Willow. Willows, also called sallows and osiers, of the genus Salix, comprise around 350 species of typically deciduous trees and shrubs, found primarily on moist soils in cold and temperate regions. 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 centimetres in height, though it spreads widely across the ground. 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. Willows all have abundant watery bark sap, which is heavily charged with salicylic acid, soft, usually pliant, tough wood, slender branches, and large, fibrous, often stoloniferous roots. The roots are remarkable for their toughness, size, and tenacity to live, and roots readily sprout from aerial parts of the plant. The leaves are typically elongated, but they might also be round to oval, frequently with serrated edges. Most species are deciduous; semi-evergreen willows with coriaceous leaves are rare, e.g. Salix micans and S. australior in the eastern Mediterranean. 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 stipules often very conspicuous, resembling tiny, round leaves, and sometimes remaining for half the summer. On some species, however, they are small, inconspicuous, and caducous. In color, the leaves show a great variety of greens, ranging from yellowish to bluish color. Willows are among the earliest woody plants to leaf out in spring and the last to drop their leaves in autumn. In the northern hemisphere, leafout may occur as early as February depending on the climate and is stimulated by air temperature. If daytime highs reach 10 °C for a few consecutive days, a willow will attempt to put out leaves and flowers. In the northern hemisphere, leaf drop in autumn occurs when day length shortens to approximately ten hours and 25 minutes, which varies by latitude. Young male catkin With the exception of Salix martiana, willows are dioecious, with male and female flowers appearing as catkins on separate plants; the catkins are produced early in the spring, often before the leaves. The staminate flowers are without either calyx with corolla; they consist simply of stamens, varying in number from two to 10, accompanied by a nectariferous gland and inserted on the base of a scale which is itself borne on the rachis of a drooping raceme called a catkin, or ament. 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 and the cells open latitudinally. The filaments are threadlike, usually pale brown, and often bald. The pistillate flowers are also without calyx or corolla, and consist of a single ovary accompanied by a small, flat nectar gland and inserted on the base of a scale which is likewise borne on the rachis of a catkin. The ovary is one-celled, the style two-lobed, and the ovules numerous. The scientific use of the genus name Salix originates with Carl Linnaeus in 1753. The modern concept of types did not exist at the time, so types for Linnaeus genera had to be designated later. The type species, i.e., the species on which the genus name is based, is Salix alba, based on a conserved type. The generic name Salix comes from Latin and was already used by the Romans for various types of willow. A theory is that the word is ultimately derived from a Celtic language, sal meaning near and lis meaning water, alluding to their habitat. Willows are classified into subgenera though what they should be is in flux. Morphological studies generally divide the species into 3 or 5 subgenera: Salix, Chamaetia, and Vetrix. Phylogenetic studies have suggested that Chamaetia and Vetrix be in one clade. The oldest fossils of the genus are known from the early Eocene of North America, with the earliest occurrences in Europe during the Early Oligocene.