ASPEN, a section of the poplar genus (Populus) of which the common aspen of Europe, P. tremula, may be taken as the type--a tall, fast-growing tree with a slender trunk, and grey bark becoming rugged when old. The roundish leaves, toothed on the margin, are slightly downy when young, but afterwards smooth, dark green on the upper and greyish green on the lower surface; the long slender petioles, flattened towards the outer end, allow of free lateral motion by the lightest breeze, giving the foliage its well-known tremulous character. By their friction on each other the leaves give rise to a rustling sound. The flowers, which ap pear in March and April, are borne on pendulous hairy catkins, 2-3in. long; male and female catkins are, as in the other species of the genus, on distinct trees.
The aspen is found in moist places, sometimes at more than 1,600ft. in Scotland. It is an abundant tree in northern Britain, and is occasionally found in the coppices of the southern counties; throughout northern Europe it abounds in the forests, while in Siberia its range extends to the Arctic Circle. The wood is light and soft, though tough ; it is employed for pails, herring-casks, butchers' trays, packsaddles, and various articles for which its lightness recommends it ; in mediaeval days it was valued for arrows ; the bark is used for tanning ; cattle and deer browse on the young shoots and suckers. Charcoal prepared from it is light and friable, and has been employed in gunpowder manu facture. The powdered bark is given to horses as a vermifuge; it possesses tonic and f ebrif ugal properties, containing salicin. The aspen is propagated by cuttings or suckers. The American aspen (P. tremuloides), called also quaking-asp, closely allied to the European species, is the most widely distributed tree in North America. It ranges from Labrador to the mouth of the Mackenzie river and the valley of the Yukon and southward to Pennsylvania, Missouri and Nebraska, and in the Rocky Moun tains and the Sierra Nevada to Chihuahua and Lower California, often ascending to i o,000 ft. altitude. With the exception of the black willow (Salix nigra), it is the only native tree common to both California and Maine. It attains a maximum height of ioo ft. and a trunk diameter of 3 f t., but is usually much smaller. The large-leaved American aspen (P. grandidentata) has ovate or roundish leaves deeply and irregularly serrated on the margin. The wood of both these species is manufactured into wood-pulp.