274: GRAVITATIONAL SENSITIVENESS OF THE ROOT-TIP. 
I have found it impossible to make sure whether this supposed 
curvature could actually occur, but from experiments on the 
power of reversal in the curved region of bean-roots, I believe it 
could not. 
It is, however, possible to test the matter in another way, 
namely, to ascertain whether the part of the root which curves 
has any direct independent geotropic sensitiveness—that is, any 
sensitiveness independent of the stimulus transmitted from the 
tip. 
A seedling is placed horizontally in damp sawdust until the 
root has curved geotropically: the seed is then fixed to the 
lever with the tip of the root in a vertical tube (fig. 10). If the 
region C is directly and independently sensitive to gravitation, it 
ought to continue to curve so that the cotyledons would descend. 
But this is not what happens. In my experiments, only one 
root showed increased curvature, seven showed distinct diminu- 
tion, and two slight or doubtful diminution. The balance of 
evidence is thus clearly against the existence of independent 
sensitiveness in the motile part of the root. This of course agrees 
entirely with Pfeffer’s and Czapek’s results ; and if this is granted, 
the only conceivable explanation of the continued curvature of 
the root in a horizontal tube is that the tip is the percipient 
region. It has been already pointed out that this explanation 
is consistent with the observed facts. 
The experiments illustrated by fig. 10 are of value also ip 
another way. In these experiments I found that though there 
is a tendency to the diminution of the existing curve, yet that 
the increment to the root due to new growth remains vertical. 
This disposes of the possible objection that the continuous 
curvature, occurring when the tip of the root is fixed in a hori- 
zontal tube, is due to contact-irritation, and is only a form of 
the curvature produced by injuring one side of a root-tip*. If 
this were the cause of the curvature, it should occur whether the 
tube is horizontal or vertical ; and since this is not the case, we 
are supported in referring the continuous curvature to the 
continued stimulation of the tip. 
* Darwin, ‘ Power of Movement in Plants.’ 
