340 TURESSON: SLOPE EXPOSURE AND PSEUDOTSUGA 
perfoliata, a Humid Transition plant here having an isolated 
station. The formation is of a somewhat open character, but in 
deeper shade the ground is generally covered by a continuous mat 
of mosses. Epiphytic lichens are common. On the steep de- 
clivity no humus has been able to accumulate, and it is only on 
the lower part of the bank that a rather thick layer of humus is 
to be found, owing its existence to the mass of dead matter which 
is in continuous downward motion. 
The opposite south-facing bank has a very different flora; in 
fact, the contrast is so striking as to make it seem almost unnatural. 
The heavily Pseudotsuga-forested north-facing slope faces a vegeta- 
tion on the opposite bank, which in xerophily rivals the desert flora! 
The greater part of the bank is devoid of trees. Populus tricho- 
carpa_ fringes the edge of the river, but Alnus tenuifolia has 
disappeared. Shrubs of quite different kind from those in the 
corresponding belt of the northern bank form the undergrowth. 
The shrubs are: 
Amelanchier florida Prunus demissa 
Clematis hirsutissima Ribes aureum 
The dominant herbs at the water-edge are: 
Artemisia ludoviciana Coleosanthus grandiflorus 
Asclepias mexicana Euthamia grandiflora 
Aster laevis Geyeri Steironema ciliatum 
Aster oregana 
The declivity proper has two very marked belts, a lower closed 
formation, and an upper and open formation, the difference being 
largely due to the obvious fact that the lower zone is more shel- 
tered against insolation and drying winds than is the upper. A 
continuous turf of various grasses covers the lower belt, and many 
of the plants recorded in the list from the northern slope reappear 
in this relatively sheltered zone. More or less dominant are the 
following additional: 
A pocynum androsaemifolium Erysimum asperum 
Astragalus Purshii Lomatium triternatum 
Delphinium Menziesii Sphaeralcea rivularis 
The most remarkable feature of this zone, however, is the 
