Habit and leaf form. Small trees, or shrubs (with silky young shoots); resinous. ‘Normal’ plants. Leaves deciduous; alternate; petiolate; simple. Lamina entire; elliptic oblong, or lanceolate; pinnately veined; cross-venulate; cuneate at the base. Leaves exstipulate. Lamina margins entire. Vegetative buds scaly.
Leaf anatomy. Stomata present; anomocytic. Hairs present; eglandular and glandular; multicellular. Unicellular hairs unbranched. Complex hairs (glands) present; clavate.
Lamina with secretory cavities. Secretory cavities containing resin (in the veins, also in the petioles). The mesophyll containing calcium oxalate crystals. The mesophyll crystals druses.
Stem anatomy. Secretory cavities present; with resin (in the pith). Cork cambium present; initially superficial. Nodes tri-lacunar. Internal phloem absent. Secondary thickening developing from a conventional cambial ring. The secondary phloem stratified into hard (fibrous) and soft (parenchymatous) zones. ‘Included’ phloem absent. Xylem with libriform fibres; with vessels. Vessel end-walls simple. Primary medullary rays narrow (uniseriate, or a few biseriate). Wood parenchyma scanty paratracheal (to vasicentric and terminal). Sieve-tube plastids S-type.
Reproductive type, pollination. Plants dioecious (mostly), or polygamodioecious (occasionally having perfect flowers intermingled with males). Gynoecium of male flowers absent. Pollination anemophilous.
Inflorescence, floral, fruit and seed morphology. Flowers precocious, aggregated in ‘inflorescences’; in catkins. The ultimate inflorescence unit cymose. Inflorescences axillary; erect, complex catkinlike spikes, each of numerous reduced dichasia; pseudanthial (the staminate flowers interpreted as borne in three-flowered cymules of 1–4(–5) stamens each, the cymules forming clusters of 3–12(–15) stamens which could be taken as single flowers), or not pseudanthial (female inflorescences). Flowers bracteate; ebracteolate, or bracteolate (if the female ‘perianth’ is interpreted as bracteoles); small (inconspicuous). Free hypanthium absent. Hypogynous disk absent.
Perianth sepaline (female flowers), or absent (in male flowers, and in females if the ‘sepals’ are interpreted as ‘bracts’); of the female flowers (3–)4(–8); of female flowers joined; 1 whorled. Calyx (i.e. the female ‘perianth’, alternatively interpretable as ‘bracts’ or ‘bracteoles’) (3–)4(–8); gamosepalous (of minute, unequal and irregularly disposed, glandular-fimbriate scales, more or less connate basally).
Androecium 1–4(–5), or 3–12(–15) (if the theoretical ‘cymules’ are taken as flowers). Androecial members free of one another. Androecium exclusively of fertile stamens. Stamens 1–4(–5), or 3–12(–15); shortly filantherous. Anthers basifixed; dehiscing via longitudinal slits; tetrasporangiate. Endothecium developing fibrous thickenings. Anther epidermis persistent. Microsporogenesis simultaneous. Anther wall initially with one middle layer. Tapetum glandular. Pollen shed as single grains. Pollen grains aperturate; 3–6 aperturate; colporate; 2-celled.
Gynoecium 1 carpelled (ostensibly), or 2 carpelled (theoretically, on the evidence of teratological flowers with bilocular ovaries and two styles). The pistil 1 celled. Gynoecium (pseudo) monomerous, or syncarpous (depending on interpretation); of one carpel (ostensibly), or eu-syncarpous (if considered pseudomonomeric); superior. Carpel (if viewed as monomeric) stylate (with a linear, distally expanded style and decurrent stigma, the stigmatic groove and the placenta turned away from the axis and towards the subtending bract); 1 ovuled. Ovary if viewed as pseudomonomeric, 1 locular; sessile. Gynoecium stylate. Styles 1 (slender, curved, caducous); apical. Stigmas 1 (decurrent). Placentation parietal (the ovule attached to the side of the ovary). Ovules in the single cavity 1; ascending; anatropous to amphitropous; bitegmic; crassinucellate. Outer integument contributing to the micropyle. Embryo-sac development Polygonum-type. Antipodal cells formed; 3; not proliferating; ephemeral. Synergids broadly triangular. Endosperm formation nuclear. Embryogeny of no clear type.
Fruit fleshy to non-fleshy. The fruiting carpel indehiscent; drupaceous. Fruit (if considered ‘syncarpous’, indehiscent; a drupe (a slightly asymmetric, somewhat flattened, ‘dry drupe’, with a conspicuously reticulate endocarp). The drupes with one stone. Fruit 1 seeded. Seeds thinly endospermic. Endosperm starchy. Seeds with starch. Embryo well differentiated. Cotyledons 2. Embryo straight (linear).
Physiology, biochemistry. Not cyanogenic.
Geography, cytology. Holarctic and Neotropical. Sub-tropical. Southeast U.S.A. 2n = 32.
Taxonomy. Subclass Dicotyledonae; Crassinucelli. Dahlgren’s Superorder Rutiflorae; Sapindales. Cronquist’s Subclass Hamamelidae; Leitneriales. APG 3 core angiosperms; core eudicot; Superorder Rosanae; malvid; Order Sapindales (as a synonym of Simaroubaceae).
Species 1. Genera 1; only genus, Leitneria.