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). 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 (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. Abaxial epidermis papillose, or not papillose. Mucilaginous epidermis often present. Stomata mainly confined to one surface; anomocytic. Hairs present; eglandular and glandular; unicellular and multicellular. Unicellular hairs branched and unbranched.
Adaxial hypodermis absent. The mesophyll containing calcium oxalate crystals. The mesophyll crystals druses. Main veins vertically transcurrent. Minor leaf veins without phloem transfer cells (Acer).
Stem anatomy. Stems with solid internodes. Cork cambium present; initially deep-seated, or superficial (usually). Nodes tri-lacunar. Primary vascular tissue in a cylinder, without separate bundles; centrifugal. Cortical bundles absent. Medullary bundles absent. Internal phloem absent. Secondary thickening developing from a conventional cambial ring. The secondary phloem stratified into hard (fibrous) and soft (parenchymatous) zones, or not stratified. ‘Included’ phloem absent. Xylem with fibre tracheids, or without fibre tracheids; with libriform fibres, or without libriform fibres; with vessels. Vessel end-walls horizontal to oblique; simple. Vessels without vestured pits. Primary medullary rays wide (occasionally), or narrow. Wood ring porous; not storied; parenchyma paratracheal. Sieve-tube plastids S-type.
Reproductive type, pollination. 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 unit 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, biochemistry. Not cyanogenic. Alkaloids present (rarely), or absent. Iridoids not detected. 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). Arbutin absent. Saponins/sapogenins present, or absent. Aluminium accumulation not found. Sugars transported as sucrose, or as oligosaccharides + sucrose, or as sugar alcohols + oligosaccharides + sucrose (but sucrose always predominating). C3. C3 physiology recorded directly in Acer.
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 (1998) Eudicot; core Eudicot; Rosid; Eurosid II. APG 3 (2009) Order: Sapindales.
Species about 200. Genera 2; Acer (including Negundo), Dipteronia.
Economic uses, etc. Commercial sources of maple sugar, timbers and numerous ornamentals notable for coloured autumn foliage.
• Technical details (Acer: B. Ent.).
• Acer campestre, A. pseudoplatanus: B. Ent. compilation.
• Acer pseudoplatanus and Acer campestre: Eng. Bot. 320–321, 1864.