The World Flora Online

Trees deciduous (rarely semi-evergreen), monoecious. Branches and leaves with branched or stellate tomentum; bark pale brown, gray, and/or white, smooth, exfoliating in plates. Terminal buds absent; lateral buds ovoid, slightly acute at apex, enclosed by a separate scale at base of petiole. Leaves alternate; stipules basally surrounding stem, deciduous; petiole long, usually enclosing axillary bud at base; leaf blade simple, large, usually palmately lobed and subpalmately veined, margin coarsely dentate. Flowering branchlets leafy only at base, pendulous at least in fruit; inflorescences 1–5(–12), globose-capitate, male and female ones homomorphic, borne on separate branchlets. Male flowers: sepals 3–8, triangular, pubescent; petals as many as sepals, oblanceolate; stamens 3–8; filaments short; anther connective enlarging into a peltate scale. Female flowers: carpels 3–8, free; ovary long ovoid, 1-locular; ovules 1 or 2, anatropous; style elongate, persistent and exserted from inflorescence. Infructescence a capitate or globose coenocarpium composed of numerous achenes. Achenes narrow and long obconical, 1-seeded, base usually with a tuft of villous hairs, apex with persistent style; style and hairs often exserted from infructescence. Seeds linear; endosperm thin; cotyledons heteromorphic.

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Trees , deciduous, to 50 m. Trunks 1-several, erect to prostrate. Bark smooth at first, exfoliating in thin plates, exposing conspicuous mosaic of chalky white to buff or greenish new bark, becoming dark, thick, and fissured with age. Axillary buds: each hidden by swollen base of petiole. Leaves alternate, simple; stipules sometimes persisting, green, prominent, sheathing stem, flaring, margins entire to serrate. Leaf blade palmately (0-)3-7-lobed, base cordate, truncate, or cuneate; surfaces tomentose or glabrescent. Inflorescences axillary, solitary, appearing with leaves; staminate inflorescences with heads soon falling, 1-5, green, sessile, globose; pistillate with heads 1-7, terminal (and in some species lateral), sessile or pedunculate, globose, the whole much elongate and pendulous in fruit. Flowers unisexual, staminate and pistillate on same plants, very crowded, 3-4(-8)-merous, inconspicuous; perianth hypogynous, minute. Achenes maturing in fall, often persisting until spring, tan, club-shaped, quadrangular, with terminal stylar beak, surrounded by numerous hairs; hairs basally attached, thin, unbranched, 2/3 to nearly equal to length of achene.

Plants anemophilous, monoecious, the numerous, tiny fls in unisexual, spherical heads; sep 3–4(–7), distinct or basally connate; staminate fls with tiny pet alternating with the sep, and with a single set of stamens opposite the sep; filaments very short; anthers elongate, the connective prolonged into a peltate appendage; pistillate fls generally without pet, often with small staminodia, and with (3–)5–8(–9) distinct carpels commonly in 2–3 whorls; stigma decurrent along the recurved, linear style and encroaching on the slender ovary; ovule solitary, pendulous, ± orthotropous; fr a spherical head of linear achenes, each achene subtended by many long hairs; seeds with slender, dicotyledonous embryo and scanty endosperm; trees with alternate, stipulate, mostly palmately veined lvs; winter buds at first concealed by the hollow base of the petiole. 1/6–7, most spp. N. Amer., but 2 Asian.

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SELECTED REFERENCES

Boothroyd, L. E. 1930. The morphology and anatomy of the inflorescence and flower of the Platanaceae. Amer. J. Bot. 17: 678-693. Manchester, S. R. 1986. Vegetative and reproductive morphology of an extinct plane tree (Platanaceae) from the Eocene of western North America. Bot. Gaz. 147: 200-226. Santamour, F. S. Jr. 1986. Checklist of cultivated Platanus (planetree). J. Arboric. 12: 78-83. Schwarzwalder, R. N. Jr. and D. L. Dilcher. 1991. Systematic placement of the Platanaceae in the Hamamelidae. Ann. Missouri Bot. Gard. 78: 962-969.

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