54 A Study of the Vegetation of 



30 per cent, of the ground appearing bare by mid-summer. Closer 

 investigation, however, reveals that much of this space was for- 

 merly occupied by prevernal and vernal plants which have by this 

 time become dry and brown. However, relict xerophytic mosses 

 and lichens indicate the more open nature of the ground cover. 



A careful census of the plant population consisting of scores of 

 list quadrats reveals striking differences, not only in the number 

 of individuals, but also in the floristic distribution of species. 

 On an average it was found that south slopes are clothed with 

 about 115 individuals per square meter, while the north slopes, 

 with only about 3 per cent, of unoccupied soil space, showed 

 more than 200 individuals in the same unit area. 



Perhaps the most pronounced difference is the smaller number 

 or total absence of more mesophytic plant-forms which thrive in 

 moist situations. Erythronium grandiflorum, Trillium petiola- 

 tum, Capnorea villosula, Vaccinium caespitosum, and Viola 

 adunca are examples of species characteristic of moist north hill- 

 sides. Numerous species, such as Sidalcea oregana, Circium 

 foliosum, Valeriana edulis, Gentiana oregana, and others seldom 

 occur on south slopes. 



The blue bunch-grass, Festuca ovina ingrata, ranking in im- 

 portance with Agropyron spicatum, is the most characteristic 

 grass of the region (Fig. 48). The slightly pale-green plants are 

 densely tufted into bunches from I to 4 inches in diameter. The 

 low, bushy, setaceous leaf blades scarcely exceed a height of 12 

 inches. It is not unusual to find as many as 10 to 13 of these 

 bunches in a single square meter. The bunch habit, an aspect 

 of prairie vegetation which is found more or less developed in 

 practically all prairie regions, and one which has succeeded in a 

 remarkable degree in withstanding xerophytic conditions, is most 

 pronounced on the drier slopes. In moist situations the clumps 

 are smaller, but I have never found Festuca producing rhizomes. 

 Like the other prairie grasses Festuca flowers by the middle of 

 June. Since its root-system draws the water supply from the 

 upper 1 8 inches of soil, its drying out in July and resumption of 

 growth upon the arrival of the autumn rains can easily be ac- 

 counted for. 



54 



