CHAPTER XV. 



THE MOVEMENTS OF PLANTS. 



268. Irritability. — Among the fundamental properties of 

 protoplasm are irritability and automatism. We know practi- 

 cally nothing of the nature of either of these properties, 

 though upon them depend all the movements executed by 

 plants. Automatism is the name given to the power in 

 virtue of which protoplasm is able to initiate internal changes 

 without the action of any external force. Irritability ex- 

 presses the power of the protoplasm to respond or react to 

 the influence of an external change. 



269. Stimuli. — The external change which brings about 

 the reaction is known as a stimulus, and its application is 

 called stimulation. External forces which may acj as stimuli 

 are light, heat, gravity, moisture, electricity, chemical sub- 

 stances, etc. Most of these act constantly upon plants. In 

 order that they may act as stimuli, therefore, a relatively 

 sudden change in intensity or direction must occur. Some- 

 times, however, a slow change will still produce a reaction. 

 For example, the gradual withdrawal of light may cause 

 movements of leaves. (See ^f 297.) 



270. Conditions limiting irritability. — Protoplasm is ir- 

 ritable only under certain conditions, which coincide in tjie 

 main with those that promote the general well-being or life 

 of the organism. The limits of temperature, moisture, 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 



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