This genus consists of only three species: the common Walnut (_J. regia_); the black Walnut (_J. nigra_); and the Butter-nut (_J. cinerea_ or _cathartica_). The male and female flowers are distinct, but on the same plant: the male flowers being produced in long, solitary, cylindrical catkins, and the female ones in pairs, or in shorter catkins. The leaves are pinnate, with the leaflets not always opposite, which is very rarely the case in other plants. In _Juglans regia_ (the common walnut), the male flowers are produced in a very thick catkin, each flower consisting of a calyx divided into five or six scale-like lobes, and generally from twelve to twenty stamens, with very long anthers and very short filaments; there is also a very curious bract to each, as shown in the magnified flower at _a_ in _fig._ 77; in which the anthers are seen at _b_. The female flowers are in pairs, as shown at _c_; and they consist of a calyx, _d_, enclosing the ovary, and toothed in the upper part, and four small petals encircling two large thick leafy-looking stigmas, _e_. [Illustration: FIG. 77—WALNUT (_Juglans regia_).] The fruit is a fleshy husk in one piece, formed of the dilated calyx; it generally retains the stigmas till it has nearly attained its full size, and when it becomes ripe it does not separate into valves, but bursts irregularly. The nut, on the contrary, is in two distinct valves, which may be easily separated from each other; and it is imperfectly divided into cells by four half dissepiments. The germ of the future plant is what children call the heart, and it is in the upper part of the kernel, with the root end uppermost, so that when a walnut is sown the sharp end should be placed downwards. The kernel is four-lobed, and deeply wrinkled; and when the young plant begins to grow, it divides into two cotyledons or seed-leaves, which drop off when the true leaves are fully developed. The kernel is covered with a thick skin, which is very astringent; and the nut is covered with a membranaceous network of strong veins, which are generally found in a withered state on opening the ripe husk, having left their impression deeply imprinted on the outside of the shell of the walnut. The leaves are impari-pinnate, consisting of four pairs of leaflets and a terminal single one; the lower pair of leaflets is much the smallest, and the other leaflets are frequently not opposite; and they are sometimes unequal at the base. The main petiole is dilated at the point where it joins the stem; and the leaves are placed alternately. The tree is large and widely spreading; and the timber is of a close grain, and takes a fine polish. [Illustration: FIG. 78.—LEAF AND FRUIT OF THE BLACK WALNUT (_Juglans nigra_).]