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A deciduous shrub or small tree, occasionally 30 to 40 ft high in a wild state; branches erect; branchlets quite glabrous. Leaves very variable in shape, usually three- but sometimes five-lobed; the lobes so deep sometimes that the leaf becomes trifoliolate, at other times quite shallow; 3 to 5 in. long and broad, coarsely toothed, quite glabrous on both surfaces; dark shining green above, pale beneath; stalk reddish, 11⁄2 to 3 in. long. Flowers few, produced towards the end of April in clusters 1 to 2 in. long, greenish yellow, 1⁄4 in. across. Fruit with incurved wings, each 3⁄4 in. long, 3⁄8 to 1⁄2 in. wide, reddish when young.
Native of western N. America; introduced about 1884. It is very distinct because of its thin, lustrous leaves, quite devoid of any down. At Kew it is thriving well, the best being now 40 × 3 ft (1967). The trees are well marked by their upright, almost fastigiate branches.
A. glabrum was originally described from the Rocky Mountains. Some authorities draw a distinction between the typical variety, with greyish branchlets and rather deeply lobed leaves up to about 23⁄8 in. wide, and the more western var. douglasii (Hook.) Dipp. (A. douglasii Hook.), with reddish branchlets and less deeply lobed leaves averaging more than 23⁄8 in. in width (Hitchcock, Vasc. Pl. Pacif. Northwest, Part 3, p. 412; B. O. Mulligan, Int. Dendr. Soc. Year Book 1970, pp. 14-17). But the difference is not at all clear cut. There are two examples of the var. douglasii at Kew, raised from wild-collected seed received from the University of British Columbia.
Also at Kew is A. glabrum var. tripartitum (Nutt.) Pax, a native of parts of Colorado, New Mexico, etc., in which the lower leaves of the stems are lobed almost to the centre.