1921] CRIBBS—TILIA AMERICANA 3Ir 
7. Sunlight affects the transpiration stream through its influ- 
ence in initiating stomatal movements, and has particular reference 
to the morning rise. Direct sunlight accelerates the water loss, 
due apparently to a rise in temperature of the mesophyll cells which 
would be followed by a greater molecular diffusion of water from 
the cell walls into the lacunae of the leaf, thus increasing the 
diffusion gradient. 
8. Relative humidity is a very potent factor in influencing the 
transpiration index. By increasing or decreasing, it affects directly 
the diffusion gradient between the external and internal atmos- 
pheres. Sudden changes in this factor almost invariably affected 
the transpiration stream, even when the atmometer failed to regis- 
ter it. When the relative humidity is great there seldom is any 
saturation deficit developed, but when it is low and the evaporation 
rate increases greatly the reserve will probably be consumed and a 
saturation deficit will follow. Relative humidity is considered a 
very potent factor leading to such depression in the curve. 
9. Growth water exerted less influence in modifying the tran- 
spiration rate than did most of the atmospheric factors. There is 
little evidence that it is a potent factor at all on the clay series. 
Its direct effect is most noticeable when it approximates the wilting 
coefficient as was recorded on the sand dunes, when the index is 
observed to fall rapidly. The growth water was always more than 
could be utilized in the clay environments, and hence never proved 
a limiting factor. 
10. The part played by soil temperature in maintaining the 
transpiration stream is considered more important than has gen- 
erally been ascribed to this factor. The close parallel of the soil 
temperature and transpiration curves is strongly indicative of its 
influence (fig. 7), and especially is this significant since there is 
strong evidence that the absorption rate often becomes a limiting 
factor. I believe that since soil temperature bears a direct relation 
to absorption it will eventually be found to be of great significance 
in limiting the foliar transpiring ability. 
11. The highest average transpiring power recorded for the 
clays, with the exception of the prairie data, was that registered for 
the swamp habitat. In this environment Tilia, while producing 
