| # The Families of Angiosperms |

L. Watson and M.J. Dallwitz

### Trochodendraceae Prantl

Excluding Tetracentraceae.

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

Leaf anatomy. The leaf lamina dorsiventral. Stomata present; each enclosed by a pair of horseshoe-shaped subsidiaries. Hairs absent. The mesophyll with sclerenchymatous idioblasts (these striking, diverse in form). Minor leaf veins without phloem transfer cells.

Axial (stem, wood) anatomy. Pith heterogeneous (with thick-walled, heavily pitted secretory cells). Cork cambium present; initially superficial. Nodes penta-lacunar (Sinnott), or unilacunar to multilacunar (depending on leaf size?). Primary vascular tissues comprising a ring of bundles (at first, separated by rays 1–3 cells wide); collateral. Internal phloem absent. Secondary thickening developing from a conventional cambial ring. Primary medullary rays mixed wide and narrow. The axial xylem without vessels.

The axial xylem with tracheids (these forming most of the wood); with fibre tracheids. The fibres without spiral thickening. The parenchyma diffuse apotracheal. The secondary phloem later stratified into hard (fibrous) and soft (parenchymatous) zones. ‘Included’ phloem absent.

Reproductive type, pollination. Unisexual flowers present (male plants), or absent. Plants androdioecious. Floral nectaries present. Nectar secretion from the gynoecium (?). Pollination entomophilous.

Inflorescence, floral, fruit and seed morphology. Flowers aggregated in ‘inflorescences’; in cymes (raceme-like). The ultimate inflorescence units cymose. Inflorescences initially terminal; short, raceme-like 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, phytochemistry. Cyanogenic, or not cyanogenic. Iridoids not detected. Proanthocyanidins present; cyanidin and delphinidin. Flavonols present; kaempferol and quercetin. Ellagic acid absent. Aluminium accumulation not found. Sieve-tube plastids S-type.

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 III core angiosperms; peripheral eudicot; Superorder Proteanae. APG IV Order Trochodendrales.

Species 1. Genera 1; only genus, Trochodendron.

General remarks. See Doweld (1998) for carpology and seed anatomy of Tetracentraceae and Trochodendraceae, discussion of taxonomic relationships, and recent relevant references. In terms of the present compilation, Trochodendron differs from Tetracentron in 35 characters, involving vegetative morphology and anatomy, sexuality of plants, numerous floral morphological features, etc., even perhaps in cytology.

Illustrations. • Trochodendron aralioides: Bot. Mag.120 (1894). • Trochodendron aralioides: Hutchinson. • Trochodendron aralioides: © Zoya Akulova (2016).

We advise against extracting comparative information from the descriptions. This is much more easily achieved using the DELTA data files or the interactive key, which allows access to the character list, illustrations, full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, distributions of character states within any set of taxa, geographical distribution, genera included in each family, and classifications (Dahlgren; Dahlgren, Clifford, and Yeo; Cronquist; APG). See also Guidelines for using data taken from Web publications.

Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 1992 onwards. The families of flowering plants: descriptions, illustrations, identification, and information retrieval. Version: 8th June 2026. delta-intkey.com’.

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