CHAPTER XV. 



THE MOVEMENTS OF PLANTS. 



268. Irritability. — Among the fundamental properties of 

 protoplasm are irritability and automatism. We know jjracti- 

 cally nothing of the nature of either of these jjroperties, 

 though upon them dejiend all the movements executed by 

 plants. Automatism is the name gi\'en to the power in 

 virtue of which protoplasm is able lo initiate internal changes 

 without the action of any external force. T-ritaliility ex- 

 presses the i)Ower of the protoiilasm 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 stiiuuhis, and its application is 

 called stimuhiiion. External forces which ma)' act 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 nnist occur. .Some- 

 times, howe\'er, a slow change will still produce a reaction. 

 For example, the gradual withdrawal of light mav cause 

 movements of leaves. (Sec ^ -97-) 



270. Conditions limiting irritability. — Protoplasm is ir- 

 ritable only under certain conditions, \\'hich coincide in the 

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

 of the organism. The limits of temperature, moisture, and 

 the su])ply of oxygen, which permit irritabilit\-, are nnich 

 narrower than those which ])ermit life. 'Phus, irritability 

 may be lost when the conditions are unfavorable, though life 



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