222 PLANT RELATIONS. 



distances. Where some barrier, such as a fence, lies across 

 the track of the wind, these tumbleweeds may accumulate 

 in great masses. This tumbling over the surface results 

 in an extensive scattering of seeds (see Fig. 120). 



The prairies, so characteristic of the United. States, are 

 regarded by some as belonging to the plains. They cer- 

 tainly are closely related to them in origin, but can hardly 

 be regarded as being included in xerophyte conditions, as 

 the conditions of water supply and soil are characteristically 

 mesophyte, under which head they will be considered. 



157. Cactus deserts. In passing southward on the 

 plains of the United States, it is to be noted that the con- 

 ditions become more and more xerophytic, and that the 

 bunch grasses and sage brush, peculiar to the true plains, 

 gradually merge into the cactus desert, which represents 

 a region whose conditions are intermediate between true 

 plains and true deserts (see Fig. 186). In the United States 

 this characteristic desert region begins to appear in West- 

 ern Texas, New Mexico, Arizona, and Southern California, 

 and stretches far down into the Mexican possessions. This 

 vast arid region has developed a peculiar flora, which con- 

 tains most highly specialized xerophytic forms. The va- 

 rious cactus forms may be taken as most characteristic, 

 and associated with them are the agaves and the yuccas. 

 Not only are the adaptations for checking transpiration 

 and for retaining water of the most extreme kind, but 

 there is also developed a remarkable armature. It is evi- 

 dent that such succulent bodies as these plants present 

 might speedily disappear through the attacks of animals, 

 were it not for the armor of spines and bristles and rigid 

 walls. Study Figs. 38, 39, 40, 187, 188, 189. 



158. Tropical deserts. In such areas xerophyte con- 

 ditions reach the greatest extreme in the combination of 

 maximum heat and minimum water supply. It is evident 

 that such a combination is almost too difficult for plants 

 to endure. That the very scanty vegetation is due to lack 



