1921] CRIBBS—TILIA AMERICANA 305 
on the prairie, where the factors in general approximate those of 
the open sands of Lake Michigan, except for the soil type. The 
high maximum is attributed to the late morning rise and the failure 
to develop a saturation deficit in the leaves; the factors tending 
to this end all being impotent throughout the day, namely, high 
temperature, high evaporation, low relative humidity, high wind 
velocity, and wide open stomata. A light precipitation in the 
early afternoon was accompanied by a lowering of temperature 
and evaporation and an increase in relative humidity to a point 
approaching saturation, while the transpiration index fell more 
slowly than was commonly true under such circumstances. 
Transpiration curve no. 2 represents the relative foliar water 
loss for July 21. This curve is characterized by a very early rise 
because the weather was at that time clear. The rapid ascent was 
interrupted at 8:00 A.M. owing to the influence of relative humidity. 
At this time there was a period of cloudiness for an hour or 
more, when a thunder shower passed just southward without 
giving precipitation at the station. At 11:00 A.M. the transpiration 
index had attained its maximum, and conspicuous depreciation 
between the curves of water loss and evaporation was then devel- 
oped. There occurred then a period of six hours when the factors 
for high transpiration increased, but the index nevertheless con- 
tinued to fall throughout the period. No visible wilting took 
place on this day, and there appears no evident reason why the 
transpiration index should not have exceeded that for July 16, 
judging from the relative humidity, evaporation, and temperature. 
The difference might be attributed to the less amount of growth 
water (4.537 per cent) in comparison with that of the latter date 
(5.910 percent). I do not believe, however, that the quantity of 
_ available water influenced the transpiration very strongly in this 
instance, for, as already stated, higher indices were regularly 
attained on the open sands where the growth water was as low as 
1.25 per cent. It would seem that the depression takes place at 
the time the reserve water, accumulated during the night, and held 
by the translocating system and leaves, plus that continually being 
added by absorption from the root system, is equalized by diffusion 
from the leaf. If the water content of the soil is not a limiting 
