CHAPTER XIII 



DIA-PHOTOTROPISM AND NEGATIVE PHOTOTROPISM 



I HAVE explained how under the unilateral action of 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 organ may be found in one of the 

 three different conditions determined by the light intensity, 

 viz. (i) a condition of positive heliotropism, (2) a condition 

 of indifference, (3) a condition of negative heliotropism.' ^ 

 No satisfactory explanation has, however, been found as to 

 why the same organ should exhibit at different times a 

 positive, a neutral, and a negative response. The exhibition 

 of these different effects by an identical organ is incom- 

 patible with the theory of specific sensibility, often assumed 

 in explanation of characteristic differences of phototropic 

 response. 



Oltmanns found that the seedling of Lepidium sativum 

 assumed a dia-phototropic position under intense and long- 

 continued action of light of 600,000 Hefner lamps. He 

 regards this as the indifferent position. But the neutralisa- 

 tion of curvature is, as will be presently explained, not due 

 to a condition of indifference, but to antagonistic effects 

 induced at the two opposite sides of the organ. 



Phenomenon of Neutralisation 



Neutralisation, partial or complete, is principally due 

 to the transverse conduction of excitation across the stem. 



^ Jost, ibid. p. 462. 



