Southeastern Washington and Adjacent Idaho. a 
community instead of a climax one. Like the society it is a case 
of subdominance within a dominance. 
The following table shows the relation of climax and seral or 
development units to the formation, as well as the relation of 
the units of each series to each other and the correspondence of 
units in each series. 
FORMATION 
Climax Units: Seral Units: 
Association Associes 
Consociation Consocies 
Society Socies 
EVAPORATION AND SOIL MOISTURE IN RELATION TO 
SUCCESSION 
Without question, the water relations of the habitat are the 
most important cause of succession. This is especially true of 
grass land communities, and even in forests where light is a con- 
trolling factor, it is well known that the water relation has a 
marked effect upon the tolerance of forest trees. As Fuller (4, 
5) has pointed out, in a study of water conditions, two phases 
of the subject are of importance. 
They are the direct source and amount of the supply and the region 
and cause of the loss. The latter is a climatic, the former largely an 
edaphic problem, for it is evident that plants derive their moisture from 
the soil and lose it into the air, and for the quantitative solution of these 
problems it is necessary to measure the power of the air to extract water 
from the plant; in other words, the evaporating power of the air, and the 
amount of moisture in the soil available for the use of plants. 
Fuller (4, 5) and others have pointed out that the evaporation 
rates in the lower aérial stratum and the range of water content 
in the upper subterranean stratum are the most critical ones. 
Within this (aérial) stratum develop all the seedlings,-and upon their 
death or survival depends the character of the succeeding vegetation. 
Therefore, if the vegetation of an association so affects the evaporation 
rate of this stratum that it permits the survival of seedlings of more 
mesophytic species, it is evident that a more mesophytic association will 
develop, and succession will be accomplished. 
Measurements of the water content of the soil and the evap- 
21 
