This is a very small genus, only containing two or three species, of which only one, the Sweet Chestnut (_Castanea vesca_) is common in England. This plant was included by Linnæus in the genus Fagus, but it appears very distinct. The male flowers are produced round a central axis, but so far apart as hardly to be like a catkin (see _a_ in _fig._ 88). These flowers in the bud look like little knobs, but when they open the stamens burst out, as shown at _b_. Each flower has a large and a small bract, and from ten to fifteen stamens. The female flowers are disposed in a tuft as shown at _c_, surrounded by a number of bracts and scales, which afterwards grow together and form a spiny involucre (see _fig._ 89 _a_,) which forms the husk of the ripe nuts (_b_), and opens into four valves as shown at _c_. Each female flower has a closely-fitting calyx, toothed at the tip, which afterwards becomes the hard brown skin that envelops the kernel of the ripe nut; and each flower is furnished with six styles, having as many cells with two ovules in each, though generally all the cells unite into one, and most of the ovules wither before the fruit ripens. There are three female flowers in each involucre, which lie nestling together like birds in a nest. When ripe the involucre or husk opens naturally into four valves (as shown in _fig._ 89), and drops the one or two Chestnuts which it contains. Each nut, when ripe, is enveloped in a brittle shining skin formed of the metamorphosed calyx, and consists of only one cell, in which are one, two, or at most three kernels, which are the seeds. THE GENUS CORYLUS. [Illustration: FIG. 90.—THE HAZEL (_Corylus Avellana_.)] [Illustration: FIG. 91.—MALE FLOWER OF THE HAZEL.] The Hazel Nut (_Corylus Avellana_) has the male and female flowers on the same tree; the male being in long catkins and the female ones in little oval buds, something like those of the Oak, (see _a_ in _fig._ 90,) which are so small that they would hardly be seen on the tree, if it were not for their bright red stigmas. The male flowers (_fig._ 91) have each three bracts, one behind the two others, to the inner ones of which are attached eight stamens. As the buds containing the female flowers expand, two or three small leaves make their appearance between the scales (_b_ in _fig._ 90), so that gradually the bud becomes a little branch, bearing the female flowers at its tip. Each flower has two long stigmas, and the ovary is enclosed in a closely-fitting calyx (_c_) toothed at the upper part, the whole being enveloped in a deeply cut involucre (_d_), which afterwards becomes the husk of the ripe nut. This involucre is not closed, as in Fagus and Castanea, but it is open at top; the nut, as in all cupuliferæ, adhering to it, when young, by the hilum or scar visible at its base. There are about seven species of Corylus, of which the most remarkable is the Constantinople Nut (_Corylus Colurna_.) The Filbert is only a variety of _Corylus Avellana_.