Acer glabrum, Rocky Mountain maple

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Intermountain Flora
Freely branching, sometimes abhorescent shrub up to 5 or even 10 m tall, usually with several stems from the base, essentially glabrous except for the loosely tomentose inner surface of the inner 2 pairs of scale of the winter-buds, the twigs of the season (and often of the past 1 or 2 seasons) generally red, the older ones gray; leaves turning bright red in the autumn, the petiole often red, the blade simple and palmately 3-5-lobed to 3-foliolate, the sinuses generally narrow and sharp, the lobes or leaflets typically with more or less numerous sharp, unequal teeth, often double toothed or shallowly lobulate with the lobules again toothed, but in small-leaved forms, especially in var. diffusum, often with fewer, blunter, mainly distal teeth; flowers opening with the leaves, borne in loose, terminal, corymbiform cymes arising from preformed red lateral buds that also produce 1 or 2 pairs of leaves below the inflorescence, functionally unisexual but with reduced parts of the opposite sex (the pistil of the staminate flowers minute, but the stamens of the pistillate flowers more evident), the plants monoecious or often dioecious; perianth only obscurely biseriate, the sepals and petals (4) 5 (6) each, rather slender, pale yellowish-green, 3-5 mm long, or the petals reputedly rarely wanting; stamens mostly 8, individually set in the lobulate disk; mericarps of the samara each mostly 1.5-3 cm long, broadly winged, the wings often as much as 1 cm wide, varying from scarcely divergent to divergent at a little more than a right angle.

Cronquist, A., Holmgren, J.L., and Holmgren, P.K. (1997) "Intermountain Flora" pg. 310, vol. 3 part A, The New York Botanical Garden New York.