314 Introduction to Botany. 



rare, so plants may be expected to have need under like 

 circumstances of increasing their surfaces for the more 

 rapid absorption of oxygen. But in nature, at high eleva- 

 tions, there are many circumstances which must be taken 

 into account by plants. The rarefaction of the atmosphere 

 increases illumination, radiation, and transpiration, and 

 these circumstances tend to modify the influence of 

 decreased pressure. 



Fig. 164. 



Showing the north slope (on the right) of a hill densely wooded, while the south 

 slope (on the left), which is exposed to the drying summer winds, is destitute of 

 trees. 



198. Effect of Winds. — The wind has much to do in 

 determining the kinds of plants which shall grow in a wind- 

 swept locality, and the wind also brings about adaptive 

 changes by causing a diminished growth of the plant as a 

 whole, and notably of the leaves ; but these modifications 

 are probably brought about indirectly through the increase 

 in transpiration caused by winds. Instances are common 

 where the windward side of a hill is nearly or quite 

 destitute of trees, being clothed merely by low-growing 

 herbs, grasses, and shrubs (Fig. 164). 



