Most species are known as willow, but some narrow-leaved shrub species are called osier, and some broader-leaved species are referred to as sallow. Some willows are low-growing or creeping shrubs, for example, the dwarf willow rarely exceeds 6 cm in height, though it spreads widely across the ground. Willows all have abundant watery bark sap, which is charged with salicylic acid, soft, usually pliant, tough wood, slender branches. The roots are remarkable for their toughness, size, and tenacity to life, the leaves are typically elongated, but may also be round to oval, frequently with serrated edges. Most species are deciduous, semievergreen willows with coriaceous leaves are rare, e. g. Salix micans, all the buds are lateral, no absolutely terminal bud is ever formed. The buds are covered by a single scale, usually, the bud scale is fused into a cap-like shape, but in some species it wraps around and the edges overlap. The leaves are simple, feather-veined, and typically linear-lanceolate, usually they are serrate, rounded at base, acute or acuminate. The leaf petioles are short, the often very conspicuous, resembling tiny, round leaves. On some species, however, they are small, inconspicuous, in color, the leaves show a great variety of greens, ranging from yellowish to bluish. Willows are dioecious, with male and female flowers appearing as catkins on separate plants and this scale is square, entire, and very hairy. The anthers are rose-colored in the bud, but orange or purple after the flower opens, they are two-celled, the filaments are threadlike, usually pale brown, and often bald. The ovary is one-celled, the style two-lobed, and the ovules numerous, almost all willows take root very readily from cuttings or where broken branches lie on the ground. The few exceptions include the willow and peachleaf willow. One famous example of such growth from cuttings involves the poet Alexander Pope and this twig was planted and thrived, and legend has it that all of Englands weeping willows are descended from this first one. Willows are often planted on the borders of streams so their interlacing roots may protect the bank against the action of the water, frequently, the roots are much larger than the stem which grows from them. Willows are very cross-compatible, and numerous hybrids occur, both naturally and in cultivation, a well-known ornamental example is the weeping willow, which is a hybrid of Peking willow from China and white willow from Europe. The hybrid cultivar Boydii has gained the Royal Horticultural Societys Award of Garden Merit, Willows are used as food plants by the larvae of some Lepidoptera species, such as the mourning cloak butterfly. Ants, such as ants, are common on willows inhabited by aphids, coming to collect aphid honeydew
Image: Salix alba Morton
At the base of the petiole a pair of stipules form. These may fall in spring, or last for much of the summer or even for more than one year (marcescence).