Plants of Different Regions. 347 



stems bore large leaves which remained green throughout 

 the summer. It seemed to possess no special provision to 

 prevent transpiration, for the leaves on severed branches 

 soon withered. It was able to survive and remain green 

 and turgid because of its extraordinarily long roots, which 

 absorbed moisture at great depths. 



The deserts of our own country are characterized by 

 Yuccas, thick-leaved Agaves, and cacti, whose succulent 

 green stems assume various forms from oval and globose 

 to columnar, the latter forms sometimes rising to the height 

 of trees (Fig. 169). In the cacti, the leaves are reduced 

 to protective spines, and the chloroplasts are borne in the 

 superficial cells of the cortex of the stems, while the major 

 part of the tissues of the stem serves as a storehouse for 

 water. 



218. Vegetation of Mountain Heights. The physical 

 conditions vary materially from the base to the summit of 

 high mountains. As a rule, the amount of precipitation 

 increases up to a certain height, the atmospheric pressure 

 diminishes as altitude increases, and the amount of heat 

 energy absorbed by the atmosphere also diminishes ; the 

 intensity of illumination, however, increases with the alti- 

 tude. It follows from these conditions that objects exposed 

 to the direct rays of the sun warm up quickly, but on the 

 withdrawal of sunlight they also quickly radiate their heat 

 into space. 



The amount of transpiration from plants increases with 

 the elevation because of the increased illumination and 

 decreased atmospheric pressure, together with the increased 

 force of the wind at successively greater heights. At the 

 same time the amount of absorption by the roots is apt to 

 be lessened by the relatively low average temperature of 

 the soil. 



