CHAPTER XVI. 



THE MOVEMENTS OF PLANTS. 



228. Irritability. — Among the inherent properties of 

 protoplasm are irritability and automatism. We know prac- 

 tically nothing of the nature of either of these properties, 

 though upon them depend all the activities of plants. They 

 seem to be merely two phases of the same property. Auto- 

 matism is the name given to the ability of protoplasm to in- 

 itiate internal changes without the action of any external 

 force. Irritability expresses the power of the protoplasm to 

 respond or react to the influence of an external change. 



229. Stimuli. — The external change which brings about 

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

 called stimulation. External forces which may act as stimuli 

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

 stances, etc. Most of these act constantly in some measure 

 upon plants. In order that they may act as stimuli, there- 

 fore, a change in their intensity or direction must occur. If 

 the change be great or sudden, the reaction is likely to be 

 more marked. Sometimes, however, a slow change will still 

 produce a distinct reaction. For example, the gradual with- 

 drawal of light may cause movements of leaves. (See ^ 255.) 



230. Conditions limiting irritability. — Protoplasm is ir- 

 ritable only under certain conditions, which coincide in the 

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

 of the organism. But the limits of temperature, moisture, 



164 



