4 OUR FORESTS AND WOODLANDS 



specific nature according to local climatic con- 

 ditions. Amid the marvellous luxuriance and 

 vegetative energy of tropical forests some woods, 

 almost evergreen in their wealth of foliage, are 

 to be found consisting of three tiers of trees 

 or shrubs, varying from about 20 to 40 up to 

 nearly 250 feet in height, while others, consist- 

 ing of characteristically deciduous trees, have an 

 underwood of lofty bamboos, which throw up, 

 within their annual growing period of about five 

 months, huge culms sometimes attaining a height 

 of over 100 feet. Within the temperate zones 

 both the variety of trees and the luxuriance of 

 their growth become very noticeably less. To- 

 wards the polar regions, and at the higher eleva- 

 tions of lofty mountain ranges, the natural 

 covering of forest consists mainly of Pines and 

 Firs, Birches, Maple, and similar hardy kinds of 

 trees. Even there, these all become of a dwarfish 

 and slow-growing habit, in marked contrast with 

 their dimensions and rate of growth under more 

 favourable conditions as to climate. In every con- 

 ceivable respect the vegetation and the habit of 

 growth found towards the polar limits of tree-forest 

 form the very antithesis of what obtains in the 



