384 VEGETABLE PHYSIOLOGY 
This form of sensitiveness is exhibited in a very striking 
way by the growing apex of a young root. If a seedling 
bean is taken, and its tip is stimulated by pressing it 
lightly against some hard particle, or if a small piece of 
cardboard is attached by a drop of 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 coiled 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 curvature 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. Ifa 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 
further back, a curvature results, which causes the root to 
bend towards the obstacle instead of away from it. This 
appears to be due to the contact injuriously affecting the 
cells which are pressed upon, so that their growth is 
