Southeastern Washington and Adjacent Idaho. 107 
Often the gregarious growths almost exclude other plants. | Its 
large, dark-green, shiny, resinous leaves, more than a foot long 
and as many as 20 on a single individual, are no less characteris- 
tic than its numerous, large yellow flowers. A single plant may 
occupy an area of more than 2 square feet although 20-30 plants 
often occur in a single quadrat. It possesses a strong tap root, 
5-9 inches in circumference, which may penetrate the soil to a 
depth of over 6 feet. 
The wet meadow is gradually replaced upon its higher lying 
borders by typical prairie species. 
It should be mentioned in this place that in the drier regions 
large areas of low-lying ground are dominated by dense growths 
of rye-grasses, especially Elymus condensatus. In low ground 
such as dried basins of shallow ponds and where the soil is alka- 
line, socies of salt grass, Distichilis spicata, or of Spartina gracilis 
are to be found. 
The Flood-plain Forest Associes 
The flood-plains along the streams, because of the seasonal dis- 
tribution of the precipitation, are peculiarly apt to be subjected to 
periodical submergence. Growing in these rich, well-watered, 
alluvial soils are to be found dense thickets of various trees and 
shrubs and occasional groves of poplars and aspens. Since the 
larger streams soon cut canyons into the basalt, the flood-plains 
are usually not extensive and the zone of tree growth is often 
limited, especially westward, to.a mere fringe along the bank of 
the streams (Fig. 35). Likewise many of the smaller streams 
are intermittent and along these thickets of woody species are 
few or entirely absent. A close relationship exists between the 
protection from wind, afforded flood-plain areas by canyon walls 
and hills, and the presence or absence of trees and shrubs. 
Although these flood-plain species usually occur in various 
mictia, however, two communities are rather well defined—the 
Populus consocies and the Salix-Crataegus mictium. 
The Populus Consocies—Small groves of poplars consisting 
of Populus tremuloides, or more rarely of Populus trichocarpa 
are frequent along the flood plains, especially in the less arid parts 
107 
