COMPARATIVE ELECTRO-PHYSIOLOGY 



TABULAR STATEMENT SHOWING COMPARATIVE EFFECTS OF EXTERNAL 

 STIMULUS AND INTERNAL ENERGY ON PULVINATED AND GROWING ORGANS 



Mechanical response 



Effect of normal turgidity : 



Normal horizontal position of leaf. 

 Local action of external stimulus : 



Contraction ; 



Diminution of turgidity ; 



and concomitant depression of leaf. 



Action of internal energy exhibited by 



(a) Recovery ; 

 Re-establishment of turgidity and 



gradual return of leaf to normal 

 horizontal position. 



(b) Increased hydrostatic pressure : 

 Erection of leaf. 



Growth response 



Effect of normal turgidity : 



Uniform rate of growth. 

 Local action of external stimulus : 



Contraction ; 



Diminution of turgidity ; 



and concomitant depression of rate 



of growth. 

 Action of internal energy exhibited by 



(a) Recovery : 

 Re-establishment of turgidity and 



gradual return of organ to normal 

 rate of growth. 



(b) Increased hydrostatic pressure : 

 Increased rate of growth. 



energy of a plant is by a moderate rise of temperature. And 

 this finds expression in the case of a motile organ, by the 

 erection of the leaf. Thus, when Mimosa is raised in tem- 

 perature, all its leaves become highly erect. A diminution 

 of energy, on the other hand, by cooling, brings about a 

 depression of the leaves. 



In the same way, the rate of growth is exalted by rise of 

 temperature. Thus in a growing flower of Crinum lily the 

 normal rate of growth at 30 C. was '0040 mm. per minute, 

 and this was exalted to '0113 mm. per minute, or nearly 

 three times, when the temperature was raised to 35*5 C. 

 Lowering of temperature, on the other hand, greatly de- 

 presses the rate, and may even, if it proceed far enough, 

 cause arrest of growth. 



As regards external stimulus, on the contrary, we have 

 seen that its effect on the motile organ is one of depression. 

 The following record (fig. 50) shows that it has a similar 

 influence on the rate of growth. The first part of the curve 

 shows the normal rate of elongation. But after the applica- 

 tion of stimulus of light, growth is not only retarded, but 

 there is an actual shortening of the organ. On the cessation 

 of stimulus, the normal rate of growth is gradually re- 

 established. 



