1921] CRIBBS—TILIA AMERICANA : gor 
stations M and N (fig. 7) show a variation from the behavior recorded 
for the other stations. The readings for these two were taken 
on one day only, and hence, not being an average of several days’ 
data, are not considered typical. This is especially true as these 
records were taken in early September during a period of dry hot 
weather, and hence represent maximum figures for these stations. 
There is no doubt that the average transpiration and soil 
temperatures would have been very close to those of station J 
had readings been taken in the late spring and early summer, as 
was done at the other stations. To what extent this low soil tem- 
perature is associated with low transpiring power and high soil 
temperatures with high transpiring power, cannot be adequately 
stated at present, but it is known that low soil temperatures produce 
an inhibiting effect on water absorption by the root system, and 
would therefore be expected to lead to a lower rate of water loss 
from the leaf. 
The average soil temperature of station E is slightly more than 
double that recorded for station J, and the fact that the transpira- 
tion index is likewise a little more than doubled is very suggestive. 
The close parallel recorded between average soil temperatures and 
average transpiration indices throughout these experiments may 
be considered evidence that this factor is influential i in limiting the 
foliar water loss. 
SunuticHT.—Sunlight directly or indirectly produces an effect 
upon transpiration, inducing a rise in its index. This was pointed 
out by Lrvrncston (s) as the result of studies on the effect of light 
intensities upon the transpiration rate in Xanthium, Physalis, and 
Martynia. Fig. 6 shows in the morning rise a response to sunlight. 
The transpiration index in this instance did not show the charac- 
teristic response until two hours after daylight, whereas usually 
the rise begins about dawn. On this day transpiration increased 
rapidly, beginning when the sunlight fell directly upon the station. 
The failure to develop an earlier response is attributed to the very 
low temperature, which was at that time but slightly above the 
freezing-point. Sunlight falling upon the leaf is followed by the 
absorption of certain rays which leads to an increase in temperature 
sufficient to accelerate the diffusion of water from the mesophyll 
