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A deciduous tree with glabrous, angular young shoots. Leaves three-lobed, the lobes ovate-lanceolate, pointed, toothed or nearly entire, the lateral ones wide-spreading and forming a truncate or slightly heart-shaped base to the leaf; 11⁄2 to 3 in. long, often rather more wide; glabrous, dark green with pale veins above, slightly glaucous and downy beneath, especially on the veins; stalk reddish, 3⁄4 to 11⁄2 in. long, downy. Flowers few in a cluster or short raceme; males and females in separate clusters. Fruit 3⁄4 in. long, with downy nutlets and parallel, forward-pointing wings.
Native of N. China; discovered in Kansu by Dr Piasezki in 1875; introduced to the Arnold Arboretum in 1911 from the same province by W. Purdom, under whose No. 767 it may be in cultivation.
This species was described by Bean (B222) and Krüssmann (K91), but it seems probable that var. pilosum is not in cultivation.
A. stenolobum var. megalophyllum W.P. Fang & Y.T. Wu
The leaves of var. stenolobum have shorter lobes than in var. pilosum, which are only 0.5 cm wide (2.5 cm in the type variety). The samaras are 2–2.5 cm long with wings spreading at an obtuse or right angle. Van Gelderen et al. 1994, van Gelderen & van Gelderen 1999, Xu et al. 2008. Distribution CHINA: northeast Gansu, southwest Nei Mongol, southeast Ningxia, northern Shaanxi. Habitat Forests between 1000 and 1500 m asl. USDA Hardiness Zone 5–6. Conservation status Not evaluated.
Acer pilosum var. stenolobum is regarded as one of the rarest maples in cultivation, all stock being said to be derived from a tree at the Arnold Arboretum introduced by William Purdom in 1911 (van Gelderen et al. 1994, van Gelderen & van Gelderen 1999). It makes a small tree of no great beauty, although the angular leaves turn a good red in autumn. It is now cultivated in the major maple collections in Europe as well as North America, where it is fully hardy, but it would seem to need reintroduction. A wider sample of material would be helpful, to establish whether var. stenolobum really is distinct.