138 CHAP. XIII. DIA-PHOTOTROPISM 



light. In the present case, very strong light from an arc- 

 lamp was applied from the beginning, and a continuous 



Fig. 76, Phototropic reversal under continued action of 

 strong light L (Vicia Faba). 



record taken of the change in the sign of response of the 

 young shoot of Vicia Faba. The first effect was a positive 



curvature which attained its 

 maximum in the course of 

 5 minutes ; after this there was 

 neutralisation, succeeded by nega- 

 tive curvature (fig. 76). 



Experiment 85. Transformation 

 of positive into pronounced negative 

 in Oryza. — Since transverse con- 

 duction is more rapid in a thin 

 specimen, I expected to obtain 

 a quicker reversal in Oryza. 

 Light from an arc-lamp produced 

 maximum positive curvature in 

 the course of 2 minutes, after 

 which there was neutralisation. 

 Subsequently complete and pro- 

 nounced negative phototropic 



Fig. 77. Positive, dia- and ■, j • ^^i 



negative phototropic re- reversal occurred m the course 

 sponse of Oryza under of a further minute and a half 



continued unilateral 



stimulus of intense light. (hg. 77/' 



