1885.] A TALK ABOUT SOME TREE LEAVES. 291 



A TALK ABOUT SOME TREE LEAVES. 

 No. I. — Plates III. and IV. 



DESriTE several scientists at our shoulders, no formidable 

 words such as they delight in wiU figure in our two talks 

 with the above title. The twelve leaves depicted either in the two 

 accompanying plates, or in those to follow, are supposed to be 

 scrutinized by a young student as he collects them in the woods, as 

 to their differences, resemblances, or teachinirs. 



Fewer delusions having hold of the popular mind, come oftener 

 under the ken of an editor of such a journal as this, than the belief 

 that the specific name of a tree can at once be determined by the 

 leaf alone. The leaves in the plates now before us bear markedly 

 on this question. In the two plates are depicted the balsam 

 poplar, Populus lalsamifera (Plate iii. fig. 1), and a species of 

 buckthorn, Bhamnus alpina (Plate iv. fig. 3), with leaves practi- 

 cally similar, as well as two species of hazel, Corylus avellana and 

 C. colurna (Plate iv. figs. 1 and 2), with similar outlines. The 

 single leaf of the mulberry tree. Moms nigra (Plate iii. fig. 2), large- 

 lobed, segmented, and hairy, though at first sight clearly specializing 

 the only British representative of its silkworm-feeding neighbour, 

 after all only strictly enforces the theme proposed to be demonstrated. 

 For more than one of the eight cultivated species of poplar have 

 leaves with different characteristics. Thus the mere leaf student 

 notes the similarity betwixt the balsam and black poplars ; both 

 have a like leaf-form shaped like a hen's egg towards the stalk, but 

 tapering to a point at the apex ; in both also the colour of the under 

 surface is pale, reticulated with a network of hairs. But, as Sir 

 John Lubbock has shown, the leaf stalk of the black poplar thickens 

 at its junction with the leaf, thereby allowing the latter to hang not 

 in a horizontal but pendulous fasliion, and so exposing all its 

 surfaces to the influences of the atmosphere. And thus the leaf 

 would appear to be less a characteristic difference of the species, 

 and more an adaptation to its physiological requirements, in some 

 cases marked by geographic peculiarities. For so it is that Professor 

 Schiibeler of Christiania, from whose work on Norwegian physical 

 geography the figures are reduced, first showed that leaves from 

 so-called typical southern forms, like Bhamnus alpina (Plate iv. 

 fig. 3), assume a development both in size and intensity of colour in 

 the Scandinavian summer unknown in their native habitats. Gaston- 

 Bonnier and others have subsequently obtained rich results in 



