Plants of Different Regions. 349 



Up to a certain height the vegetation may be more 

 hygrophilous than in the lowlands, on account of increased 

 rainfall; but higher up the factors which accelerate tran- 

 spiration and lessen the rate of absorption necessitate 

 special provision for the reduction of transpiration, and 

 xerophytes begin to predominate similar to those which 

 characterize desert and polar regions. Trees are stunted 

 in their growth beyond a certain height, and finally an 

 elevation is reached beyond which they are unable to 

 exist. The line where the growth of trees ceases is 

 known as the timber line (see Fig. 190). 



One finds in ascending mountains in the tropics, that 

 the vegetation gradually changes in its aspect, the plants 

 at the base of the mountains being tropical, those farther 

 up are warm temperate and temperate forms, until, at the 

 height of perpetual snows, polar plants appear. Indeed, 

 some of the same species of plants that occur in the po- 

 lar regions may be found near the summit of tropical 

 mountains. 



Where representatives of the same species occur at 

 different elevations, it is found that at successively greater 

 heights the internodes become more and more shortened, 

 the leaves smaller, thicker, and tending to cluster in rosettes, 

 and the roots longer, so that the identity of the same 

 species at the two extremes would not be recognized but 

 for the intervening forms (see Fig. 191). 



When we compare the plants of tropical, temperate, and 

 frigid zones, and of desert and mountain regions, we find 

 that their chief modifications of form and structure have 

 special reference to the available water, and that the same 

 general modifications are found to serve wherever reduction 

 of transpiration becomes necessary. The modification of 

 the protoplasts of arctic plants, so that they are able to 



