Chap. VII] COLD TEMPERATE FOREST FORMATIONS 565 



A tropophilous tree in districts with cold winters must display in its peren- 

 nial parts xerophilous structure; if the tree is summer-green, the xerophily 

 is confined to its axes and buds ; if, on the contrary, it is winter-green, 

 the leaves also require an effective protection against transpiration during 

 the winter months. Hence, in their histology, the needles of the larch 

 approach the hygrophilous type of summer foliage of broad-leaved trees 

 more closely than do the needles of the silver-firs and spruces. The slight 

 relic in them of xerophilous structure must be regarded as hereditary, and 

 of a character opposed to their present conditions of existence. 



Like the majority of conifers, the evergreen broad-leaved woody plants of 

 cold-winter districts, with few exceptions, are thoroughly tropophilous in 

 their conditions of existence. Ivy and holly, for instance, thrive best in 

 humid air, and consequently attain their greatest dimensions in Europe near 

 the coasts of the Atlantic Ocean, especially in the west of England, where 

 the famed Forest of Dean consists to some extent of magnificent holly 

 trees. Yet the foliage of all these woody plants is without exception xero- 

 philous, and is constructed on the sclerophyllous plan (Fig. 28). 



2. SPECIAL ILLUSTRATIONS, 

 i. THE FORESTS OF NORTH AMERICA. 



The most extensive and most richly differentiated summer-forest district 

 is that of North America, and, despite the already far-gone destruction, it 

 is still sufficiently preserved, in contrast with old cultivated countries, to 

 permit of a reconstruction of its original physiognomy without the assistance 

 of dubious hypotheses. An account of this kind has been given in a masterly 

 manner by Sargent, the leading authority on the North American forest, 

 from whose work the following considerations are essentially taken, excepting 

 when otherwise specified \ 



A broad strip of conifers, traversing the whole continent from south- 

 east to north-west, from the south of the peninsula of Labrador to Alaska, 

 represents the most northern forest, the north boundary of which coincides 

 everywhere with that of tree-growth. This subpolar forest is thin and 

 poor, its trees never attain large dimensions, owing to the shortness and the 

 low temperature of the vegetative season and to the moderate annual rainfall. 

 In contrast with the more southerly parts of the North American forests, 

 there are but few species of trees. The black spruce (Picea nigra) and the 

 white spruce (Picea alba) predominate. Broad-leaved trees scarcely appear 

 except in valleys, where poplars, dwarf birches, and willows occur. The 

 subpolar strip of forest throughout its whole breadth exhibits the same 

 oecological stamp; but the composition of its flora in the eastern and 



1 Sargent, op. cit. 



