XXX. -DIA-PHOTOTROPISM AND NEGATIVE 

 PHOTOTROPISM 



By 



Sir J. C. BosE, 

 Assisted by 

 (tURUprasanna Das. 



I have explained how under the action of unilateral 

 light the positive curvature attains a maximum. There 

 are, however, cases where under the continued action of 

 strong light the tropic movement undergoes a reversal. 

 Thus to quote Jost : " Each organism may be found in 

 one of the three different conditions determined by the 

 light intensity, viz. (1) a condition of positive heliotropism, 

 (2) a condition of indifference, (3) a condition of nega- 

 tive heliotropism"*. No explanation has however been 

 offered as to why the same organ should exhibit at 

 different times, a positive, a neutral, and a negative irrita- 

 bility. These changing effects exhibited by au identical 

 organ is thus incompatible with the theory of specific 

 sensibility, assumed in explanation of characteristic differ- 

 ences in phototropic response. 



In regard to this I would draw attention to an 

 important factor which modifies the tropic response, namely, 

 the eff'ect of transverse conduction of excitation. I shall 

 presently describe in detail a typical experiment of the 



fjQet— /fc»rf-p.462, 



