| # The Families of Angiosperms |

L. Watson and M.J. Dallwitz

### Aceraceae Juss.

~ Sapindaceae- Hippocastanoideae p.p.

Habit and leaf form. Trees and shrubs; leptocaul. Leaves deciduous; opposite; flat; petiolate; simple, or compound; when compound, ternate (e.g. A. griseum), or pinnate (e.g. Dipteronia, A. negundo); when pinnate, imparipinnate. Lamina when simple, dissected, or entire; when dissected, pinnatifid, or palmatifid; pinnately veined, or palmately veined. Leaves exstipulate. Lamina margins entire, or crenate, or serrate, or dentate. Vegetative buds scaly (mostly), or not scaly (A. negundo and relatives). Vernation plicate. Domatia occurring in the family (seen in many species of Acer); manifested as pits, or pockets, or hair tufts.

General anatomy. Plants with laticifers, or without laticifers. The laticifers when present, in stems, or in leaves and in stems (there being elongated secretory cells or cell series in the axial and leaf phloem, occasionally also in the mesophyll, these sometimes but not always containing detectable latex).

Leaf anatomy. The leaf lamina dorsiventral (usually), or centric. Abaxial epidermis papillose, or not papillose. Mucilaginous epidermis often present. Stomata usually mainly confined to one surface (abaxial); anomocytic. Hairs present; eglandular and glandular; unicellular and multicellular. Unicellular hairs branched and simple. Adaxial hypodermis absent. The mesophyll containing crystals. The crystals druses. Main veins vertically transcurrent. Minor leaf veins without phloem transfer cells (Acer).

Axial (stem, wood) anatomy. Young stems with solid internodes. Cork cambium present; initially deep-seated (in species secreting wax from the branches), or initially superficial (usually). Nodes tri-lacunar. Primary vascular tissues in a cylinder, without separate bundles; collateral. Internal phloem absent. Cortical bundles absent. Medullary bundles absent. Secondary thickening developing from a conventional cambial ring. Primary medullary rays wide (occasionally), or narrow.

The wood ring porous. The vessels moderately small; solitary, radially paired, and in radial multiples (in Acer), or clustered (in Dipteronia). The vessel end-walls horizontal to oblique; simple. The vessels without vestured pits; with spiral thickening. The axial xylem with tracheids, or without tracheids; without vasicentric tracheids; with fibre tracheids, or without fibre tracheids; with libriform fibres, or without libriform fibres; without septate fibres. The fibres without spiral thickening. The parenchyma paratracheal. The secondary phloem stratified into hard (fibrous) and soft (parenchymatous) zones, or not stratified. ‘Included’ phloem absent. The wood not storied.

Reproductive type, pollination. Unisexual flowers present, or absent. Plants andromonoecious, or dioecious, or androdioecious. Gynoecium of male flowers vestigial, or absent. Pollination entomophilous.

Inflorescence, floral, fruit and seed morphology. Flowers aggregated in ‘inflorescences’; in fascicles, in racemes, and in corymbs. The ultimate inflorescence units racemose. Inflorescences terminal, or axillary; racemose, corymbose or fasciculate. Flowers regular; 4 merous, or 5 merous; cyclic. Floral receptacle with neither androphore nor gynophore. Free hypanthium absent. Hypogynous disk present, or absent (rarely); of separate members, or annular (annular or lobed, or reduced to teeth, rarely absent).

Perianth with distinct calyx and corolla, or sepaline (rarely, the corolla missing); 4–5, or 8–10(–16); 1 whorled, or 2 whorled; isomerous. Calyx 4, or 5(–8); 1 whorled; polysepalous, or gamosepalous. Calyx lobes when gamosepalous, markedly longer than the tube. Degree of gamosepaly (maximum length joined/total calyx length) 0–0.25. Calyx regular; not persistent; imbricate. Corolla when present, 4, or 5, or 8–10 (rarely); 1 whorled; (when present, as is usual) polypetalous; imbricate; green to white; plain. Petals shortly clawed.

