Plants of Different Regions. 341 



the early settlers were obliged to make clearings with ax 

 and grub hook in order to obtain land suitable for agri- 

 culture. 



But as the rainfall diminishes toward the west and 

 north, we find the forests giving way to prairies, the grasses 



FIG. 185. 



Tillandsia usneoides covering and pendent from Quercus virens. After photograph 



by WEBBER. 



and herbaceous plants which characterize them being able 

 to obtain sufficient moisture from the summer rains to 

 flourish where trees, with their larger demands on moisture, 

 are unable to subsist, excepting along water courses. West- 

 ward from the Mississippi to the Rocky Mountains the rain- 

 fall, which occurs for the most part in the spring, early 

 summer, and fall, steadily diminishes, and the grasses and 



