THE MOVEMENTS OF PLANTS. l6S 



and the supply of oxygen, which permit irritability, are much 

 narrower than those which permit life. Thus, irritability 

 may be lost when the conditions are unfavorable, though life 

 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 even to a greatly increased 

 stimulus. Upon the return of suitable conditions, or after 

 sufficient rest, irritability may be regained. 



231. 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. 



232. Reaction time. — The observable reaction does not 

 follow instantly upon stimulation. The interval, which is 

 known as the reaction time or the latent period, is ordinarily 

 much longer in plants than- in the higher animals. In ex- 

 treme cases no reaction may be manifest until several hours 

 after stimulation. In other cases, however, as in the well- 

 known sensitive plant, the movements of the leaves follow 

 almost instantly upon stimulation. 



233. 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- 

 lopsis) 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. 



