1918] BAKKE—WILTING 109 
settled. Regarding the stomatal diffusion as a minor factor during 
intense wilting, the problem resolves itself to the point where the 
resistance to the passage is considered. From the data given in 
this paper and in a previous publication the resistance is exceed- 
ingly great. This will give further information, therefore, upon 
the strength of the evaporating force and that of cohesion. 
Discussion 
In comparing the results obtained during the summer of 1915 
with those of 1916, considerable additional evidence is set forth 
which substantiates the argument advanced by BAKKE that wilting 
occurs at a definite point and is readily determined by the use of 
standardized hygrometric paper. In the series of 1915 the average 
of 3 leaves were used in plant Ia and 2 leaves for plant Ib. No 
effort was made during the 1915 season to obtain the difference in 
the time of wilting for leaves of different ages. The difference, 
however, was probably very slight, as the evaporation was exceed- 
ingly low. At no time during the entire run was the evaporation 
as high as 0.7 cc. per hour, and usually it was below 0.5 cc. per 
hour. The temperature of the greenhouse was seldom over 28° C. 
In contrast, the evaporation during the season of 1916 was high 
and during the time the experiments were being performed was 
exceedingly uniform. It may be added that during the progress 
of the experiment no rain fell. It would then have been preferable 
to have run the experiments outside, but in the climate of Chicago 
it is rather difficult to obtain such a continued period of clear 
weather. The usual feature will then be a low evaporation at 
night, a higher one during the forenoon, and the maximum at the 
14thhour. The high evaporation rate on July 31 is not explainable. 
It may be well to remark, however, that on July 30 the tempera- 
ture in the greenhouse was 41.2° C. and at the first hour of July 
31 it was 27° C., almost the maximum of the previous year. 
It is again brought out that for the 1915 and 1916 series a point 
1s reached where the foliar transpiring power shows very little 
fluctuation. In the cases presented, this point can be represented 
graphically by a line that is almost straight. The ratio values are 
hot far above unity in the majority of cases, and sometimes are 
