TRANSMITTED EFFECT OF LIGHT 369 



unsuspected, is not so anomalous as would appear at 

 first sight. Indirect stimulus, unilaterally applied, has 

 been shown to give rise to two impulses : a quicker 

 positive and a slower excitatory negative. The former 

 induces a convexity on the same side, and a movement 

 away from stimulus (negative curvature) ; the excitatory 

 negative, on the other hand, is conducted slowly and 

 induces contraction and concavity, and a movement towards 

 the stimulus (positive curvature). In semi-conducting or 

 non-conducting tissues, the excitatory negative is weakened 

 to extinction during transit, and the positive reaction 

 with negative curvature persists as the initial and final 

 effect. 



But in Setaria the excitatory negative impulse is trans- 

 mitted along the parenchyma which is moderately conduct- 

 ing ; the speed of transmission of heliotropic excitation is, 

 according to Pfeffer, one or two mm. in five minutes or 

 about 0*4 mm. per minute. Thus under the continued 

 action of light, the excitatory impulse will reacl) the grow- 

 ing region, and by its predominant reaction neutralise ai\d 

 reverse the previous negative curvature. 



Inspection of figure 136 shows that this is what 

 actually took place ; the intervening distance between the 

 tip of the cotyledon and the growing region in hypocotyl 

 was about 20 mm,, and the beginning of reversal from 

 negative to positive curvature occurred 29 minutes after 

 application of light. The velocity of transmission of ex- 

 citatory impulse under strong light is thus 0*7 mm. per 

 minute. The positive curvature continued to increase for 

 a very long time and became comparatively large. This 

 is for two reasons : (1) because the sensibility of the 

 tip of the cotyledon is very great, and (2) because the 

 positive curvature induced by longitudinally transmitted 



