152 CHAP. XIV. THE PHOTOTROPIC CURVE 



The employment of sensitive recorders has, however, 

 enabled me to discover the important fact that subminimal 

 stimulus is bv no means ineffective. It induces a definite 

 reaction of expansio7i, antecedent to the normal contrac- 

 tion. A critical intensity demarcates the subminimal from 

 the minimal stimulus, the respective responses being of 

 opposite signs, an expansive positive and a contractile 

 negative. 



I have already explained that the critical intensity of 

 stimulus varies in different species of plants. Thus the 

 same intensity of light which induces a retardation of growth 

 (negative variation) in one species may cause an enhance- 

 ment of the rate (positive variation) in another. The critical 

 point, moreover, depends on the tonic level of the organ ; 

 in an optimum condition the critical point is low, inas- 

 much as a feeble stimulus induces the excitatory reaction. 

 In a subtonic tissue, on the other hand, the critical point 

 is high, necessitating a relatively strong and long-continued 

 stimulation to induce the normal reaction. Stimulation 

 itself is found to raise the tonic level of the tissue, so that 

 the response is transformed from expansion to normal 

 contraction. 



The physico-chemical processes underlying these opposite 

 responses have, for convenience, been distinguished as the 

 * A ' and ' D ' changes. The expansive assimilatory ' build- 

 ing-up ' process A is associated with an increase of potential 

 energy of the system. The contractile dissimilatory ' break- 

 down ' D is, on the other hand, concomitant with an 

 evolution or run-down of energy.^ 



Returning to the consideration of the action of uni- 

 lateral stimulation by light in inducing phototropic curva- 

 ture of an organ in a slightly subtonic condition, the directly 

 stimulated side exhibits expansion at the first stage, causing 

 a negative curvature away from light. But by the con- 

 tinuous action of stimulus the subminimal becomes minimal 

 and then maximal. The negative will thus be transformed 



^ The Motor Mechanism of Plants (1928), p. 56. 



