342 TURESSON: SLOPE EXPOSURE AND PSEUDOTSUGA 
Machaeranthera canescens Phacelia heterophylla 
Menitzelia laevicaulis Physaria Geyeri 
Mentzelia integrifolia Ptiloria exigua 
Oreocarya sericea Scutellaria grandiflora 
The vegetation of the upper belt of the south-exposed bank, as 
recorded in the above list, corresponds closely to the Upper 
Sonoran flora, to follow Merriam’s terminology. In Washington 
the Upper Sonoran life area is confined to the interior southern 
part of the State with a rainfall less than 10 inches. However, 
many of the Upper Sonoran plants follow the valley of Columbia 
River and its tributaries, and extend far up in the Arid Transition 
Area. Through the valley of Okanogan River they even reach 
British Columbia. The warm and sunny south-facing slope of the 
river banks is well fitted to support this vegetation of a more 
southern character. They all exhibit xerophily, protecting them 
from desiccation. A well-developed coating of hair is the most 
common means of protection, reaching an extreme development 
in such forms as Eriogonum niveum and Eriophyllum lanatum, which 
have a thick felt on leaves as well as stem. One of the forms 
included in the above list is of more than common interest, namely, 
Physaria Geyeri. This beautiful crucifer is known to occur only 
in the upper region of Spokane Valley, here usually occupying 
the southern river banks. The dense coating of hair is apparently 
a very effective means for depressing transpiration, but it also 
constitutes, in connection with the cushion-like and tufted char- 
acter of the plant, a method of protection against the low winter 
temperature. Flowering in the latter part of April and the whole 
month of May, reflorescence very often takes place in August, 
and this reflorescence lasts often through all winter. A few hours 
of sunshine even in the most severe winter months, December and 
January, is enough to induce it to open its yellow flowers. 
Where the slope terminates upwards and level ground com- 
mences this characteristic flora ceases abruptly, and the dry 
Pinus ponderosa forest with its scanty undergrowth takes its 
place. In some places where the slope is gentle it merges into the 
flora of the gravelly prairie dominated by Balsamorhiza sagittata 
and Gaillardia aristata. 
From the above we find how profoundly slope exposure in- 
