284 ELEMENTARY STUDIES IN BOTANY 



veloped a peculiar flora, which contains our most highly 

 specialized drought plants. The numerous forms of cactus 

 are the most characteristic, and associated with them are 

 the yuccas and agaves. Not only is the equipment for check- 

 ing transpiration and for retaining water of the most ex- 

 treme kind, but also there is developed a remarkable armature 

 of spines. 



The dune associations, plains, and cactus deserts are illus- 

 trations of xerophytic associations, but even more extreme 

 xerophytic conditions occur in connection with bare, exposed 

 rocks, and in the subtropical desert regions (as the desert of 

 Sahara) . 



Three conspicuous kinds of mesophytic associations may 

 be selected as illustrations, associations that require a medium 

 amount of moisture, a more or less evenly distributed pre- 

 cipitation, and a soil rich in humus. 



A very characteristic mesophytic association is the meadow, 

 by which is meant areas of natural meadow, not to be con- 

 fused with artificial areas of the same name under the con- 

 trol of man. The appearance of such an area hardly needs 

 description, as the vegetation is a well-known mixture of 

 grasses and flowering herbs, the former usually predominat- 

 ing. Such meadows, of large or small extent, are very 

 common in connection with forest areas and on the flood-plains 

 of streams (Fig. 236). 



The greatest meadows of the United States are the prairies, 

 which extend in general from the Missouri eastward to the 

 forest region of Illinois and Indiana. The vegetation of the 

 prairies is usually composed of tufted grasses and peren- 

 nial flowering herbs. Unfortunately, most of the natural 

 prairie has been replaced by farms, and the characteristic 

 prairie plants are not easily seen. The flowering herbs are 

 often very tall and coarse, but have brilliant flowers, as 

 asters, goldenrods, rosinweeds, lupines, etc. The origin of 

 the prairie has long been a vexed question, which has usually 



