INTERNAL ENERGY AND EXTERNAL STIMULUS 439 



The laws of growth are therefore : 



(1) The response of a growing organ is the same as that of a 

 stationary organ. Direct application of stimulus, inducing con- 

 traction, retards the rate of growth. 



(2) The effect of indirect stimulation is to increase the internal 

 energy, and thus augment the rate of growth. 



Summary 



The multiple response of growth is characterised by all 

 the peculiarities seen, for instance, in the autonomous response 

 of Desmodium. 



The rhythmic responses of growth exhibit periodic 

 groupings. 



External stimulus is found to renew growth in organs in 

 which, owing to the deficit of internal energy, it had come to 

 a temporary standstill. 



The increase of internal hydrostatic pressure, up to an 

 optimum, increases the rate of growth. 



The effect of increased internal hydrostatic pressure is 

 exhibited in the case of the leaf of Mimosa by an erectile 

 response. In a growing organ the effect of increased turgidity 

 is shown by growth-elongation. A drought-rigored Mimosa 

 on being supplied with water responds by erection of the 

 leaf ; and similarly, a plant in which growth, owing to drought- 

 rigor, has come to a standstill, responds, on being supplied 

 with water, by renewed growth-elongation. When the root 

 of Mimosa is supplied with ice-cold water, the responding 

 leaf, owing to the consequent arrest of ascent of sap, becomes 

 depressed. The increased suctional activity, again, which is 

 caused by a supply of warm water to the root, induces an 

 erectile response of the leaf. Similarly, corresponding varia- 

 tions in the rate of ascent of sap, brought about in a growing 

 plant by the application of cold and warm water to the roots, 

 cause respective depressions and accelerations of the rate of 

 growth. 



The energy supplied at the roots is hydraulically trans- 



