82 BOTANICAL GAZETTE [AUGUST 
no. oo—11 cm.)' are impregnated with 3 per cent solution of cobalt 
chloride and are later cut into small squares. Just before using, 
these squares are heated over a bicycle lamp, or on a granite pie- 
plate suspended by a clamp over the flame of an alcohol lamp, until 
they become blue. One of these squares is placed between the 
jaws of a “transpiration clip,” and as quickly as possible applied 
to either the upper or lower surface of a leaf. The time required 
to change the paper square from blue to pink is determined in 
seconds. The time which it takes to change a similar piece of 
cobalt paper from -lue to pink when placed over a moist blotting 
paper surface blanketed by a millimeter of air (2, 4, 28, 30, 34, 42) 
is recorded. The water apparatus is the same as used by BAKKE 
and Livincston (4). TRELEASE and LivincsTton (42) have 
developed the relations of the temperature to vapor tension as 
first shown by BAKKE (2). These authors have presented a formula 
whereby the time interval may be ascertained on knowing the 
temperature. Livincston and SHREVE (30) have recently im- 
proved and modified this method. The principal improvement is 
in the adoption of permanent color standards. Instead of the 
simple square of cobalt chloride paper, a composite slip is em- 
ployed consisting of a small piece of the hygrometric paper in 
juxtaposition with two slips having permanent color standards. 
These provide both an initial and an end point for the color change. 
For use in the laboratory they advocate and describe a simpler 
form of standard water evaporating apparatus. These modifica- 
tions were not used in this study. 
The possibilities of using the original method of standardized 
hygrometric paper in determining the extent of wilting and the 
permanent wilting point was first suggested to me by its author, 
B. E. Livincston, at the Desert Laboratory of the Carnegie Institu- 
tion during the summer of 1913. In 1914 the writer (3), working at 
the Desert Laboratory, performed a series of measurements upon 
sunflower plants lifted from the soil and later brought into the 
laboratory to wilt. The results of this series of tests show that 
? Livincston and SHREVE in a more recent publication (Improvements in the 
method for determining the transpiring power of plant surfaces by hygrometric paper. 
ed 
Plant World 19: 287-309. 1916) have recommended Whatman’s filter no. 30 (11 cms.) 
circles as being superior to the Swedish paper. 
