1900| MECHANISM OF ROOT CURVATURE 57 
contact by allowing water to escape from cells on the concave 
side, and the reaction may take as long as 1% hours (38, p. 376). 
Roots often curve 90° within two hours. 
The shortening of the cells on the concave side by allowing 
water to escape is also in harmony with the results of Noll’s 
bending experiments, since, as 1 have previously shown, those 
results are merely the expression of the tissue tensions in stimu- 
lated organs, and indicate nothing as to the particular method by 
which those tensions are produced. 
It has already been mentioned that Noll and Kohl both 
found, in the early stages of the curve, an increase of the curve 
on first putting the organ into a plasmolyzing solution, and this 
plus curve was followed only later by the straightening of the 
organ. Each of these men claimed that the plus curve was in 
harmony with his theory of the curvature; but the criticism of 
-Noll’s argument compelled him to admit that the problem was 
more complex than he had supposed and that he had not 
explained the plus curve. I am totally unable to see the logic 
in Kohl’s argument, or to see how his theory explains the plus 
curve at the beginning of plasmolysis. On the other hand it 
seems to me easily explainable on the theory that the protoplasm 
in the cells of the concave side becomes more permeable to 
water. If we suppose that the protoplasm becomes more perme- 
able by an increase in the size of its molecular interstices, thena 
given amount of energy, whether exerted by the elasticity of the 
cell walls or by a plasmolyzing solution outside of the cell, will 
withdraw a given amount of water from the cell 7 @ shorter time. 
Hence when a curving root or stem is placed in a plasmolyzing 
solution a plus curve necessarily follows at first, if the protoplasm 
of the concave cells is more permeable; but while the convex 
side plasmolyzes more slowly, since its protoplasm is less perme- 
able, the cells on that side finally shorten more than the cells on 
the concave side, and the end result is a straightening of the 
whole organ. 
Some observations of Kohl seem to contradict the view that 
the cells on the concave side plasmolyzed more rapidly than on 
