730 PLANT RESPONSE 



conductivity is feeble, the stimulus remains localised on the 

 side of the organ acted upon. Thus the stimulation of either 

 upper or lower side induces a positive response, or movement 

 towards stimulus. 



(e) Dorsi-ventral response which may become negative. — 

 When the transverse conductivity of the organ is con- 

 siderable, and the excitability of the lower half relatively 

 great, then the strong stimulation of the upper side will, by 

 internal diffusion, cause contraction in, and concavity of, the 

 more excitable lower. The responsive movement will then 

 be negative, or away from stimulus ; but feeble or moderate 

 stimulation of the upper half, not being transmitted to the 

 lower half, causes a positive response. Direct excitation of 

 the more excitable lower half will always give rise to a 

 movement towards stimulus, or positive response. 



2. Indirect effect of unilateral stimulation : (a) 

 Negative response. — When moderate stimulus is unilaterally 

 applied at a distance from the responding organ, it is the 

 indirect effect that is transmitted to that region, causing 

 convexity of the same side, with consequent movement 

 away from stimulus, or negative response. This is very 

 well illustrated when the tip of either shoot or root is sub- 

 jected to moderate unilateral stimulation. 



(b) Positive response. — But when the unilateral stimulus 

 at the distant point is strong or long continued, the excitatory 

 effect is transmitted by conduction, and induces a contraction 

 and concavity of the same side, resulting in a movement 

 towards stimulus, or positive response. 



From what has been said it will be understood that 

 moderate unilateral stimulation of the tip of root or shoot 

 induces negative, and excessive stimulation positive, while 

 between these two extreme cases there may be intermediate 

 or neutral, response of the responding region. 



These effects are induced by stimulation of all forms, and 

 it is thus clear that there is no specific sensitiveness of the 

 dorsi-ventral as distinguished from the radial organ, nor is 

 there any polar difference between the response of root or 



