THE INDEX OF FOLIAR TRANSPIRING POWER AS 
AN INDICATOR OF PERMANENT 
WILTING IN PLANTS 
A. L: BAKKE 
(WITH ONE FIGURE) 
The index of transpiring power as put forward by LIvINGsTON* 
is taken as a measure of the power of leaves to supply moisture to 
the surrounding air. The reciprocal of this index is a measure of 
the power of leaves to retain water against the drying influence of 
the surroundings. The index of transpiring power for any leaf as 
found by means of cobalt chloride paper represents the power of the 
leaf to give off water, this being measured in terms of the similar 
power possessed, at the same temperature, by a saturated blotting 
paper surface blanketed by a millimeter of air. The index for an 
entire leaf is obtained by averaging the indices obtained for the 
two leaf surfaces. Indices less than unity represent leaf surfaces 
with lower power of giving off water than is possessed by the 
standard evaporating surface. Those greater than unity can give 
off water faster than does the standard surface at the same tempera- 
ture. The work of Briccs and SHANTz,? that of CALDWELL,’ and 
that of Suive and Livineston’ have recently brought forth the con- 
cept of permanent wilting in plants. This is that stage of progres- 
t Livincston, B. E., The resistance offered by leaves to transpirational water 
loss. Plant World 16:1-35. 1913 
2 Bricos, L. J., and SHANTz, = , The wilting coefficient and its indirect deter- 
mination. Bot. GAz. §3:20-37. 
, The relative wilting ticle for different plants. Bot. GAz. 53:229- 
235. 1912. oe 
, The wilting coefficient for different plants and its indirect dete: tion. 
U.S. Dept. Ketle:; Bur. Pl. Ind. Bull. 230. 1912. 
'3 CALDWELL, J. S., The relation of environmental conditions to the phenomenon 
‘ of permanent wilting in plants. Physiol. Res. 1:1-56. 1913. 
4Sutve, J. W., and Lrvrneston, B. E., The relation of atmospheric evaporating 
power to soil moisture content at permanent wilting in plants. Plant World 17: 
Si-121. I9td. 
Botanical Gazette, vol. 60] [314 
