The leaves of _Acer rubrum_ become red in autumn. The Tatarian Maple differs from the other species in having entire leaves, and the samaras are red when young; but all the other kinds of Acer common in British gardens bear a strong family likeness to each other. The Ash-leaved Maple is now made into a separate genus, and is called _Negundo fraxinifolia_. This tree is easily distinguished from the Maples by its compound leaves, which resemble those of the Ash, and its long pea-green shoots, which have very few buds. The male and female flowers of the Negundo are on different trees, and they are so small as to be seldom seen, though the racemes of samaras or keys which succeed the flowers are very conspicuous. The Negundo is a native of America, and its leaves turn yellow in autumn. ORDER XLV.—HIPPOCASTANEÆ, OR ÆSCULACEÆ. THE HORSE-CHESTNUT TRIBE. [Illustration: FIG. 127.—FLOWERS OF THE HORSE-CHESTNUT.] [Illustration: FIG. 128.—HORSE-CHESTNUT.] This order contains only two genera; viz., Æsculus, the Horse-chestnut, and Pavia, the Buckeye; both of which are generally called Horse-chestnuts, though the genera are easily distinguished by their fruit, the husk of which is smooth in the Pavias, but rough in the true Horse-chestnuts. The buds of all the species of both genera are covered with bracted scales, most of which fall off when the leaves and flowers expand; and those of the common Horse-chestnut (_Æsculus Hippocastanum_) are very large, and covered with a kind of gum. Four large compound leaves, each consisting of five or seven leaflets, and a raceme of sixty-eight flowers, have been unfolded on dissecting one of these buds, before the leaves unfold in spring. The flowers of this species are produced in large, upright panicled racemes (see _a_ in _fig._ 127); and the leaves (_b_) are compound, consisting of five or seven leaflets, disposed in a palmate manner. Two of the inner bracts, which remain after the outer scales (which are very numerous) have fallen, are shown at _c_. I mention this particularly, as these remaining bracts have very much the appearance of stipules, and it is one of the characters of the Horse-chestnuts that their leaves are without stipules. The flowers consist of five petals, two of which (_d_ in _fig._ 128) are somewhat smaller than the others. Each petal consists of a broad blade or limb (_e_), and a very narrow claw (_f_). There are seven stamens, three of which (_g_) are shorter than the others. The filaments are inserted in the receptacle (_h_), and surround the pistil, which is hairy, and has a long style and a curved stigma (_i_). The ovary is two-celled, and each cell contains two ovules, but seldom more than one seed ripens. The nut (_k_) is large, and covered with a shining brown skin, which is strongly marked with the hilum. When put into the ground, the cotyledons do not appear in the shape of seed-leaves, but remain in the ground, and the plumule and radicle are protruded as shown in _fig._ 129.