TH E PHOTOTROPIC CURVE AND ITS CHARACTERISTICS 359 



(Fig. 134). After this the response became transformed 

 into negative. 



Fig. 134. — Complete phototiopic curve of a growing organ {Zea Mays). 



In a complete phototropic curve we may thus distin- 

 guish 4 distinct stages : — 



(1) The stage of sub-minimal stimulation. 



(2) The stage of increasing positive curvature culmin- 



ating in a maximum. 



(3) The stage of neutralisation. 



(4) The stage of complete reversal into negative. 



The curve thus crosses the zero line of the abscissa 

 twice ; the first crossing takes places upwards at the 

 critical point of stimulation which demarcates the sub- 

 minimal from the minimal. The second crossing down- 

 wards occurs beyond the point of complete neutralisation. 



In a tissue in which transverse conductivity is absent, 

 and the stimulus applied from the beginning is above the 



28 



