BOTANICAL BEGIONS. 675 



none more than the. beauty and comparatively large size that usually cha- 

 racterize their flowers. They are mostly of perennial growth, since, 

 although the severe cold prevailing throughout the greater part of the 

 year is unfavourable to the maturation and preservation of seeds, the 

 thick covering of snow protects established plants from the severe frost ; 

 and it is known that they are arrested in warmer regions where winter 

 frosts prevail without great accumulations of snow, precisely because 

 they are then incapable of bearing the cold, to which they are directly 

 exposed. 



The great discrepancies existing between mountains occurring in the 

 same zone indicate that local circumstances must have most powerful in- 

 fluence in determining the altitudes attained by the various classes of 

 vegetation. We are not in a position to give the real temperatures of 

 regions of altitude with any accuracy in most cases, or these would pro- 

 bably greatly assist in ascertaining the direct causes of aberration ; for 

 differences of temperature certainly accompany the difference of elevation 

 attained by particular forms of plants. Good examples of the influence 

 of the form and local conditions of mountains are furnished by Teneriffe, 

 Ararat, the Himalayas, and the Rocky Mountains of North America. 

 The first is an isolated mountain, exposed to the equalizing influence of 

 the ocean ; the second an isolated mountain situated in the interior of a 

 continent ; the two chains are portions of enormous systems of mountains 

 extending over large regions in the interior of continents. To work out 

 this subject thoroughly, however, it is necessary to observe not only the 

 conditions of different mountains, but those of the different declivities of 

 the same mountain ; since, when great elevations are attained, chains of 

 mountains form the boundaries of local climates, and present different 

 conditions on the two faces. 



Sect. 3. DIVISION or THE GLOBE INTO BEGIONS 

 CHABACTEBISTIC VEGETATION. 



The character of the vegetation of different regions is influenced 

 not merely by climate, but by the more remote causes referred to 

 in the last Chapter, which have led to the distribution of plants 

 over more or less extensive areas, and their restriction within 

 narrow limits in other cases ; further, by the habit of plants, as 

 by a social mode of growth, by size, &c. 



Many attempts have been made to divide the earth's surface 

 into Botanical Eegions, according to their characteristic vegetation. 

 None of these can be regarded as satisfactory ; but perhaps the 

 generalizations of Schouw and Grisebach are, on the whole, those 

 which suggest most to the student. We therefore introduce here 

 a brief account of the regions into which those authors divide the 

 globs. 



Phyto-yeoyraphic Regions. The regions established by Schouw 

 are founded on the following principles : 



2x2 



