[Illustration: FIG. 92.--FRUIT OF THE HORNBEAM.]
Some botanists include this genus and that of Ostrya in the order Betulaceæ, instead of placing them in Cupuliferæ, as the nut of the Hornbeam is not surrounded by a cup or husk, but by a leaf-like involucre as shown in _fig._ 92, at _a_, _b_ being the nut. Both the male and the female flowers are produced in long catkins, which have an exceedingly light and elegant appearance on the tree. The male flowers consist each of one bract, with twelve or more stamens attached to its base; and the female flowers have each two very long stigmas, and a ribbed calyx, which adheres to the ripe nut and assumes the appearance of a hard brown skin. The leaves are feather-nerved and persistent, like those of the Beech, frequently remaining on the tree, though in a withered state, till spring.
The nut appears ribbed when ripe, from the remains of the metamorphosed calyx, and it contains only one seed; though, as in the other allied genera, the ovary had two cells, with an ovule in each.
The Hop Hornbeam (_Ostrya vulgaris_) was included in the genus Carpinus by Linnæus; and indeed the general construction of the flowers is the same. The male catkins are, however, very much longer, and the female catkins much shorter, and closely resembling those of the Hop.
This order formerly included the Plane trees and the Liquidambar; but many botanists now put the latter tree in a separate order, which they call Balsamaceæ.
In the Oriental Plane (_Platanus orientalis_) the male and female flowers are both in globular catkins. The male flowers are composed of very small, but rather fleshy bracts, which remain on after the stamens fall; and the female flowers are each furnished with bracts, and have two long stigmas. Both kinds of flowers are so small as not to be seen without a microscope. The fruit is covered with fine hair. The globular catkins retain the bracts, and these remain on after the seed has fallen, giving the tree a very singular appearance even in winter. The Occidental Plane (_P. occidentalis_) differs principally from the preceding species in the leaves being more downy beneath; the buds are also so downy that the tree in America is called the Cotton-tree. Both kinds are remarkable for the manner in which the bark becomes detached from the main trunk and peels off.