ECOLOGY 



147 



Desert Conditions. — In 

 the deserts of Central Africa 

 and those of the Western 

 United States the conditions 

 of temperature, and especially 

 lack of moisture, are essen- 

 tially the same. We find in 

 both regions plants the leaves 

 of which are either very small 

 or entirely lacking, their place 

 having been taken by spines 

 or thorns. In some plants, 

 Agave, for example, leaves are 

 present, but "are thick and 

 fleshy to hold water. 



Cold Regions. — Here 

 plants, which in lowland re- 

 gions of greater warmth and 

 moisture have a tall form and 

 luxuriant foliage, are stunted 

 and dwarfed; the leaves are 

 smaller and tend to gather 

 in rosettes or are otherwise 

 closely placed for warmth and 

 protection. As we climb a 

 mountain we find the average 



Two plant societies: in the foreground plants living 

 in conditions of much moisture; in the back- 

 ground true mesophytes, a tree society. 



Polar limit of trees, northern Russia. 



