XXXV. ACERACEÆ.
Trees or rarely shrubs, with limpid juice, terete branches, scaly buds, their inner scales accrescent and marking the base of the branchlets with ring-like scars, and fibrous roots. Leaves opposite, or on vigorous shoots rarely in whorls of 3, long-petiolate, simple, palmately 3—7-lobed and nerved or pinnately 3—7-foliolulate, usually without stipules, deciduous, in falling leaving small U-shaped narrow scars showing the ends of 3 equidistant fibro-vascular bundles. Flowers regular, diœciously or monœciously polygamous, rarely perfect or diœcious, in fascicles produced from separate lateral buds appearing in early spring before the leaves or in terminal and lateral racemes or panicles appearing with or later than the leaves; bracts minute, caducous; calyx colored, generally 5-parted, the lobes imbricated in the bud; petals usually 5, imbricated in the bud, or 0; disk annular, fleshy, more or less lobed, with a free margin; stamens 4—10, usually 7 or 8, inserted on the summit or inside of the disk, hypogynous; filaments distinct, filiform, commonly exserted in the staminate, shorter and generally abortive in the pistillate flower; anthers oblong or linear, attached at the base, introrse, 2-celled, the cells opening longitudinally; ovary 2-lobed, 2-celled, compressed contrary to the dissepiment, wing-margined on the back; styles 2, inserted between the lobes of the ovary, connate below and divided into 2 linear branches stigmatose on their inner surface; ovules 2 in each cell, collateral, rarely superposed, ascending, attached by their broad base to the inner angle of the cell, anatropous or amphitropous; micropyle inferior. Fruit composed of 2 samaras separable from a small persistent axis, the nut-like carpels compressed laterally, produced on the back into a large chartaceous or coriaceous reticulated obovate wing thickened on the lower margin. Seed solitary by abortion, or rarely 2 in each cell, ovoid, compressed, irregularly 3-angled, ascending obliquely, without albumen; seed-coat membranaceous, the inner coat often fleshy; embryo conduplicate; cotyledons thin, foliaceous or coriaceous, irregularly plicate, incumbent or accumbent on the elongated descending radicle turned toward the hilum.
A family of two genera, one widely distributed, the other, Dipteronia, distinguished by the broad wings encircling the mature carpels, and represented by a single Chinese species.