Androecium (4–)8(–12) (commonly with the two median members suppressed). Androecial members free of the perianth, or adnate (to the corolla); all equal; free of one another; 1 whorled (by suppression of the inner whorl), or 2 whorled. Androecium of male flowers exclusively of fertile stamens. Stamens (4–)8(–10); isomerous with the perianth, or diplostemonous, or triplostemonous. Anthers dorsifixed (slightly), or basifixed; versatile, or non-versatile; dehiscing via longitudinal slits; introrse; tetrasporangiate. Endothecium developing fibrous thickenings. Anther epidermis persistent. Microsporogenesis simultaneous. The initial microspore tetrads tetrahedral, or isobilateral. Anther wall initially with one middle layer, or initially with more than one middle layer. Tapetum glandular. Pollen monosiphonous; shed as single grains. Pollen grains aperturate; mostly 3 aperturate; porate, or colporate; 2-celled.

Gynoecium (in female flowers) 2 carpelled. The pistil 2 celled. Gynoecium syncarpous; synovarious to synstylovarious; superior (usually compressed laterally, at right angles to the septum). Ovary 2 locular. Gynoecium median. Ovary sessile. Styles 2; apical. Stigmas 2; dry type; papillate, or non-papillate; Group II type. Placentation axile. Ovules 2 per locule; pendulous; when not orthotropous, apotropous (Engler); with dorsal raphe; collateral, or superposed; non-arillate; orthotropous to anatropous; bitegmic; crassinucellate. Outer integument not contributing to the micropyle. Endothelium not differentiated. Embryo-sac development Polygonum-type. Antipodal cells formed; 3; not proliferating; ephemeral. Synergids at least sometimes hooked. Endosperm formation nuclear.

Fruit non-fleshy; a schizocarp. Mericarps 2; samaroid (each conspicuously one-winged, from one side only in Acer, all round in Dipteronia). Dispersal unit the mericarp. Dispersal by wind. Seeds non-endospermic. Embryo well differentiated. Cotyledons 2 (flat or plicate); irregularly folded, or rolled. Embryo chlorophyllous (1/8); curved. The radicle lateral. Polyembryony recorded (in one species).

Seedling. Germination phanerocotylar, or cryptocotylar (rarely). Primary root persistent.

Physiology, phytochemistry. C3. C3 physiology recorded directly in Acer. Sugars transported as sucrose, or as oligosaccharides + sucrose, or as sugar alcohols + oligosaccharides + sucrose (but sucrose always predominating). Not cyanogenic. Alkaloids present (rarely), or absent. Arbutin absent. Iridoids not detected. Saponins/sapogenins present, or absent. Proanthocyanidins present; cyanidin, or cyanidin and delphinidin. Flavonols present; kaempferol and quercetin, or kaempferol, quercetin, and myricetin. Ellagic acid present (definitely or dubiously, in 5 Acer species), or absent (one Acer species, Dipteronia). Aluminium accumulation not found. Sieve-tube plastids S-type.

Geography, cytology. Temperate. North temperate, and tropical mountains. X = 13.

Taxonomy. Subclass Dicotyledonae; Crassinucelli. Dahlgren’s Superorder Rutiflorae; Sapindales. Cronquist’s Subclass Rosidae; Sapindales. APG III core angiosperms; core eudicot; Superorder Rosanae; malvid. APG IV Order Sapindales (as a synonym of Sapindaceae).

Species about 200. Genera 2; Acer (including Negundo), Dipteronia.

General remarks. A segregate of Sapindaceae (q.v.), differing from them in the opposite leaves, racemose inflorescence units and the lateral seed radicle, supported by ‘esoteric characters’ of doubtful taxonomic value in view of restricted sampling (non-storied wood, dry stigmas).

Economic uses, etc. Commercial sources of maple sugar, timbers and numerous ornamentals notable for coloured autumn foliage.

Quotations.

Under the cool shade of a sycamore
I thought to close mine eyes some half an hour
(‘Love’s Labour’s Lost’, v., 2)

A living river by the door,
A nightingale in the sycamore
(Robert Louis Stevenson, ‘Wishes’)

Illustrations. • Le Maout and Decaisne: Acer. • Acer tenellum: Hook. Ic. Pl. 19 (1889). • Acer campestre, A. pseudoplatanus: B. Ent. compilation, 1824–35. • Acer pseudoplatanus and Acer campestre: Eng. Bot. 320–321, 1864. • Acer rufinerve var. albo-limbatum: Bot. Mag. 95 (1869). • Acer velutinum, as A. insigne: Bot. Mag. 109 (1883). • Dipteronia sinensis: Hook. Ic. Pl. 19 (1897).

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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