STIMULATION AND ITS RESULTS 393 
mid-rib were a hinge, bringing together the upper surfaces 
on each side so as to imprison the body which touches it. 
This form of sensitiveness is exhibited in a very striking 
way by the growing apex of a young root. If the tip of a 
seedling bean is stimulated by pressing it lightly against 
some hard particle, or if a small piece of cardboard is attached 
by a drop of water or very dilute gum to one side of its 
apex, a curvature speedily results, which causes the root to 
bend away from the irritating body. If the movement 
takes the sensitive part away from the latter the curvature 
is slight; but if, as in the case of the attached cardboard, 
the foreign body accompanies it in its displacement, the 
curvature will continue until the root is curved completely 
round. The stimulus in the case of this movement must be 
prolonged, differing thus from the cases already noted, in 
which a mere touch is sufficient to bring it about. 
Wounding one side of the apex of the root by bruising 
it, or applying an irritant poison such as lunar-caustic, 
brings about the same movement. Indeed such wounding 
may be regarded as an exaggeration of mere contact. 
The cause of this curvature must be the sensitiveness of 
the protoplasm to the stimulus of contact or of injury. 
The part which curves is some little distance from the apex, 
at which the capacity for receiving the stimulus is located, 
and the mechanism of the curvature is a modification of the 
distribution of turgescence of the cells in the zone of growth. 
It is only while that part is actively growing that the curva- 
ture can be caused. 
Another kind of curvature can be detected in the course 
of the growth of young roots, which differs fundamentally 
from the one just described, and the two must be carefully 
distinguished from each other. If a young root comes into 
contact with an obstacle such as a small stone, so that the 
latter presses, not upon the tip as in the case described, but 
upon the region of the growing cells some little distance 
farther back, a curvature results, which causes the root to 
bend towards the obstacle instead of away from it. This 
