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A tree attaining 100 feet in height and 13 feet in girth; branches massive and forming an oval crown, which becomes flattened at the top in old trees. Bark smooth, grey, or dark brown, and marked by longitudinal, narrow, pale-coloured fissures, which are very characteristic.

Leaves deciduous, compound, 1–3 feet long, imparipinnate, with 7–9 (sometimes even 20) pairs of leaflets, which are either opposite or nearly so, shining above, pale and glabrous (occasionally slightly pubescent) beneath, and unequally divided by the midrib. Each leaflet is stalked, ovate, or ovate-lanceolate, acute or acuminate at the apex, cordate or truncate at the base, entire in margin, except that near the base there are 1–4 pairs of glandular teeth. Stipules absent. The leaves appear late in spring, and exhale when rubbed a disagreeable odour which renders them distasteful to animals. They fall off late in autumn, absciss layers being formed at the base of the leaflets as well as of the main stalk; the former usually drop first.

Flowers appearing in July and August in large panicles at the summit of the branchlets, either unisexual or hermaphrodite; but as a rule the trees are practically diœcious, and those bearing staminate flowers give off an objectionable odour.

Fruit, 1–5 keys, resembling those of the ash, linear or oblong, membranous veined, with a small indentation above the middle on one side, close to where the seed is located; and the wings on both sides of the seed are slightly twisted, so that the fruit in sailing through the air moves like a screw. The keys are bright red or purplish brown in colour, and are very conspicuous amidst the green foliage.

Seedling: the cotyledons appear above the soil on a caulicle about an inch long and are foliaceous, coriaceous in texture, oboval, obtuse, shortly stalked, entire in margin, and pinnate in venation. The stem above them is pubescent, and at a short distance (about ½ inch) up bears two leaves, which are trifoliolate and long-stalked, the terminal leaflet being lanceolate, acuminate, and entire, the two lateral shorter and toothed. Higher up ordinary pinnate leaves are borne. Plate 15 shows a seedling raised by Elwes from seed ripened on a tree overhanging Dr. Charles Hooker's garden at Cirencester in 1900; sown November 26, germinated under glass in May 1901, and photographed on August 28 of the same year, when it measured about a foot high; the roots, which were very succulent and brittle, were 13 inches long. The seedlings were planted out in May 1902, and grew very rapidly, attaining 5 feet in height, but did not ripen their wood, which was killed back in some cases nearly to the ground. They are now (January 1905) 4–6 feet high.