailanthus, etc., the midvein, and in the case of the Kentucky coffee-tree (Gymnocladus) the primary lateral veins also, serve in place of terminal branches of the stem. By the increase in the outline of the leaf and the reduction of its surface between the larger veins, the tree has attained the same leaf development that it would were the larger veins replaced by stems bearing simple leaves. The tree as it is, however, has the advantage of being able to cast off for the winter period a layer of what otherwise would have been a portion of the stem system, to retain which through the winter would use more energy than with the present reduced stem system, and the stouter stem is less liable to dry out.