DIA-PHOTOTROPISM AND NEGATIVE PHOTOTROPISM 335 



the exhibition of reversal effect. I took a hypocotjl of 

 Sinapis nigra and subjected it to unilateral action of light 

 from a 10 candle-power incandescent electric lamp placed 

 at a distance of 10 cm. A maximum positive curvature 

 was induced in the course of 50 minutes. The intensity of 

 light was afterwards increased by bringing the lamp nearer 

 to a distance of 6 cm. This resulted in a process of 

 neutralit^ation of the preceding response ; after an t^xposure 

 of 70 minutes the specimen assumed a dia-pbototropic 

 position in which it remained in equilibrium. Sunlight was 

 next applied, and in the further course of 30 minutes there 

 was a pronounced reversal into negative phototropic curvature. 

 Neutralisation and reversal under continunus stiinula- 



tion : Experintent 128.— In the 

 last experiment the different 

 changes in the response were 

 brought about by successive 

 increase in the intensity of 

 light. In the present experi- 

 ment, very strong light was 

 applied from the beginning, 

 and continuous record was 

 taken of the change in the 

 response. In order to reduce 

 the period of experiment I 

 employed a mercury vapour 

 lump which emits the most 

 effective violet and ultra-violet 

 rays. The specimen used 

 was a seedling of the rice 

 Fig. i2i.-Positive and negative pja^^ (^Qryza sativa). The 



phototropic responses of Oryza under „ , ™ , ,. ■• , 



A 1 . 1 *• 1 f • f » nrst effect of light was a nosi- 



continued unilateral stimulus of intense t> "»o a jjuDi 



light from arc lamp. tive curvaiure which attained 



its maximum ; after this there was a neutralisation in less 

 than six minutes after the application of light. The further 



