CHAPTER V 
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—A pparent 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.—lf 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 
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