THE MOVEMENTS OF PLANTS. 189 



may persist under such conditions for a long time. Irritabil- 

 ity may also be lost through fatigue, as when, after repeated 

 reaction, no response occurs to even a greatly increased 

 stimulus. Upon the return of suitable conditions, or after 

 sufficient rest, irritability may be regained. 



271. Reaction.- -The response of the protoplasm to a 

 stimulus is out of all proportion to the physical or chemical 

 action of the stimulus itself. The action of the stimulus upon 

 the irritable protoplasm may be roughly compared to the 

 action of the trigger upon a primed and loaded gun. It 

 sets free forces vastly in excess of those which it exerts. 



272. Reaction time.- -The reaction does not follow in- 

 stantly upon stimulation. The interval, which is known as 

 the reaction time, is ordinarily much longer in plants than in 

 the higher animals. In extreme cases no reaction may be 

 manifest until several hours after stimulation. In other cases, 

 however, as in the well-known sensitive plant, the move- 

 ments of the leaves follow almost instantly upon stimulation. 



273. Form of reaction.- -The character of the reaction is 

 not dependent upon the nature of the stimulus, but upon the 

 nature of the organ itself. It is not in the least understood 

 what the inherent peculiarities are which determine the form 

 of the reaction. In different organs exactly opposite effects 

 may be produced by the same stimulus, and the same organ 

 at different ages may respond differently to the same stimulus. 

 Thus the young internodes of the Virginia creeper (Ampe- 

 /opsis) are sharply recurved, but become erect when older. 

 The stalk bearing the flower of the peanut is erect, but as it 

 becomes older it becomes strongly reflexed, and thrusts the 

 fruit under ground. 



274. Localization of irritability. In multicellular plants 

 irritability to certain stimuli is usually localized in certain 

 organs, and often in special parts of these organs. In many 

 tendrils, for example, the free end is curved and only the 



