486 GENERAL CONSIDERATIONS [Cn. XIX 



hour and forty minutes (during which time it acquired an 

 upright position through the action of gravity), and then 

 exposed again to diffuse light from one side for an hour, show- 

 ing the phototropic curvature. Light was now excluded, but 

 the cotyledon continued to bend for 14 minutes towards the 

 window. In these experiments there was a persistence of the 

 response to light after light had been cut off. Finally, if a 

 plant organ, which has been growing slowly at a low temper- 

 ature, is quite rapidly subjected to the optimum, the increased 

 growth is slow in making its appearance ; it may be delayed 

 for an hour or two. 



What is the significance of this after-effect ? It seems 

 clearly to show that an agent acting on protoplasm not merely 

 modifies the constitution of the protoplasm, but produces a 

 change which is more or less permanent, and is only slowly 

 obliterated upon removal of the agent, or becomes only slowly 

 overshadowed by a new stimulus. This permanency of the 

 change wrought permits the accumulation of extremely slight 

 effects. An instance of such accumulation is seen in tendrils, 

 which exhibit no response to a single soft blow but show evi- 

 dent thigmotropism to a series of such blows. 



When, however, the repeated stimuli are each great, it is 

 not an accumulation of effect which is noticed, but rather 

 an absence of any response whatever. Then appears the phe- 

 nomenon of acclimatization, or accustoming to the stimulus. 

 Examples of this have been given in the foregoing chapters. 

 Thus, in the case of thigmotropism, DARWIN found that 

 after repeatedly stimulating the tendril of the passion flower 

 (twenty-one times in 54 hours), it responded only slightly. In 

 the case of traumatropism, DARWIN ('81, p. 193) observed 

 that the radicle, to one side of which a card had been fixed by 

 shellac, eventually became so accustomed to the stimulus that 

 it no longer bent away from it, but grew vertically downwards. 

 In the case of phototropism, WIESNER noticed that after a 

 growing organ had been for some time subjected to daylight, 

 its growth was less markedly controlled by a unilateral illumi- 

 nation. 

 



Any general explanation of acclimatization, whether of meta- 

 bolic or of growth processes, to repeated irritants must, at the 



