crowned by a pilose capitate connective. Pistillate flowers; sepals, four (three to six), rounded, short; petals, four (three to six), long, acute; staminodes pilose at the apex; ovaries as numerous as the sepals, superior, sessile, surrounded at the base by long hairs, gradually narrowing above into long simple styles; ovules one, rarely two. Head of fruit composed of numerous elongated obpyramidate achenes, surmounted by the persistent style, surrounded at the base by long rigid hairs. Seed solitary, oblong, suspended, containing a thin fleshy albumen and a axile erect embryo. The fruiting heads remain hanging on the tree during winter, the component achenes being ultimately dispersed by the wind.
The dispersal of the pollen in the flowers of plane trees is effected by a peculiar mechanism, which bears some resemblance to that of the yew, and is well described by Kerner.[1]
The planes are readily distinguished by the simple alternate palmately-lobed leaves, the base of the stalks enclosing and concealing the buds. In winter, the conical buds, all lateral, with stipule-lines around the twig and the peculiar narrow sinuous leaf-scars are diagnostic.
The genus is a very ancient one, fossil species[2] having been found in North America in Cretaceous, Eocene, and Oligocene strata. In the Miocene and Tertiary epochs numerous species were spread throughout all Europe, Northern Asia, and North America as far north as the Arctic Circle. In the glacial period those became extinct in the northern parts of their area, and the existing species are confined to Canada, the United States, and Mexico in the New World, and to the Eastern Mediterranean region in the Old World. Their entire absence from Eastern Asia is remarkable, as tertiary plants of circumpolar distribution, which have survived to the present time, are usually found existing both in Eastern North America and in China and Japan.
Six species[3] are now living, which may be conveniently arranged as follows:—
I. Adult leaves glabrous or nearly so, conspicuously toothed in margin.
- 1. Platanus orientalis, Linnæus. Albania, Macedonia, Thrace, Greece, Crete, Cyprus, Rhodes, and Asia Minor.
- Leaves distinctly lobed, the sinuses extending at least one-third the length of the leaf. Fruiting heads bristly, several on the peduncle. Achenes, with long hairs arising not only at the base, but along the body of the achene; apex pyramidal or conic, acute, passing gradually into the long style.
- 2. Platanus occidentalis, Linnæus. Eastern North America from Ontario to Texas.
- Leaves indistinctly lobed, the sinuses not extending one-third the length of the leaf. Fruiting heads smooth, solitary, and terminal on the peduncle. Achenes with basal ring of long hairs, elsewhere glabrous; apex truncate or rounded, with a depression, from which arises a very short style.
- ↑ Nat. Hist. Plants, Eng. Transl. ii. 146 (1898).
- ↑ Cf. L.F. Ward, in Proc. U.S. Nat. Museum, 1888, p. 39, who states that a prominent characteristic of these archaic forms the presence of basal lobes on the leaves. These basal lobes are occasionally met with on the young shoots of the species now living.
- ↑ Platanus glabrata, Fernald, Proc. Am. Acad. xxxvi. 493 (1901), is an imperfectly known species from Coahuila in Mexico.
