68 A Study of the Vegetation of 



Alyssum alyssoides Festuca megalura 



Arabis sparsiflora Platyspermum scapigerum 



Arabis holboellii Ranunculus glaberrimus 



Athysanus pusillus Sedum douglasii 



Bromus brizaeformis Selaginella wallacei 



Cogswellia macrocarpa Tellima tenella 



Draba verna Tellima parviflora 



Eriogonum niveum Thysanocarpus curvipes 

 Eriogonum compositum 



A number of the above species form more or less well de- 

 veloped socies. Cogswellia gormani, one of the earliest of pre- 

 vernal plants, though inconspicuous as an individual, grows in 

 such great numbers on the thin soils as to become quite promi- 

 nent in early spring. When 500-800 of these plants occur in a 

 square meter, as is often the case, with perhaps a third of them 

 in blossom, they indeed make the landscape appear as if covered 

 with " salt-and-pepper." 



Cogswellia grayi is much more important than C. gormani. It 

 covers large areas of rim-rock almost to the exclusion of other 

 species. Its large umbels of yellow flowers make it very con- 

 spicuous. 



Moist places on the thin soils are dominated for a time in early 

 spring by great patches of Ranunculus glaberrimus, a small plant 

 but one with large and showy flowers. 



The Poa-Polygonum associes gives way to the community 

 dominated by Agropyron. The chief cause for the transition is 

 the accumulation of soil and the presence of rock crevices of 

 sufficient depth and width to furnish soil and water for the deep- 

 rooted bunch-grass. The early stage of the Agropyron con- 

 sociation, already described, well illustrates this change. Often 

 however, in the rim-rock throughout the less arid region, the 

 transition is more abrupt. Belt transects only a few meters long 

 often show zones of vegetation dominated by the characteristic 

 species of the Poa-Polygnum, Agropyron, and Festuca com- 

 munities respectively. These zones extend in a parallel manner 

 along the upper edge of rocky outcrops. Below the transition 



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