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A tree attaining in America 70 feet in height and 12 feet in girth; bark deeply fissured into broad rounded ridges. Young branchlets green or glaucous, glabrous. Leaves (Plate 205, Fig. 2) pinnate, turning yellow in autumn. Leaflets, three or five, stalked, ovate or oval, rounded or cuneate at the base, acuminate at the apex, serrate or toothed above the middle, often three-lobed ; upper surface bright green and glabrous; lower surface pale green and with slight pubescence on the midrib and nerves; rachis glabrous.

Flowers diœcious, without petals, appearing with the leaves, from buds in the axils of the leaf-scars of the previous season, staminate flowers in fascicles, pistillate in narrow pendulous racemes. Fruit, with narrow acute nutlets, diverging at an acute angle, and thin reticulate, straight or falcate wings.

In winter, the terminal buds are about ⅛ inch long, acute, with four tomentose ciliate external scales; lateral buds appressed to the twigs, with two outer visible scales ; opposite leaf-scars united around the twig, narrowly crescentic, three-dotted, fringed with hairs on the margin.

The species, extending over a vast territory, varies considerably in the wild state, the typical form described above occurring in the eastern part of its distribution. Farther west, in Colorado, Utah, and New Mexico, the branchlets and leaves become pubescent ; and in California, an extreme form is met with, which is often considered to be a distinct species :—


 * 1. Var. californicum, Wesmael, Bull. Bot. Soc. Belg. 43 (1890); Sargent, Garden and Forest, iv. 481 (1891), Silva N. Amer. ii. 112, t. 97 (1892), and Trees N. Amer. 643 (1905).

This is distinguished, according to Sargent, by its darker-coloured bark; buds covered with dense tomentum ; short pale persistent pubescence on the branchlets and ripe fruit; leaflets three, larger, more coarsely serrate, and more frequently lobed than in the type, and coated beneath with pale pubescence. As seen in cultivation at Kew, the leaflets are usually five and not three; and in the wild state, 5-foliolate leaves are occasionally met with, as in a specimen in the Kew herbarium collected by Lobb in California. The pubescence on the leaflets beneath is most strongly marked on the midrib and nerves, is whitish in colour, and forms prominent