t)lA-PHOTOTROPlSM AND NEGATIVE PHOTOTROPISM 333 



Oltmanns lound that the seedlirig of Lepidium sativtim 

 assumed a. transverse or dia-phototropic position under 

 intense and long continued action of light of ()00,()00 Hef- 

 ner lamps. He regards this as the inditierent position. 

 But tlie neutralisation of curvature is not, as explained 

 before, due to a condition of indifference, but to the anta- 

 gonistic effects of the two opposite sides of the organ, the 

 proximal being stimulated by the direct, and the distal 

 by the transversely conducted excitation. I obtained such 

 neutralisation with Dregea volahilis under the prolonged 

 unilateral action of arc-light. The first effect was positive ; 

 this w^as gradually and continuously neutralised under ex- 

 posure for two hours ; even then the neutralisation was 

 not complete. I shall presently adduce instances where ihe 

 neutralisation was not merely complete, but the final effect 

 was an actual reveisal into negative response. 



SUPPOSED PHOTOTROPIC INEFFECTIVENESS OF SUNLIGHT. 



I may here consider the remarkable fact that has been 

 observed, but for which no explanation has been forth- 

 coming, that "direct sunlight is too bright to bring about 

 heliotropic curvaiure, only diffuse, not direct sunlight has 

 the power of inducing heliotropic movements."* But we 

 cannot conceive of light sudcenly losing its phototropic 

 eff'ect by an increase of intensity. The experiment just 

 described will offer full explanation for this apparent 

 anomaly. Feeble or moderate stimulus remains, as we 

 have seen, localised, hence the contraction of the proximal 

 side gives ri^e to positive curvature. But the intense 

 excitation caused by sunlight would be transmitted 

 to the distal side and thus bring about neutralisation. 

 It is the observation of the final result that has misled 

 observers as to the inefficiency of direct sunlight. A 

 continuous record of the response of the organ shows, on 



* Jost— /6i</— p. 464. 



