1918] BAKKE—WILTING 89 
The breaking point occurring on August 21 is not far from the 
normal minimum value of the daily march of foliar transpiring 
power. From previous work upon the march of foliar transpiring 
power, there is more or less of a definite maximum (usually during 
the day) as well as a definite minimum (usually during the night). 
It seems that, in all probability, the minimum in the foliar tran- 
spiring power indicates approximately the greatest resistance to 
transpirational water loss. If the water content of the soil coupled 
with the evaporating power of the air is of such magnitude as to 
increase the resistance to the passage of water, so that the day 
maximum has a value as low or lower than the diurnal minimum 
(at night), the plant is then in a critical condition; at least this has 
been found to be true for Helianthus. For the entire leaf surface 
the transpiring power ratios at night are as follows: (1) Ia, —o.23, 
©.30, 0.34; (2) Ib, —o0.30, 0.31. On August 20 the respective 
values are 0.24, 0.26, 0.27 for Ia,, and 0.28 and 0.32 for Ib. The 
average ratio for the first is 2.2 and the average ratio for the test on 
August 20iso.91. On August 21 the ratio is less; on the following 
day it is a little higher. 
The entire situation as here brought forward centers about the 
amount of moisture present in the soil during the march of wilting 
when the index of transpiring power ratio of day to night comes to 
be represented by unity or less. The duration of this ratio may be 
an important factor in obtaining data that will give information 
on the relative drought resistance of plants. 
Series of 1916 
METHOD 
The method ‘of procedure in the experimentation for 1916 was 
much the same as for the previous season. The sheet iron con- 
tainers were a little deeper (7 inches instead of 6). The soil mix- 
ture was lighter than before, containing 1 part of clean pure sand 
mixed with 3 parts of rich garden soil, and the variety the same as 
before (Mammoth Russian). The seeds were planted on June 24, 
and on July 1 the seedlings were 5 cm. high and were then trans- 
Planted. Three plants were set deeply in the soil. The cultures: 
