RELATION BETWEEN EVAPORATION AND PLANT SUCCESSION 1 75 
obtained in the region, means that the evaporation from the chamae- 
phytic layer is not the fundamental factor in the ecesis of such seedlings. 
The fact that the rate of evaporation from the chamaephytic layer 
is decreased in the development of mesophytism has been demon- 
strated by many investigators: Transeau (5) at Cold Spring Harbor, 
Gleason and Gates (i) in Central Illinois, Fuller (2) near Chicago, 
Weaver (3) in southeastern Washington and adjacent Idaho, and the 
present investigation in northern Michigan all strongly bring out the 
same conclusion. If there is a causal relationship between evaporation 
from the chamaephytic layer and succession, which I believe no one 
disputes, either the decreased evaporation causes plant succession 
or plant succession causes a decrease in evaporation. Dr. Gleason 
and I (i) made the latter interpretation. Fuller (2) says: "the de- 
creased rate of evaporation caused by the heavier vegetation is the 
direct cause of succession between different associations." The 
data of the present investigation indicate that evaporation is changed 
in the course of succession and not preceding it. 
In the succession towards mesophytism a conspicuous feature is 
the fact that the seedlings of the dominant species of a genetically 
higher association commence their development under the conditions 
furnished by the existing association. Instead of a change of evapora- 
tion preceding the development of a different vegetation, that which 
is controlling and changing the rate of evaporation from the chamae- 
phytic layer is the invading dominant species which have successfully 
withstood the conditions imposed upon them by the existing asso- 
ciation. It is quite obvious that they can not change nor control 
conditions before they are present. 
An increase in density of an association, itself, likewise causes a 
decrease in the rate of evaporation from the chamaephytic layer. 
Except a change of dominant species obtain, however, succession has 
not taken place. Variations in evaporation from typical stations of a 
given association in a given area are not likely to be as great as the 
difference obtained between two genetically related associations. If 
one should add to Fuller's statement, previously quoted, to have it 
read: "The evaporation thus controlled and changed is one of the 
principal factors in permitting the development of a different lower 
story vegetation," its validity could be readily appreciated for those 
secondary species whose physiological limitations precluded their 
development in the lower genetic association. The fundamental 
