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Platanus comprises seven species, most of which occur in North and Central America, though P. orientalis is native to the eastern Mediterranean region, while P. kerrii Gagnep. is restricted to Vietnam and Laos. Platanus species are large, deciduous trees (P. kerrii evergreen) with distinctive, mottled bark. Branchlets are usually covered with branched or stellate tomentum and have swollen nodes; terminal buds are absent. The leaves are sweet-smelling; adult leaves are typically large, alternate and palmate with three to five (to seven) lobes and palmate venation; juvenile leaves (and adult leaves of P. kerrii) are typically unlobed, but serrated with pinnate venation. The petiole has an enlarged base that partially or completely surrounds the axillary bud. The stipules are typically large and foliose with parallel veins; they encircle the stem and may be lobed or serrated. Flowering occurs in spring; the inflorescences are unisexual and terminal (axillary in P. kerrii) and are composed of one to many spherical heads arranged in a raceme or spike. Each head comprises numerous, rather inconspicuous flowers; staminate heads break up after flowering, but the pistillate heads have a hard, central receptacle. The fruit is a small, hairy achene that detaches from the receptacle. A key to the species is provided by Nixon & Poole (2003).
A number of DNA-based studies have indicated that the Platanaceae and Proteaceae are very closely related, and together with the Nelumbonaceae (sacred lotus), form the order Proteales (APG 2003, Feng et al. 2005). The APG II classification goes on to suggest that the Platanaceae could be merged with the Proteaceae (APG 2003). However, this suggestion has yet to find favour in the wider botanical community.
The importance and interest of the small genus Platanus can be judged by the space afforded it by Bean (1976b), and in particular his account of the London Plane, which extends over six pages – considerably more than the comments on Quercus robur! The controversy over the correct scientific name for the London Plane continues, but the balance currently seems to be tipped in favour of P. ×hispanica Mill. ex Münchh. (see Orchard & Lepschi 2007). This name embraces both F1 hybrids between P. orientalis and P. occidentalis and their descendants in later generations. Some of these are significant clones in their own right and deserve the recognition of a cultivar name. The planes of Greater London have been studied by I.M. Chengappa, whose website (www.aranya.co.uk/planes) is a fascinating source of information about these trees, including several distinct but unacknowledged cultivars.
With the exception of P. orientalis, members of this genus are of limited significance in horticulture and are only likely to be found in specialist collections. The interesting Indo-Chinese P. kerrii and the Mexican P. gentryi Nixon & J.M. Poole and P. rzedowskii Nixon & J.M. Poole are not known in cultivation.
The consistent failure of P. occidentalis in British gardens is attributed to the effects of the anthracnose disease caused by Apiognomonia veneta on young plants already weakened by spring frosts and missing the summer heat of their native eastern North America. It has to be said, unfortunately, that the recent Mexican introductions described below are likely to suffer in a similar way, in areas where P. occidentalis does not thrive. Propagation of planes is by seed, cuttings or grafting.
The planes are very distinct from any other group of trees and constitute in themselves the family Platanaceae, which is allied to the Hamamelid- aceae. There are about eight species, all natives of N. America and Mexico except P. orientalis of S.E. Europe and S.W. Asia and the anomalous P. kerrii from Indo-China. The leaves are alternate and palmately lobed, except in P. kerrii, which has entire, pinnately veined leaves. The arrangement of the main veins in some species is complex; it is often not truly palmate, since the ribs running out to the lobes do not spring from the same point, except in three-lobed leaves, and even in these the point of origin may be well above the base of the blade. The axillary buds in Platanus are concealed in summer by the swollen, hollow base of the petiole, which is furnished with a pair of stipules united at the base into a tube and often leafy at the apex.
The flowers are arranged in dense clusters and are unisexual; the male and female clusters are borne on the same tree but on separate peduncles. The female flowers develop into achenes, each with a large tuft of erect hairs at the base. The ripe fruit-balls are a characteristic feature of the planes; sometimes they are solitary, but more often they are strung two to six together on the pendulous, fibrous peduncle, which is sometimes branched. The fruit-balls persist more or less throughout the winter and, when they break up, release vast numbers of achenes, which are carried over long distances by the wind.
The bark of most species peels off in flakes, and after a stormy spring day the ground beneath a large plane will sometimes be seen strewn with pieces of bark torn off by the wind. This feature is most evident on strongly growing trees. The trunks of old trees become scaly or even fissured, though the branches continue to flake.
Some of the noblest trees of the northern hemisphere belong to this genus. Specimens of the European P. orientalis are known to have trunks 40 ft in circumference, and the American P. occidentalis growing in the Mississippi Valley has trunks about as large, and it occasionally reaches 170 ft in height. They like a deep, moist, loamy soil, and thrive better in the south of England than in the north. They are essentially sun-lovers. Seeds ripen on the London plane and P. orientalis, and germinate readily, but the former does not breed true and is best raised from cuttings, which should be made at the fall of the leaf, of shoots 8 to 12 in. long, with a 'heel' of old wood at the base, and placed under a handlight in a sheltered spot. In nurseries they are usually propagated from stools by layers. Young plants are rather subject to being cut back by frost.
The plane trees of town streets and promenades are under suspicion of causing serious bronchial irritation by shedding the hairs from their leaves, and especially fruits. These break up into minute particles which, floating in the air, are inhaled. Although the alleged evil influences of these particles on the throat and lungs (and even on the eyes and ears as well) were suspected and written about by the ancients – among others by Galen and Dioscorides – they never appear to have deterred either them or later generations from planting the tree freely. There seems to be little doubt that on the Continent it produces, or helps to produce, a catarrhal affection analogous to hay fever. In Britain the crops of fruit are not so large, and probably our damper climate prevents the hairs travelling far from the tree; at any rate, nothing has been proved against the tree to justify its wholesale condemnation.
The planes are susceptible to the fungus disease Gnomonia veneta(platani), once known by the name Gloeosporium nervisequum, given to one of its two asexual (conidial) stages, and usually referred to as the plane-tree wilt disease or plane-tree anthracnose. The first sign of attack is the discoloration of the leaf-tissue between the principal veins; later all the young foliage may be killed, and cankers often develop on the stems if the attack has been severe. This disease is sporadic in its attacks. It is said to be favoured by cold, damp weather in spring and early summer, and to be more prevalent in country districts than in towns. It is not lethal, and the dead foliage is usually replaced later in the summer. But it weakens the tree and renders it unsightly. P. occidentalis is very susceptible, while P. orientalis is said to be immune. P. acerifolia is sometimes attacked, but the clones and seedlings probably vary in their susceptibility to the disease. Reports that P. orientalis is attacked probably refer to P. acerifolia misnamed.