TURESSON: SLOPE EXPOSURE AND PSEUDOTSUGA 341 
abundance of shrubs. They do not form thickets, but are scat- 
tered widely apart over the area. In some cases, however, one of 
them, Berberis repens, may form a pure growth of some extent. 
The stratum of shrubs is composed of: 
Philadelphus Lewisit 
Rhus Toxicodendron 
Amelanchier Cusickit 
Berberis repens 
Crataegus brevispina 
The remaining part of the bank, the upper zone, is by far the 
most interesting one. As said before, no continuous turf of 
vegetation is found, only scattered individuals often several feet 
apart. The intense insolation and the exceedingly dry winds 
which are so common in eastern Washington in the summer 
months must be considered as the chief causes for this scanty 
xerophytic vegetation. Another factor, which may play some 
part in the difficulty of establishing a closed formation, is the 
mobility of the substratum. Owing to the steepness of the bank, 
gravel is rapidly moving downward. Many plants which probably 
are able to withstand the strong insolation are unable to get a 
foothold, thus giving room to other plants more adapted to the 
moving substratum, but poor in the formation of a continuous 
turf. That many of these plants show a high degree of adaptation 
to the habitat is evident from ‘the fact that they do not occur 
outside of these south-exposed banks. The formation, which 
includes many of the most decorative and at the same time the 
rarest plants in Spokane Valley, shows a composition of the fol- 
lowing nature: 
SHRUBS 
Ramona incana Rhus glabra occidentalis 
HERBS 
Allocarya hispidula 
Carduus undulatus 
Eriogonum niveum 
Eriophylium lanatum 
Chaenactis Douglasii 
Collomia grandiflora 
Cuscuta californica* 
Eriogonum compositum 
Eriogonum heracleoides 
iN abe el ade 
* Infesting Scutellaria grandiflora. 
Cilia aggregata 
Lappula ciliata 
Lithospermum ruderale 
Lomatium Grayi 
Lupinus Pipert 
