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Acer glabrum Torr.

Rock Maple

Modern name

Acer glabrum Torr.

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, 112 to 3 in. long. Flowers few, produced towards the end of April in clusters 1 to 2 in. long, greenish yellow, 14 in. across. Fruit with incurved wings, each 34 in. long, 38 to 12 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.

From the Supplement (Vol. V)

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 238 in. wide, and the more western var. douglasii (Hook.) Dipp. (A. douglasii Hook.), with reddish branchlets and less deeply lobed leaves averaging more than 238 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.



Other species in the genus