CHAPTEB VII 



PLANT RESPONSE ON THE RELATION BETWEEN STIMULUS 

 AND RESPONSE 



Increased response with increasing stimulus Apparent diminution of 

 response with excessively strong stimulus. 



As already said, in the living tissue, molecular dis- 

 turbance induced by stimulus is accompanied by an 

 electric disturbance, which gradually disappears with 

 the return of the disturbed molecules to their position 

 of equilibrium. The greater the molecular distortion 

 produced by the stimulus, the greater is the electric 

 variation produced. The electric response is thus an 

 outward expression of a molecular disturbance produced 

 by an external agency, the stimulus. 



Curve of relation between stimulus and response. 

 In the curve showing the relation between stimulus and 

 response in nerve and muscle, it is found that the 

 molecular effect as exhibited either by contraction or 

 E.M. variation in muscle, or simply by E.M. variation 

 in nerve, is at first slight. In the second part, there is 

 a rapidly increasing effect with increased stimulus. 

 Finally, a tendency shows itself to approach a limit 

 of response. Thus we find the curve at first slightly 

 convex, then straight and ascending, and lastly, concave 

 to the abscissa (fig. 30). 



In muscle the limit of response is reached much 

 sooner than in nerve. As will be seen, the range 

 of variation of stimulus in these curves is not very 



BS 



