MECHANICAL RESPONSE TO STIMULUS 



13 



Differential response. — But the responsive movement 

 in plants is more generally produced by differential contractile 

 movement, and a mechanical model again will clearly show 

 how such movements are brought about. We take two 

 equal strips of unequally contracting 

 substances, which are glued together 

 throughout their length. The two 

 strips consist of ebonite and the 

 relatively more contractile caoutchouc. 



Fig. 9. Series of Contractile 

 Responses in Muscle 



Fu;. 10. I'hotographic 

 Record of Longitudinal 

 Contractile Response 

 in ordinary Stamens 

 (Bro7C'/i/a ariza) 



If such a compound strip be held horizontally, with the 

 more contractile element below, and if we subject it to 

 thermal stimulation in the manner described above, the 

 result will be a responsive curvature do\vnv\'ards, the more 

 contractile caoutchouc forming the concave surface. Thermal 



Fig. II. Differential Lateral Response of Compound Strip 

 Thermal stinuili applied at intervals of three minutes. 



stimulus may be applied, as in the last case, by sending a 

 momentary heating current through an enclosing spiral of 

 German-silver wire, the responses being recorded in the usual 

 manner (fig. 1 1). 



In such cases, where the upper and lower elements are 



