XXXVI. HIPPOCASTANACEÆ.
Trees or rarely shrubs, with stout terete branchlets conspicuously marked by triangular leaf-scars, fetid bark, thick fleshy roots, and large scaly winter-buds, the inner scales accrescent with the young shoots and often brightly colored. Leaves opposite, digitately compound, without stipules, deciduous; leaflets 3—9, lanceolate or ovate, serrate, pinnately veined. Flowers polygamo-monœcious, showy, white, red, or pale yellow, on stout jointed pedicels from the axils of minute caducous bracts, racemose or nearly unilateral on the branches of large terminal thyrsi or panicles, appearing later than the leaves, only those near the base of the branches of the inflorescence perfect and fertile; calyx 5 or rarely 2-lobed, the lobes imbricated in the bud, unequal, campanulate or tubular, the lobes imbricated in the bud, mostly oblique or posteriorly gibbous at base; disk hypogynous, annular, depressed, lobed, more or less gibbous posteriorly; petals 4 or 5, imbricated in the bud, alternate with the lobes of the calyx, deciduous, the anterior petal often abortive, unguiculate, the margins of the claw commonly involute; stamens 6—8, rarely 5, generally 7, inserted on the disk, free, unequal; filaments filiform; anthers ellipsoid, glandular-apiculate, attached on the back below the middle, introrse, 2-celled, the contiguous cells opening longitudinally; ovary sessile, oblong or lanceolate, 3-celled, echinate or glabrous, rudimentary in the staminate flower; style slender, elongated, generally more or less curved; stigma terminal, entire, mostly acute; ovules 2 in each cell, borne on the middle of its inner angle, amphitropous, the upper ascending, the micropyle inferior, the lower pendulous, the micropyle superior. Fruit an echinate or smooth coriaceous capsule, 3-celled and loculicidally 3-valved, the cells 1-seeded by abortion, often by suppression 1 or 2-celled, and then 1 or 2-seeded, the remnants of the abortive cells and seeds commonly visible at its maturity. Seeds without albumen, round when one is developed, or, when more than one, flattened by mutual pressure; seed-coat coriaceous, dark chestnut-brown or pale orange-brown, smooth and lustrous, with a broad opaque light-colored hilum; embryo filling the cavity of the seed; cotyledons very thick and fleshy, often conferruminate, unequal, incurved on the short conic radicle, remaining under ground in germination; plumule conspicuously 2-leaved.
The Horsechestnut family is composed of the widely distributed genus Aesculus and of Billia Peyr., a genus of two species of Mexican and Central American trees, differing from Aesculus in its 3-foliolate leaves.