RESPONSE TO STIMULATION 423 



growing organs is retardation of the rate of growth ; if the 

 stimulus be intensified or the stimulation prolonged, the 

 contractile reaction of retardation is more marked, and, at 

 a critical point, causes complete arrest. 



The independent method of electrical investigation fully 

 confirms the results obtained by the mechanical method. 

 The closest parallelism has been established between the 

 mechanical and electric responses in both non-growing and 

 growing organs under stimulation. Conditions which evoke 

 negative mechanical and electric response in a non-growing 

 organ, also give rise to negative variation and retardation 

 of the rate of growth. Other conditions which cause 

 positive mechanical and electric response in a non-growing 

 organ bring about positive variation or enhancement of 

 the rate of growth. The physiological machinery is the 

 same in pulvinated and in growing organs. 



Effects of feeble and strong stimulation. — It may be said 

 in general that two opposite effects are induced in growth 

 under feeble and strong stimulation. There is a critical 

 intensity below which there is an acceleration and above 

 which there is a retardation. This critical intensity varies 

 in different species of plants. These opposite effects are 

 exhibited in all modes of response to diverse methods of 

 stimulation. 



Effect of electric stimulation. — In normal conditions, 

 direct stimulation induces incipient contraction exhibited 

 by a retardation of the rate of growth ; this is sometimes 

 effected by an intensity of stimulus below the range of human 

 perception. Under increasing intensity of stimulation the 

 contractile reaction becomes more and more pronounced. 

 At a critical intensity of stimulus, growth becomes arrested ; 

 under a still stronger intensity there is an actual shortening 

 of the organ. A continuity thus exists between incipient 

 and actual contraction. 



Effect of mechanical stimulation. — Moderate frictional 

 stimulation induces incipient contraction, manifested as 

 retardation of growth, recovery being completed within a 



