SALICALES

the willow order of flowering plants, belonging to the class called dicotyledon (characterized by two seed leaves). It consists of the Salicaceae family, having two genera, Salix (willow) and Populus (poplar, aspen, cottonwood), with about 320 and 30 species, respectively. All are trees and shrubs, distributed largely in the cooler regions of the Northern Hemisphere. Only S. tetrasperma, found in Asia, is native to tropical and subtropical regions. Most members of Salicales have extended ranges. Tree species of both Salix and Populus are commercially important. Some Salix species are large trees, but many are tall shrubs that grow along streams or are prostrate plants of the Arctic. The wood of Salix trees is fashioned into furniture, caskets, cabinets, and boxes. Pliable twigs of several species of willow are used in wicker furniture and baskets. Aspirin is a derivative of salicylic acid, which is obtained from willow bark. The soft, white wood of Populus is fabricated into plywood, pulpwood, excelsior, boxes, and matches. Poplars are often used to line avenues and form windbreaks. Common features of the Salicales include rapid growth, woodiness, leaves with simple blades, and a pair of leaflike stipules borne on either side of the base of the petiole (leaf stalk). This order is different from other catkin-bearing families in that male (staminate) and female (pistillate) flowers are borne on separate plants and are accompanied by bracts (scale leaves); in addition, seeds are covered with many long hairs for wind dispersal. The Salix genus is distinguished from Populus by linear leaves, winter buds covered with a single scale, catkins (elongate, compact flower clusters) appearing with (or before) leaves (and these erect with a simple scale associated with each flower), and few stamens. Poplars possess broadly triangular, pointed leaves and a flattened petiole that permits leaves to flutter. A sticky resin covers the numerous bud scales. The catkins are well-developed before leaves expand, and associated catkin scales are finely divided. They have many stamens. Most willows and many poplars produce roots when larger stems are placed in wet ground. Following forest fires, roots of aspens produce suckers (sprouts) around the parent stump. In addition to vegetative means of propagation, seeds are produced prodigiously; in the European aspen (Populus tremula), for example, as many as 54,000,000 seeds are produced each season on one tree. Flowers in the Salicales lack sepals and petals. In Populus, stamens and pistils arise from a conspicuous disk in their respective catkins. In Salix, one or two glands may develop. In both genera the pistil is composed of two or four joined carpels that are differentiated into a basal, one-chambered ovary, a style, and pollen-receptive stigmas. A fringed bract, characteristic of the genus, arises from the stalk of each flower. Solitary bees arrive as the small flowers of willows emerge; other insects also pollinate these plants. Some species of Populus are wind-pollinated. In this order, all fruits, derived from the ovary, are capsules that open to release the hair-tufted seeds.