1915] BAKKE—PERMANENT WILTING 319 
stage of wilting represents simply the most intense wilting possible 
without serious rupture of the water columns of the plant. At any rate, 
this rupture should represent a rather definite critical point in the 
march of the power of the plant to extract water from the soil, 
probably the same critical point. about which the concept of 
permanent wilting has been developed. If this suggestion be cor- 
rect, then the graphs of fig. 1 show that this critical point occurred 
in the particular case under discussion about hour 13:30, or five 
hours after the roots actually ceased to absorb moisture from the 
soil. In other words, it appears that the plant in question probably 
attained the stage of permanent wilting about five hours after it 
was uprooted and taken into the laboratory. 
To test the validity of this supposed relation between the mini- 
mum in foliar transpiring power and the attainment of permanent 
wilting, it will of course be necessary to employ the hygrometric 
paper tests upon plants in which permanent wilting is subsequently 
established by the method of Briccs and SHantz—the 24-hour 
exposure to a saturated atmosphere, the plants being still rooted in 
the soil. The press of other experimentation prevented the writer 
from undertaking this comparison, and the foregoing suggestions 
have been made with the hope that such a study as that just men- 
tioned may be accomplished at no distant time. Enough has 
been done to render it highly probable that the index of foliar 
transpiring power determined by means of cobalt chloride paper 
may furnish a somewhat precise criterion for the determination of 
that state of plants heretofore vaguely defined as permanent 
wilting. 
Iowa STATE COLLEGE 
Ames, Iowa 
