6i2 The Trees of Great Britain and Ireland 



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 an 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.* ^ 



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 ^ 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 * are now living, which may be conveniently arranged as follows : 



I. Adult leaves glabrous or nearly so, conspicuously toothed in margin, 



1. Platanus orientalis, Linnaeus. 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, Linnaeus. 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. 



1 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 is 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. 



