IIenrv and Flood — The Hislory of the London Plane. 13 



111 all species and varieties of planes, the leaves are remarkably inconstant 

 ill the outline ol: the lobes, which are sometimes entire in margin, sometimes 

 minutely toothed, and at oiher times with large sinuate teeth or lobes. The 

 leaves, two to five in number on a single branch, are all somewhat different 

 in outline. The variation in the occurrence, size, and number of the teeth 

 does not seem to constitute even a varietal character, and is due to unknown 

 causes. The Oriental Plane, judging from numerous cultivated trees iu 

 Britain and from dried specimens of wild trees preserved at Kew, is singularly 

 variable in this respect; and no satisfactory division of this species into 

 geographical forms is possible. Peculiar entire small leaves characterize 

 some planes in Cyprus, but other trees in the island have very dentate leaves. 

 The plane of Kashmir has very large leaves, while that of Greece and Asia 

 Minor is intermediate in size between the Kashmir and Cyprus forms. In 

 P. occidentalis there are several types of foliage which cannot be correlated 

 either with the age of the tree or with the region of distribution, or with any 

 known cause. Some adult trees, for example, bear small leaves, with three 

 distinct lobes, entire in margin except for the terminal point of each lobe. 

 Other adult trees bear large leaves, with indistinct lobes, having numerous 

 small teeth on the margin. 



The base of the leaf, which may be cordate, truncate, or cuneate, cannot 

 be relied on for the discrimination of species, as it is an inconstant character, 

 apparently dependent on the vigour of the branch or of the tiee. In some of 

 the hybrids the form of the base is comparatively fixed ; thus in P. pyrami- 

 dalis it is scarcely ever cordate, while iu P. acerifolia the terminal leaf has a 

 very cordate base. 



What is really specific in the shape of the leaf is the depth of the lobes. 

 The significance of lobed leaves in the life of a tree is obscure; but lobing 

 may have some relation to the demand of the foliage for light, as the gaps 

 between the lobes allow illumination of the layer of leaves beneath. A study 

 of the habitats of variously lobed planes, maples, &c., might elucidate this 

 subject. Three species of Platauus with deeply lobed leaves — P. orientalis, 

 P. racemosa, and P. Wrightii — appear to grow habitually on the banks of 

 streams in full sunlight. The species with the shortest lobes, P. occidentalis, 

 grows in the midst of the broad-leaved forests of the I'niled States, where it 

 seems to be able to bear a considerable amount of shade. 



The extent of the lobing of the leaves being an important character in the 

 discrimination of the various species and hybrids, its accurate measurement 

 is desirable. This is affected by the use of a significant number, referred to 

 as A, which is fixed for any given plane figure, with perimeter jy and area a, by 



the formula A = — • For a circle, which has the minimum perimeter of all 



