216 PLANT [RELATIONS. 



same general character are common in almost all temper- 

 ate regions. 



156. Plains. Under this head are included great areas 

 in the interior of continents, where dry air and wind 

 prevail. The plains of the United States extend from 

 about the one hundredth meridian westward to the foot- 

 hills of the Rocky Mountains. Similar great areas are 

 represented by the steppes of Siberia, and in the interior of 

 all continents. These regions have been regarded as semi- 

 desert areas, but they are found for the most part to be 

 far from the real desert conditions. They are certainly 

 areas of comparative dryness, on account of the dry winds 

 which prevail. 



Taking the plains of the United States as a type, a very 

 characteristic plant physiognomy is presented (see Fig. 

 184), In general, there is a meadow-like expanse, but the 

 vegetation is much more sparse than in meadows, and is 

 much more dense than in deserts. The two characteristic 

 plant forms are the bunch grasses, that is, grasses which 

 grow in great tufts ; and low grayish shrubs, predomi- 

 nantly " sage brush. " Under the shelter of the sage brush 

 or other bush forms, many low herbs succeed in growing. 

 In such areas the growing season is very short, during 

 which time the vegetation looks vigorous and fresh ; but 

 during the rest of the year it is very dull. In some parts 

 the plain is dry enough to permit the growth of the prickly- 

 pear cactus (Opuntia), which may take possession of ex- 

 tensive areas (see Fig. 185). 



Usually there are two rest periods during the year, 

 developed by the summer drouth and the winter cold. As 

 a consequence, the plants of the area are partly spring 

 plants, which are apt to be very brilliant in flower ; and 

 partly the later, deep-rooted forms. Over such areas the 

 transportation of seeds by the wind is very prominent, as 

 the force of the wind and the freedom of its sweep make 

 possible very wide distribution. It is in such areas that 



