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A tree 80 to 100 ft high, with a trunk up to 3 ft in diameter, supporting a narrow roundish head of branches. Branchlets often becoming corky. In a young state the trees are pyramidal; winter buds and young shoots downy. Leaves oval to obovate, with an abrupt, slender point and an unequal, oblique base, 2 to 41⁄2 in. long, 11⁄4 to 23⁄4 in. wide, doubly toothed, glabrous, dark glossy green above, downy beneath; side veins in often over twenty pairs; stalk up to 1⁄4 in. long, sometimes partially covered by the overlapping bases of the blade. Flowers in racemes 1 to 2 in. long. Samaras oval, 1⁄2 to 3⁄4 in. long, downy all over as well as on the thickened margins, with a slight open notch at the apex, the two points erect.
A native of N. America from southeast Canada and New England west to Minnesota and Iowa, south to Tennessee. It produces a hard timber, whence ‘rock elm’, but is of no economic importance and grows slowly even in its native country. An example at Kew, although almost half-a-century old, measures only 38 × 13⁄4 ft (1970). The distinctive points of this species are the large, downy winter-buds, 1⁄4 or 3⁄8 in. long, the racemose inflorescence and the shallowly notched hairy fruits.
U. alata Michx. Wahoo or Winged Elm. – Allied to U. thomasii, differing in its smaller stature, shorter almost sessile leaves (to 21⁄2 in. long) and the narrower ovate-lanceolate samaras. A native of the southern and southern-central USA, attaining its best development in the Mississippi delta. The twigs of this species soon develop a pair of opposite corky wings, though these are sometimes absent; in U. thomasii the cork appears later and is less regular. As in U. thomasii the leaves are downy beneath and the samaras downy on the surface and edge.
This elm is rare in Britain, where it has been confused with the corky-barked form of U. carpinifolia, which differs obviously in its inflorescences and samaras, and in having longer-stalked leaves glabrous beneath.