1900] MECHANISM OF ROOT CURVATURE 51 
that there is an increased turgor on the convex side of the curv- 
ing organ. Wiesner’s view that when the root tip is injured on 
one side the cell membranes on that side above the wound are 
made more extensible, does not account for the changes on the 
opposite side which are indicated by’my experiments. Wiesner 
makes a distinction between geotropic and traumatropic curva- 
tures. In the case of the former he supposes an increase of 
ductility of the cell membranes on the side becoming convex, 
and also an increase in the elasticity of the cell membranes on 
the side which becomes concave. Even though we admit that 
a sufficient increase in the elasticity of the membranes on the 
concave side could shorten the cells by forcing water out through 
the protoplasm, we may well doubt whether that is the method 
by which the cells are shortened, until further evidence has been 
presented in favor of that view. We must believe that the 
elasticity of the membranes is brought about only by the activity 
of the protoplasm; and it does not seem to be an economical 
expenditure of energy for the protoplasm to increase the elasti- 
city of the walls until their pressure forces water out through 
the protoplasm, when by its activity it can change its permea- 
bility to water and allow the cells to shorten without increasing 
the elasticity of the,cell walls. It has already been shown (p. 44) 
that the concave side shortens in relation to the axial cylinder 
when compression by the cortex of the convex side is out of the 
question. 
Noll supposed he had proved the greater extensibility of the 
membranes on the convex side of curving stems by his bending 
experiments. With the same pull the stems bent more toward 
the concave than toward the convex side. It has been shown 
(p. 26) that this is the necessary result of the tensions in stimu- 
lated stems; therefore it is evidence only of those tensions, and 
not at all in favor of any particular method by which those 
changes from the normal tension are brought about. Noll 
showed by other methods that the size, and the optical and 
staining properties of membranes on the convex side were differ- 
ent from those of unstimulated stems; and it may be that the 
