1914] VESTAL— PRAIRIE VEGETATION 397 



similar 



diffusa Wats 



may 



represent a distinct association of strongly alkaline or saline habi- 

 tats, or perhaps a primitive growth of alkaline soil. 



THE PLAINS RUDERAL ASSOCIATION 



Open waste formation. — Pound and Clements 13, p. 412. 

 Plains ruderal formation. — Shantz 17, p. 182. 



The first plants to invade disturbed ground in the area studied 

 are annuals, practically all of the plains region. The more impor- 

 tant species are Argemone intermedia Sweet, Euphorbia marginata 

 Pursh, Solanum rostratum Dunal, Festuca octoflora Walt., Lappula 

 occidentalis (Wats.) Greene, Plantago Purshii R. and S., Ambrosia 

 psilostachya DC, Boebera papposa (Vent.) Rydb., Helianthus 



# 



petiolaris Nutt. Ambrosia trifida integrifolia (Muhl.) T. and G. is 



moist 



mountains 



sweet, clover (Melilotus alba Desv.) is becoming very frequent in 

 disturbed situations. In such habitats, as in the eastern part of 

 the prairie, it is apparently more successful than native ruderals. 

 The Russian thistle (Salsola Tragus L.) behaves as if it were a 

 successful native ruderal. having: become thoroughly established. 



THE PRIMITIVE BUNCH-GRASS ASSOCIATION 



Calamovilfa longifolia consocies. — Shantz 17, p. 46. 

 Sporobolus cryptandrus (growth). — Shantz 17, p. 46. 



Certain of the grasses of the prairie region are very abundant 

 in new growths or in open or sterile situations, as contrasted with 

 other grasses which are found only in more permanent, closed asso- 

 ciations, or in mesophytic stations or in soils of considerable humus 

 content. Among these xerophytic grasses distinctive in primitive 

 growths are Elymus canadensis L., Eriocoma cuspidata Nutt., Cala- 

 movilfa longifolia (Hook.) Hack., Sporobolus cryptandrus (Torr.) 

 Gray, and Panicum virgatum L. Stipa Vaseyi Scribn., though 

 infrequent in the immediate area, occupies many similar situations 

 throughout the Colorado mountain-front. All of these can grow 



