286 EREMOPHYTES SECT, xi 



Although recent investigations have enriched our information in 

 regard to the prairies and have supplied floristic descriptions of diverse 

 associations, very little has been done that bears on the oecology of the 

 growth-forms present. Among the different prairies considerable differ- 

 ences reveal themselves. Certain eastern prairies should perhaps be 

 properly regarded as mesophytic. Those west of Chicago, according 

 to Cowles, 1 are rather allied to meadow, and differ entirely from steppe. 

 Mayr 2 states that in places there is sufficient moisture for prairie to bear 

 forest, and he is of opinion that the eastern parts were originally clad 

 with forest which has been destroyed by prairie-fires ; and it is a fact that 

 west winds prevail precisely at the season of prairie fires, namely in 

 September and October. 



According to Pound and Clements 3 the northern prairies are not 

 essentially different from the southern. But in passing from west to 

 east different belts reveal themselves, and correspond to the degree of 

 humidity. The foot-hills at the base of the Rocky Mountains are clothed 

 with Artemisia-steppe. To the east of these there succeeds a belt of 

 sand-hills forming typical steppe, with ' bunch-grasses ' and some 

 xerophytic shrubs and undershrubs. The dominant grasses are Andro- 

 pogon scoparius, Aristida purpurea, A. basiramea, and Sporobolus cuspi- 

 datus. Still farther eastwards lies true ' prairie ' formed by tufted grasses. 

 Pound and Clements describe this vegetation as mesophilous, and only on 

 the high hills as xerophytic. They mention several associations : 



Prairie-grass association, with Sporobolus asperifolius, Koeleria 

 cristata, Eatonia obtusata, and Panicum Scribnerianum. 



Buffalo-grass association, with Bulbilis on loam, and Bouteloa on 

 sand. 



Many associations are apparently to be distinguished. For example, 

 Shantz 4 mentions associations (which he terms 'formations') of Bou- 

 teloua oligostachya, of grama-grass, and of Muhlenbergia gracillima ; in 

 these he recognizes smaller communities, and shows that they change 

 in appearance with the season. 5 Tuberous plants are common on prairies, 

 according to Thornber. 6 



In reference to the various types of western grasslands ('plains') 

 in Texas, reference should be made to Bray's works ; 7 but it may be 

 mentioned that in these plains ' pure formations of prairie annuals ' occur, 

 that an annual vegetation ' sweeps as a wave over the prairie in the 

 early spring ', and that the annual individuals ' mass themselves in pure 

 formations of incredible compactness and extent '. 



Grass-steppe (Pampas) in South America. 



The pampas represent another great grass-steppe tract. The name 

 is a Quichua term, and signifies 'grass-clothed, completely treeless, level 

 plains '. 8 The pampas occupy the vast, rockless, alluvial, South- American 

 plains that stretch from the Atlantic coast to the Andes, and from Pata- 

 gonia to the forests of Paraguay and Brazil. The boundless, level or 



1 Cowles, 1901 b. l Mayr, 1890. 3 Pounds and Clements, 1898. 



4 Shantz, 1906. 5 Concerning prairies in Kansas, see Hitchcock, 1898. 



6 Thornber, 1901. ' Bray, 1901. 8 Brackebusch, 1893. 



