392 ' Sir J. C. Bose and Mr. G. Das. 



(a) Eadial thick organ. Transverse conduction absent. Kesponse positive. 



(b) Eadial thin organ. Presence of dots represents possibility of trans- 

 verse conduction. Sequence of response : positive, neutral, and negative. 



(c) Anisotropic thick organ. Thick line represents the more excitable 

 opposite side. Sequence of response : positive, neutral, and pronounced 

 negative. 



(d) Anisotropic thin organ. High transverse conductivity. Sequence of 

 response : positive, quickly masked by negative. 



When light strikes on the opposite side, the sign of response in (a) and (b) 

 will remain unchanged. In (c) and (d) the effect will be only positive. 



The demonstration of these different growth-reactions is given in the 

 following series of experiments. 



15. Dia-pkototropism mid Negative Fhototropism. 



Oltmanns found that the seedling of Lepidium sativum assumed a trans- 

 verse or dia-heliotropic position under intense and long-continued action of_ 

 light of 600,000 Hefner lamps. He regards the transverse as the indifferent 

 position. In reality it is the expression of balanced neutralisation caused by 

 transversely conducted excitation. I obtained such neutralisation with 

 prolonged unilateral exposure to arc-light. The first effect was positive. This 

 was gradually neutralised under continued exposure for two hours ; even then 

 the neutralisation was not complete. 



Uxpt. 13. — From the theoretical considerations that have been advanced it 

 would appear that a reversal can only take place under intense light and 

 where there is a possibility of conduction in a transverse direction. Acting 

 on this idea, I employed light from a mercury-vapour lamp, which 

 emits the most effective violet and ultra-violet rays. The specimen 

 was a thin seedling of the rice plant, Oryza sativa. The first effect was a 

 positive curvature ; this was neutralised and reversed with increasing trans- 

 verse conduction. The neutralisation was completed in six minutes. The 

 response was afterwards reversed to pronounced negative* by the continued 

 action of light (fig. 15, a). 



There is a difficulty in connection with the reversal of response which cannot 

 be explained by mere conduction of excitation to the opposite side, for in a 

 radial organ the contraction of the distal side due to transverse conduction of 

 excitation cannot be greater than that of the directly excited proximal side. 

 The only explanation that remains is that a relaxation takes place on the 



* For explanation of negative phototropism of certain roots cf. ' Plant Response,' 

 p. 601. 



