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 
