1. Platanus occidentalis L. (sycamore, American plane tree)

P. occidentalis f. attenuata Sarg.

P. occidentalis var. glabrata (Fernald) Sarg.

Pl. 492 d, e; Map 2248

Plants large trees with widely spreading branches, monoecious. Trunks to 50 m tall, the bark flaking off as large scales or thin plates, otherwise smooth, white where recently exposed, greenish gray with age, the trunk thus a patchwork of white and gray areas. Twigs 2–4 mm thick, brown to dark brown or gray, finely hairy, becoming nearly glabrous with age. Leaves alternate, simple. Stipules to 5 cm wide, leaflike, coarsely toothed or lobed, shed during leaf development and leaving a scar visible as a narrow ring extending all the way around the twig. Petioles 1–7 cm long, their bases swollen, hollow, and completely enclosing the buds. Leaf blades 9–25 cm long, 12–33 cm wide, broadly triangular to nearly kidney-shaped, with 3–7 main veins from the base, palmately 3–7-lobed, each lobe broadly triangular and tapered toward the sharply pointed tip, the margins coarsely toothed or entire, the sinuses extending no more than halfway to the leaf base, sometimes mere shallow indentations, the leaf base rounded or broadly angled to deeply cordate, the surfaces pubescent when young with minute stellate hairs, at maturity becoming nearly glabrous or hairy only along the main veins. Inflorescences unisexual heads, these dense, globose, usually occurring singly from the tips of short lateral twigs, pendant; the staminate heads 0.8–1.2 cm in diameter, on stalks 1–2 cm long, greenish yellow; the pistillate heads 0.8–1.2 cm in diameter at flowering, 2.5–3.0 cm in diameter at fruiting, on stalks 3–10 cm long, reddish brown at flowering, tan to yellowish brown at fruiting, usually persisting through the winter. Flowers actinomorphic, hypogynous, the calyx of 3–6 sepals, these 0.3–0.5 mm long, scalelike, free or sometimes united toward the base, hairy, the corolla absent or (on some staminate flowers) of 3–6 extremely minute segments. Staminate flowers with 3–6 stamens and sometimes a reduced abortive pistil, the filaments absent or very short, the anthers attached at the base, with 2 locules, the connecting tissue prolonged into a small hairy peltate appendage at the tip. Pistillate flowers with 3 or 4 small staminodes and 3–9 pistils. Pistils with the ovary superior, 1-locular, the placentation apical. Styles 1 per pistil, elongate, linear, the stigmatic area lateral toward the tip. Fruits achenes, 7–8 mm long, club-shaped, light brown, each with a basal tuft of hairs almost as long as the fruit. 2n=42. April–June.

Common throughout the state (eastern U.S. west to Wisconsin, Nebraska, and Texas; Canada). Banks of streams and rivers, margins of ponds and lakes, and bottomland forests; also roadsides, railroads, and moist disturbed areas.

In natural vegetation, this species is mostly confined to riparian and floodplain forests. However, its ability to colonize bare mineral soils at sunny sites has made it a successful invader of bulldozed habitats, such as roadsides and railroad grades. The achenes, which may remain on the trees in winter, are eaten by many species of birds. Plants in which the main lobes of the leaves are entire or nearly so have been called var. glabrata, a taxon of uncertain merit (Hsiao, 1973; Schwarzwalder, 1986) that has been collected from a few scattered counties in southern Missouri. Plants with obtuse leaf bases have been called f. attenuata, but this variant seems unworthy of formal taxonomic recognition. Older trees develop massive trunks (up to more than 14 m in circumference, in historical reports), and early descriptions of the forests often mentioned massive sycamores. The wood is very resistant to splitting and has been used to make things that must stand up to a great deal of stress, such as ox yokes and butcher’s blocks. The fruits are sometimes inadvertently dispersed by children, who throw the mature, fruiting “itchy-bombs” at each other for entertainment.

The London plane tree, P. ×acerifolia (Aiton) Willd., is a common street tree that apparently does not escape from cultivation. It is probably descended from hybrids between P. occidentalis and P. orientalis L. of Asia Minor. London plane was very heavily used for urban plantings earlier in the last century because of its ability to grow in areas polluted by coal smoke, but it is not nearly as commonly planted as it once was. It differs from P. occidentalis in having somewhat more deeply lobed leaves and pistillate heads in pairs on each stalk.