308 BOTANICAL GAZETTE [APRIL 
There is the same tendency on the prairie toward an early rapid 
rise in the index, and for a maximum which occurs from one to 
two hours earlier than is characteristic of Tilia at the other stations 
on clay soils. The maximum is relatively very high on the prairie, 
and is followed quite regularly by a saturation deficit which is more 
pronounced than that recorded for any other habitat except upon 
the open dune complex. | 
9 
x 29 
Z 27 
b 25 
5160 23 
tbo 21 
s is 
20 
2 dh 
A | 
ee ie rEetps ad station L for September 6; note exceptionally high transpira- 
f prairie tension zone; transpiration is great although relative 
humidity is high and evaporation low. 
a: 
The prevailing environmental factors show a close similarity to 
those of the open sands. The growth water is low during much of 
the summer, the insolation is correspondingly high, the influence 
of winds is strong and quite constant, prevailing temperatures are 
higher and are associated with a relative humidity which is low 
(fig. ro). Thus the features of these two types of associations 
show remarkable similarity, with the exception of the soil composi- 
tion and growth water. The growth water falls rapidly during the 
months of August and September and approaches the dune con- 
dition. These factors in conjunction exert an influence which 
