The Problems of Plant Distribution 271 



plant growth, and only particularly hardy plants 

 can survive. First in importance are the grasses, 

 which cover the entire eastern portion with a close 

 turf, but among these there also grow many beau- 

 tiful flowers which in spring and summer dot the 

 prairie with spots of vivid color. Only along the 

 water courses, or in sheltered gullies, can trees find 

 a foothold. The soil of these eastern prairies is ex- 

 tremely fertile, and now they are the granary of 

 the whole country, immense fields of corn and wheat 

 covering the plains which a generation ago were 

 unbroken prairie sod. With the rapidly diminishing 

 rainfall westward, the close turf of the eastern prai- 

 ries gives place to arid expanses, dotted with 

 bunch-grasses mingled with low cacti, sage-brush, 

 and other outposts of the true desert lying still 

 further west. These arid plains which have risen 

 very gradually, end abruptly in many places, the 

 Rocky Mountains rising steeply from the plain 

 and forming the beginning of the great complex of 

 mountain and desert which reaches from the great 

 plains to the Pacific. 



The Western Flora. This western third of the 

 continent is in many ways the most interesting of 

 all to the botanist, as it presents a far greater va- 

 riety of conditions than prevails in the eastern half 

 of the continent. For the most part it is a region 

 of light rainfall, and much of it is a true desert 

 where such plants as can survive are extremely 

 modified. Some mountain valleys are well watered, 



