1^0 CHAP. XII. MECHANISM OF PHOTOTROPIC CURVATURE 



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intervening distance was 20 mm. the response was one of 

 galvanometric positivity. Reduction of the distance to 

 10 mm. gave rise to a diphasic response, positive followed 

 by negative. When the intervening distance was further 

 reduced to 3 mm. the response was one of galvanometric 

 negativity (fig. 72). This is an independent proof that 

 positive and negative impulses are generated under 

 indirect stimulation. 



Fig. 73. Electric response at distal and proximal sides under 



unilateral stimulation. 



a, electropositive response at distal, and b, electronegative 

 response at proximal side (Helianthus annuus). 



Experiment 77. Electric response at proximal and distal 

 sides under unilateral photic stimulation. — I took a young 

 flower-stalk of Sunflower (Helianthus) and made suitable 

 electric connections with diametrically opposite sides of the 

 organ for the record of the electric response. On applica- 

 tion of strong light on one side of the organ, the electric 

 response at the distal side was positive, indicative of 

 expansion and increase of turgor ; the electric response at 

 the proximal side was, on the other hand, negative, demon- 

 strating contraction and diminution of turgor (fig. 73). 



The results obtained by widely different methods of 

 investigation are thus found to be essentially similar ; the 



