TRANSMITTED EFFECT OF LIGHT 875 



convexity and movement away from stimulus (negative 

 curvature). This is diagrammatically shown in Fig. 189 

 Had the intervening tissue been non-conducting, the 

 slow excitatory negative impulse would have failed 

 to reach the responding region, and the negative 

 curvature induced by the positive impulse would 

 prove to be the initial as well as the final effect. 

 In the case of Setaria^ however, the excitatory 

 impulse reaches the right side of the organ after 

 the positive impulse ; the final effect is therefore 

 an induced concavity and positive curvature 

 (movement towards stimulus). 



Fk4. 139. — Diagrammatic representation of the effects of direct and indirect 

 stimulus on the response of Setaria. Direct stimulation, represented by thick 

 arrow gives rise to antagonistic concavities of opposite sides of responding 

 hypocotyl, resulting in neutralisation. 



Indirect stimulus represented by dotted arrow gives rise to two impulses 

 the quick positive impulse represented by a circle, and the slower negative 

 impulse represented by crescent (concave). 



The results given above enable us to draw the follow- 

 ing generalisations : — 



1. In an organ, the tip of which is highly excitable, 

 the balanced state of neutralisation, induced by direct 

 stimulation of the responding region, is upset in two 

 different ways by two impulses generated in consequence of 

 indirect stimulation at the tip. Hence arises two types of 

 resultant response : — 



Type A. — If the intervening tissue be semi-conducting, 

 the positive impulse alone will reach the growing region 

 and induce convexity of the same side of the organ 

 giving rise to a negative curvature. 



Type B. — If the intervening tissue be conducting the 

 transmission of the excitatory impulse will finally give 

 rise to a positive curvature. 



29 



