1921] CRIBBS—TILIA AMERICANA 207 
largely due to the effect of winds, which, without affecting the 
humidity, accelerate the loss of water from the porous cup, and may 
to a less degree affect the transpiration rate. The atmometer shows 
greater sensitiveness to the influence of wind than does the leaf. . 
TEMPERATURE.—Temperature exerts a very important influence 
in the transpiration process by affecting the surface tension films of 
the mesophyll cells and changing their rate of molecular diffusion. 
Livincston (6) has referred to temperature as the probable con- 
trolling factor in causing fluctuations in the transpiration stream. 
An increase in temperature accelerates the diffusion from the meso- 
phyll cells, setting up an increase in the vapor pressure within the 
leaf. At the same time it also increases the saturation deficit of the 
external atmosphere, and sets up between the internal and external 
atmospheres a steeper diffusion gradient, thus leading to a more 
rapid diffusion of water vapor through the stomata. 
Temperature, evaporation, and relative humidity do not explain 
in full the variation recorded in the foliar transpiring ability of 
the plant from day today. Figs. 2 and 5 show transpiration curves 
typically alike except for a slight depression in the latter at noon. 
The averages of the relative foliar transpiring power in each of 
these two cases are almost identical, being 0.198 and 0. 193 respec- 
tively, yet the atmospheric factors were all exerting the- greater 
physical demand for water at station K. Again, if we compare figs. 
2 and 3, we find two transpiration curves of the same type, that in 
fig. 3 being throughout the day approximately 0.05 higher than 
that at station -K. Despite this discrepancy the physical factors 
would call for a transpiration rate considerably lower at station NV 
than at station K. Moreover, if we compare the graphs represented 
in figs. 1 and 3, we find that fig. 1 has a higher transpiration rate 
throughout the afternoon, although the relative humidity is almost 
the same, and both evaporation and temperature are lower, thus 
theoretically calling for a lower rate. This failure of the transpira- 
tion stream to bear a consistent relationship to the factors of temper- 
ature, evaporation, and relative humidity was formerly interpreted 
as being the influence of stomatal movements, but the fallacy of 
such interpretation has been pointed out by Lioyp (7) and others. 
Direct investigation has shown that the stomata open quite early 
