570 LIFE MOVEMENTS IN PLANTS 



Stimulation there is a fatigue-decline, as seen in the 

 relaxation following normal contraction in animal muscle. 

 The positive tropic curvature, and the induced galvano- 

 metric negativity may thus undergo a decline, and neu- 

 tralisation. This neutralisation is also favoured, in certain 

 cases, by transverse conduction of excitation to the distal 

 side. 



The character of the after-effect will presently be 

 shown to be modified by the duration of previous stimula- 

 tion, the different phases of which will for conveni- 

 ence, be distinguished as pre-maximum, maximum and 

 post-maximum. Since stimulus simultaneously induces 

 positive " A " and the negative " D " changes (p. 1415), 

 their intensities will undergo relative variation during the 

 continuance and cessation of stimulus. The after-effect 

 will therefore exhibit unequal persistence of the expansive 

 " A " and contractile " D " reaction at different phases of 

 stimulation. 



ELECTRIC AFTER-EFFECT. 



Confining our attention to the electric response, it is 

 found that under continued action of light the excitatory 

 galvanometric negativity increases to a maximum, after 

 which there is a decline, and neutralisation. Figure 205 

 gives the galvanographic record of the electric response of 

 the leaf stalk of Bryophyllum under light ; the up-curve 

 represents increasing negativity which, after attaining a 

 maximum, undergoes neutralisation as seen in the down- 

 curve. I shall, with the help of the diagram given in 

 the next figure, describe and explain the various after- 

 effects I observed on sudden stoppage of light : before the 

 attainment of maximum, at the maximum, and after the 

 maximum. 



After-effect of pre'inaximwn stimulation : Exjjeriment 

 214, — Light is applied at arrow and stopped in different 



