CURVE OF GROWING ORGAN 155 



10 minutes, and neutralisation was completed after a further 

 exposure of 43 minutes. The subsequent response became 

 completely transformed into negative (fig. 86). 



In the complete phototropic curve of pulvinated and of 

 growing organs, four distinct stages may be distinguished : 



1. The stage of subminimal stimulation giving abnormal 



negative curvature ; 



2. The stage of increasing positive curvature which 



reaches a maximum ; 



3. The stage of neutraHsation ; and 



4. The stage of complete reversal into pronounced 



negative curvature. 



Fig. 86. Complete phototropic curve of a growing organ 



(Zea Mays). 



The phototropic curve crosses the zero line of the abscissa 

 twice ; the first crossing takes place upwards at the critical 

 point of stimulation which demarcates the subminimal 

 from the minimal. The second crossing downwards occurs 

 beyond the point of complete neutralisation. 



Neglecting the preliminary negative due to the action 

 of subminimal stimulus, the tropic curve under photic 

 stimulation obeys what is known as Weber's Law. But this 

 is appHcable only for a limited range of stimulation ; it fails 



