STIMULATION AND ITS EESULTS 385 



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. 



The cause of this curvature must be the sensitiveness 

 of the protoplasm to the stimulus of contact. 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 distri- 

 bution 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. 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 

 further back, the resulting curvature causes the root to 

 bend toioards 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 

 retarded or stopped. The cells on the other side of the 

 root not being affected, a curvature results from their con- 

 tinued growth. These two capacities for curvature are of 

 great assistance to a root during its growth downwards 



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