45 8 PLANT RESPONSE 



inspection, represented upwards (fig. 185). On the cessation 

 of external stimulus, if the recovery of the excited region be 

 merely passive, it is evident that this ascending line will 

 gradually return to the horizontal, as in the third record of 

 fig. 185 ; that is to say, the retarded will be exchanged by 

 degrees for the normal rate of growth. But if some portion 

 of the external stimulus be held latent in the tissue, this will 

 go to increase the internal energy of the plant. Now, we 

 have already seen that the effect of augmented internal 

 energy is exhibited in an increase of the rate of growth 

 above the normal, shown in a balanced response-curve by an 

 opposite movement to that of retardation, constituting the 

 negative after-effect. Such a negative after-effect, consisting 

 of an enhanced rate of growth, will persist until the energy 

 thus held latent is exhausted, when the curve will again 

 return to the horizontal. Thus, the up curve will represent 

 the direct effect of external stimulus, and the down curve the 

 acceleration of growth due to absorbed stimulus, or the nega- 

 tive after-effect. 



Extreme delicacy of the Method of Balance. — Such 

 transient variations in the rate of growth, occurring as the 

 expression of the absorbed fraction of incident stimulus, 

 would have been incapable of detection by the ordinary 

 auxonometric method of growth-record ; for here, owing to 

 the relatively slight magnification which is possible, it takes 

 nearly half an hour to obtain data from which the normal 

 rate of growth may be inferred. Another half-hour's obser- 

 vation would be necessary before we could infer the occur- 

 rence of variation under changed conditions, and it is clear 

 that, during a period relatively so long, the plant may 

 undergo spontaneous changes. The after-effects, however, 

 which we now wish to detect, are found to take place imme- 

 diately, and to last for a few minutes only, in the case of 

 moderate stimulation. Even with our crescographic arrange- 

 ment, though the usual magnification is a thousand times, the 

 variation constituting the after-effect is seen only in a slight 

 change of the slope of the curve ; but when the Method of 



