284 JOHN EKNST WEAVEK 



while during 68% of the time it was from some southerly direction 

 which struck the southwest slopes. Readings from anemometers 

 placed at these prairie stations (fig. 4) showed, for example, 

 that during the period from May 2 to June 3, 1120 miles of wind 

 passed over the northeast slope at a distance of 50 cm. from the 

 ground while 3.1 times as much (3497 miles) passed over the 

 southwest exposure. In this region, where the annual precipi- 

 tation is only 56 cm., most of which falls in the winter and early 

 spring, were it not for the high water-holding capacity of the 

 silty loam soil, the drying winds would convert the prairies into a 

 semi-desert. In fact the prairies may be divided into the more 

 xerophytic and less xerophytic types, the former occupying 

 exposed south and southwest slopes, the latter north and north- 

 east slopes and the narrow intervening valleys. An examination 

 of 70 sq. m. quadrats, carefully selected at Pullman, Colfax, 

 and Whelan, to represent typical conditions on exposed and pro- 

 tected hillsides, reveals marked differences in the floristic com- 

 position. In table 1 the first horizontal group includes plants 

 equally or nearly equally represented on the exposed and sheltered 

 slopes. It will be noted that the grasses — Festuca ovina ingrata, 

 Agro'pyron spicatu7n, and Koeleria cristata — are not only about 

 equally distributed but occur in all the quadrats, while Poa 

 sandbergii has a tendency to drop out on the north slopes. The 

 second group (A) shows plants which, while occurring on the 

 north slopes are more abundant in the drier situations. It also 

 includes plants (B) that (with the exception of Wyethia amplexi- 

 caulis) rarely occur on the moist hillsides. Group 3 lists those 

 plants which are more abundant in the less xerophytic situations ; 

 and the last group includes plants which rarely occur except 

 on sheltered north and northeast slopes. While the area here 

 considered is very limited in extent, the floristic conditions which 

 it reveals bear out in the main those repeatedly observed in the 

 various prairie areas of the region. 



The atmometers at station No. 4 on the southwest slope were 

 operating in a situation where the plants averaged 112 individuals 

 per sq. m. with 19% bare ground, while at station No. 5, 200 

 plants occurred in each unit area with only 3% of the soil surface 



