STIMULATION AND ITS EESULTS 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 



