DETECTION OF LATENT STIMULUS 475 



has already been demonstrated that growth is a phenomenon 

 of excitatory reaction, being at its maximum when the 

 excitabihty of the tissue is greatest. Thus varying expres- 

 sions of growth at the two seasons, as seen in the production 

 of different sized' cells during spring and autumn, would 

 appear natural,' if they could be correlated to differences of 

 excitability, characteristic of those seasons. Now all modes 

 of testing degrees of excitability lead to the conclusion that 

 while it is very great in spring and summer, it is very much 

 enfeebled in autumn and winter. Thus, in the latter season, 

 contractility under stimulation, velocity of transmission of 

 excitation, and the electrical response of a tissue, are all 

 found to undergo a marked diminution, as compared with 

 spring and summer. 



Summary 



On the cessation of strong stimulus the responsive 

 movement continues for a time in the same direction. This 

 is the positive after-effect. 



A portion of the incident stimulus is absorbed and held 

 latent, thus increasing the latent energy of the plant. On 

 the cessation of stimulus this latent component, either 

 immediately or after a time, finds expression in an opposite 

 responsive movement. This is the negative after-effect. 



In the case of growth-response, the positive after-effect 

 consists in the persistence for a time of the retardation of 

 growth, and the negative after-effect exhibits itself as an 

 acceleration of the rate of growth above the normal. 



With moderate stimulus and under normal conditions 

 the sum of the direct effect and the negative after-effect (due to 

 the latent component) remains constant up to the optimum ; 

 that is to say, the sum of the external work (direct effect) 

 and internal work (negative after-effect) done by the stimulus 

 is the same. The direct and the negative after-effect of 

 stimulus are thus complementary. 



At the exact optimum almost the whole stimulus will 

 find expression in direct response, there being little or no 



