484 BOTANICAL GAZETTE [MAY 
these differences are sufficient to be important factors in causing succession, 
at — through the earlier stages, where light does not play an important 
These conclusions are supported by adequate data obtained in a region 
eatabiting a wide range of conditions, with successions comprising a consider- 
e number of stages, and agree closely with the conclusions of the reviewer 
drawn from data obtained in northern Indiana.” These conclusions meet 
with the approval of CLEMENTs,”* who admits evaporation to be a cause of 
succession since it affects the available moisture supply of the habitats. 
Another investigation of the same moisture factors by WEAVER and 
THEIL,” while primarily concerned with contrasting the evaporating rates 
and soil moisture conditions of forest and grassland and demonstrating the 
greater xerophytism of the latter in both Minnesota and Nebraska, agrees 
perfectly in its conclusions regarding the relationship of these factors to suc- 
cession with those of WEAVER already cited. It would also appear from the 
data contained in this report that the rather high evaporating power of the 
air in these grassland communities, together with the frequent lack of growth 
water during the growing season, may in a large measure account for the 
absence of trees in these regions except along the streams or in other more 
humid situations. The investigation thus forms a contribution to our 
scanty knowledge of the factors involved in causing the development of 
prairies. 
GATES,’ measuring the evaporating power of the air in various plant asso- 
ciations in Michigan, has obtained data that are quite similar to those of the 
investigators cited, but he reaches an almost directly opposite conclusion that 
the different rates of evaporation are the result and not the cause of succession. 
This disagreement with the conclusions of WEAVER and with those of the 
reviewer, both supported by larger quantities of data, seems to be due not so 
much to a confusion of cause and effect as to the facts that (1) GATEs’s investi- 
gation was conducted in a region much more humid than those studied by the 
other workers, as shown by maximum rates of evaporation obtained by WEAVER 
being three times and those by the reviewer at least twice those shown in 
Michigan; (2) the more humid climate exhibits a successional series much 
shorter than those in Washington and Indiana; and (3) GATEs does not con- 
sider soil moisture conditions which would probably show all of his habitats 
to be decidedly mesophytic. 
20 Bot. Gaz. 58:232. 1914. 
2* CLEMENTS, F. E., Recent investigations on evaporation and succession. Plant 
World 20:357-361. 1917. 
22 WEAVER, J. E., and Turet, A. F. Piet ae studies in the tension zone between 
prairie and woodland. Bot. Survey Neb. N.S. 1 1917. 
@ Gates, F. C., The relation between evaporation and plant succession in a given 
area. Amer. Jour. Bot. 4:161-178. 1917. 
