All the species of the genus have their male and female flowers on different plants, both kinds of flowers being placed on short catkins which are either erect or spreading sideways. The male flowers have each from one to five or more stamens, with no petals or calyx, but as a substitute a bract or scale, which is entire and hairy, and which has one or more glands at its base. The female flower has a similar bract or scale, and it is also without either petals or calyx; there are two stigmas, each of which is sometimes two-lobed. The capsule has only one cell, but many seeds which are covered with down or longish hairs, and which are very conspicuous from the capsule opening naturally into two valves when ripe. The leaves of the Osiers and Willows are generally lanceolate, and serrated at the margin, and they are always furnished with stipules; but the leaves of the Sallows are generally much broader, and sometimes roundish; and they are always of a thick velvety texture. Though the number of the stamens varies in the different species, two are by far the most common. [Illustration: FIG. 81.—THE WILLOW (_Salix_).] _Fig._ 81 shows the female flower of _Salix fragilis_ at _a_, _b_ is the honey gland, _c_ the stigma, which is divided into four equal parts, and _d_ the bract or scale with its hairy fringe; _e_ is the male flower with its two stamens, two glands, and hairy scale. This species is a tall, bushy-headed tree, with the branches crossing each other frequently, being set on obliquely; and it is called the Crack Willow, from the young branches separating from the trunk in spring with the slightest blow or jerk, their bases being as brittle as glass. The leaves are of a deep green. The White Willow (_Salix alba_) differs from the preceding species in the branches being widely spreading and somewhat drooping, the old bark cracked into deep fissures, and the foliage of a silvery grey, owing to the silky hairs with which the leaves are more or less covered. The wood of the Tree Willows is soft and white, and very elastic; it is therefore used for cricket-bats, mallets, and other purposes where wood is wanted to resist a hard blow. _S. vitellina_, the Golden Osier, is so called from its golden-coloured bark; and _S. purpurea_, the Purple Willow, is so called from the colour of its branches. This last species has only one stamen; but as the anther is four-celled, it is probably two stamens grown together. All the species that have only one stamen have a four-celled anther, as for example the Rose Willow (_S. Helix_), which has the female catkins red. _Salix caprea_, the great round-leaved Sallow or Palm Willow, is perhaps the handsomest species, from the great abundance and golden hue of its flowers. THE GENUS POPULUS. [Illustration: FIG. 82.—TREMBLING POPLAR OR ASPEN (_Populus tremula_).]