1915] BAKKE—PERMANENT WILTING 315 
sive wilting at which the plant cannot recover turgidity if inclosed 
for 24 hours (without watering of the soil) in a chamber of saturated 
air. Interest in this stage of wilting centers mainly about the deter- 
mination of the residual moisture content of the soil at the time when 
permanent wilting is attained. It is unnecessary here to review the 
important contributions already made in this connection, but it 
may be safely stated that the concept of permanent wilting promises 
to be of great importance’ in soil moisture studies for ecological 
and agricultural interpretation. Besides the authors above men- 
tioned, ALwaAys has studied the relation of plants to soil moisture, 
with results bearing upon this general question. 
It is somewhat difficult to determine just when the stage of 
permanent wilting is attained; an observer can never be quite sure 
whether a plant is permanently wilted or not until after the 24-hour 
exposure in the moist chamber, and there is always considerable 
danger that the wilting may be carried too far. This difficulty sug- 
gested to the writer the possibility of employing the cobalt chloride 
paper test of foliar transpiration power as an indicator of attain- 
ment of permanent wilting. 
In the summer of 1913, at the Desert Laboratory of the Carnegie 
Institution, some preliminary studies were carried out upon the 
relation of the index of foliar transpiring power to the stage of 
wilting in plants. It was the writer’s privilege to carry out these — 
tests upon plants actually employed in the work of SHIvE and 
Livincston, then in progress. The results of some of these tests 
have already been published. They clearly indicate a direct rela- 
tion between the value of the index and the extent of wilting. 
During the summer of 1914 it was again the writer’s privilege 
to carry out studies in this connection at the Desert Laboratory, 
and he wishes here to express his appreciation of the kindness of 
Dr. D. T. MacDoveat, director of the Department of Botan- 
ical Research of the Carnegie Institution, in placing at his 
disposal the facilities of the Laboratory. At the suggestion of 
s Atway, A. J., Studies on the relation of the arenas water of the soil to 
the hygroscopic coefficient. Nebraska Agric. Exp. Sta. Research Bull. 3. 1913. 
6 Baxke, A. L., Studies on the transpiring power nip plants as indicated by the 
method of standardized hygrometric paper. Jour. Ecol. 2:145-173. 1914 (see pp, 
166-168). 
