1921] CRIBBS—TILIA AMERICANA 303 
relative humidity and evaporation show curves which indicate 
greater mesophytism at station 7 than at station H, but contrary to 
expectation the transpiration record at the latter is higher through- 
out the period than in the mesophytic forest. The transpiration 
curve was higher than that recorded for the much more open forest 
on the old Lake Chicago bed, where there is a strong prairie influence. 
Fig. 7 shows that, although the transpiration is relatively high, as 
on the sand series, the factors of soil temperature and growth water 
at station J are excessive in their extreme opposite relation to the 
dune condition. Insolation is probably the most influential factor 
leading to a higher transpiration at this place. It would seem 
from the relations worked out between transpiration and stomatal 
aperture by various investigators that the stomata should not. 
prove a limiting factor at the forest station, for the light intensity 
would seem adequate to insure an opening there which is sufficient 
to permit a much greater diffusion from the leaves than actually 
occurs. That increased exposure to light is accompanied by the 
absorption of light rays by the mesophyll which produce heat and a 
greater molecular pressure on the water films in the cell walls, and 
hence leads to a more rapid diffusion of water, seems a reasonable 
explanation and one which is substantiated by the work of Lrv1nc- 
STON (5) on the effect of this factor on transpiration. 
Daily variation in transpiring ability | 
In the former discussion of transpiration on the dunes it was 
pointed out that if the index of foliar transpiring power were to 
be utilized in an endeavor to measure the mesophytism of a plant, 
it should be based on the records of more than a single day, since 
the variability in the foliar transpiring power is very noticeable 
from day to day. There is a great daily fluctuation in the tran- 
spiring power, just as there is a great hourly fluctuation, and both 
hourly and diurnal variations are more pronounced on the dunes 
and prairie than on the forested clays. 
Fig. 9 indicates the transpiring ability as recorded at station 
C on the dunes for July 16, 21, and August 26, and it will be observed 
that on these three days there chanced to be striking differences 
in the course of the curves representing water loss from the leaves. 
