Family Trochodendraceae

Trochodendraceae Prantl

Excluding Tetracentraceae

Habit and leaf form. Small trees (without short-shoots, of araliaceous aspect). Leaves evergreen; alternate (but aggregated into pseudowhorls at the branch tips); leathery; long petiolate; non-sheathing; simple. Lamina entire; pinnately veined; cross-venulate. Leaves exstipulate. Lamina margins serrate (serrulate).

Leaf anatomy. Stomata present; each enclosed by a pair of horseshoe-shaped subsidiaries. Hairs absent.

Lamina dorsiventral. The mesophyll with sclerencymatous idioblasts (these striking, diverse in form). Minor leaf veins without phloem transfer cells.

Stem anatomy. Cork cambium present; initially superficial. Nodes unilacunar to multilacunar (depending on leaf size). Internal phloem absent. Secondary thickening developing from a conventional cambial ring. ‘Included’ phloem absent. Xylem with tracheids; without vessels. Primary medullary rays mixed wide and narrow. Wood parenchyma apotracheal (diffuse). Sieve-tube plastids S-type.

Reproductive type, pollination. Plants androdioecious. Floral nectaries present. Nectar secretion from the gynoecium (?). Pollination entomophilous.

Inflorescence, floral, fruit and seed morphology. Flowers aggregated in ‘inflorescences’; in cymes (racemelike). The ultimate inflorescence unit cymose. Inflorescences initially terminal; short, racemelike cymes, the flowers long pedicelled. Flowers bracteate, or ebracteate (?); bracteolate (subtended by several tiny, adnate-decurrent scales at the top on the swollen pedicel tip); small to medium-sized; regular to somewhat irregular; acyclic. The androecium acyclic (spiral). Floral receptacle not markedly hollowed (the torus subconical). Hypogynous disk absent.

Perianth absent (unless represented by the small bracts).

Androecium 40–100 (‘many’ commonly about 70). Androecial members maturing centripetally; free of one another; spiralled. Androecium exclusively of fertile stamens. Stamens 40–100 (commonly about 70); filantherous. Anthers basifixed; non-versatile; dehiscing by longitudinal valves; latrorse; tetrasporangiate; unappendaged (the connective not or scarcely prolonged). Endothecium developing fibrous thickenings. Anther epidermis persistent. Pollen shed as single grains. Pollen grains aperturate; 3(–4) aperturate; weakly colporate (colporoidate); 2-celled.

Gynoecium (4–)7–11(–17) carpelled. The pistil basally (4–)7–11(–17) celled. Gynoecium apocarpous to syncarpous; semicarpous (the carpels in one whorl, laterally connate but distally distinct, conduplicate, nectariferous on the back); partly inferior. Carpel shortly stylate; with a ventrally decurrent stigma; 25–30 ovuled. Placentation marginal. Ovary basally (4–)7–11(–17) locular. Stigmas dry type; papillate; Group II type. Ovules funicled; pendulous (from the broad, oblique top of the carpel); anatropous; bitegmic; crassinucellate. Outer integument not contributing to the micropyle. Endosperm formation cellular.

Fruit non-fleshy; not an aggregate, or an aggregate (more or less, consisting of the laterally coherent but distally distinct carpels). The fruiting carpels more or less coalescing into a secondary syncarp. The fruiting carpel dehiscent; a follicle (the recurved follicles laterally cohering). Fruit if considered syncarpous, dehiscent; a capsule (dorsiventridehiscent). Seeds copiously endospermic. Endosperm oily (and proteinaceous). Embryo well differentiated (but small). Cotyledons 2. Embryo achlorophyllous (1/1); straight. Testa dark brown, or black.

Seedling. Germination phanerocotylar.

Physiology, biochemistry. Cyanogenic, or not cyanogenic. Iridoids not detected. Proanthocyanidins present; cyanidin and delphinidin. Flavonols present; kaempferol and quercetin. Ellagic acid absent. Aluminium accumulation not found.

Geography, cytology. Holarctic. Temperate to sub-tropical. Japan to Formosa. 2n = 38, 40.

Taxonomy. Subclass Dicotyledonae; Crassinucelli. Dahlgren’s Superorder Rosiflorae; Trochodendrales. Cronquist’s Subclass Hamamelidae; Trochodendrales. APG 3 core angiosperms; peripheral eudicot; Superorder Proteanae; Order Trochodendrales.

Species 1. Genera 1; only genus, Trochodendron.

See Doweld (1998) for carpology and seed anatomy of Tetracentraceae and Trochodendraceae, discussion of taxonomic relationships, and recent relevant references.