RESPONSE OF LEAVES 235 



thus consist in the greater galvanometric negativity of this 

 more excitable lower, in relation to the less excitable upper half 

 If the stimulus, however, be applied at some considerable 

 distance, so that true excitation cannot reach the responding 

 point, then we have an erectile or positive mechanical 

 response of the leaf. This is brought about by the relatively 

 greater expansion of the more excitable. The corresponding 

 electrical response will be the galvanometric positivity of 

 this more excitable, in relation to the less excitable half of 

 the organ. Between these two extremes lies that experi- 

 ment in which stimulus is applied at some intermediate 

 point, the consequence of which is that the hydro-positive 

 ave, with its greater velocity, reaches the responding organ 

 earlier than true excitation, thus bringing about a pre- 

 liminary erectile or positive response, followed by the ex- 

 citatory negative or fall of the leaf. The corresponding 

 electrical response would therefore be diphasic, positive 

 followed by negative. 



But the occurrence of this second or negative phase is 

 inly possible when the conductivity is so great as to allow 

 the wave of true excitation to reach the organ. We may 

 imagine that in a very vigorous plant, with its great con- 

 ductivity, we have found a point, at the maximum distance 

 from which the true excitatory effect of a given stimulus is 

 capable of transmission to the organ. With such a speci- 

 men, in its ' prime,' we shall observe a diphasic effect pre- 

 liminary positive followed by negative. But if we took a 

 less vigorous specimen, and applied the stimulus at the same 

 distance from the responding point, the true excitatory wave 

 would fail to reach the responding organ, and we should see 

 there, only the positive effect due to hydro-positive action. 

 Hence, two different specimens, treated in exactly the same 

 way, may exhibit two different effects, one diphasic, and the 

 other positive alone ; this difference being due to their 

 unequal vigour, and concomitant inequality of excitability 

 and conductivity. This will account for the diphasic and 

 positive responses which were exhibited by the more and 



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