68 A Study of the Vegetation of 
Alyssum alyssoides Festuca megalura 
Arabis sparsiflora Platyspermum scapigerum 
Arabis holboellit Ranunculus glaberrimus 
Athysanus pusillus Sedum douglasiu 
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 gormami, 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. gormam. 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 
68 
