35 



CHAPTER Y 



PLANT RESPONSE ON THE EFFECTS OF SINGLE STIMULUS 

 AND OF SUPERPOSED STIMULI 



Effect of single stimulus Superposition of stimuli Additive effect 

 Staircase effect Fatigue No fatigue when sufficient interval between 

 stimuli Apparent fatigue when stimulation frequency is increased 

 Fatigue under continuous stimulation. 



Effect of single stimulus. In a muscle a single 

 stimulus gives rise to a single twitch which may be re- 

 corded either mechanically or electrically. If there is 

 no fatigue, the successive responses to uniform stimuli 

 are exactly similar. Muscle when strongly stimulated 

 often exhibits fatigue, and successive responses therefore 

 become feebler and feebler. In nerves, however, there 

 is practically no fatigue and successive records are 

 alike. Similarly, in plants, we shall find some exhibit- 

 ing marked fatigue and others very little. 



Superposition of stimuli. If instead of a single stimu- 

 lus a succession of stimuli be superposed, it happens that 

 a second shock is received before recovery from the 

 first has taken place. Individual effects will then be- 

 come more or less fused. When the frequency is suffi- 

 ciently increased, the intermittent effects are fused, and 

 we find an almost unbroken curve. When for example 

 the muscle attains its maximum contraction (corre- 

 sponding to the frequency and strength of stimuli) it 



D 2 



