IRRITABILITY 227 



plant or a Daisy-plant in a dark place, the leaves of the former 

 and the capitula of the latter will close up. Here the sudden 

 removal of light acts as a stimulus, and the irritable part of the 

 plant responds by closing. Bringing these plants back into the 

 light, the leaves and inflorescences open. Again, if we put a 

 Tulip-plant with fully-developed but closed flowers into a warm 

 place, we can cause the flowers to open. When the flowers 

 are open and the plant is transferred to a cold place, the flowers 

 commence to close. Here the rise or fall of temperature acts as 

 a stimulus. (Similar observations may be made on the Crocus.) 

 Finally, we know that the flowers of Anagallis arvensis (the 

 Poor-man's Weather-glass) will close at daytime if the air be 

 damp and the weather dull. 



These movements induced by stimuli enable us to illus- 

 trate the meaning of irritability. If we kill the plants be- 

 fore trying these experiments, the leaves and flowers will not 

 respond to the stimuli (of light, heat, etc.). This illustrates 

 the fact that only living plants (and anirrials) are irritable. But 

 we can render plants non-irritable .without killing them. If, 

 for instance, we expose them to chloroform-vapour, or keep 

 them in a cold place, or in darkness for a considerable time, 

 they are rendered incapable of responding to stimuli : they are 

 no longer irritable. Restoring these plants to their ordinary 

 surroundings, they may soon re-acquire their irritability and 

 execute movements when stimulated by light, heat, etc. 

 These experiments incidentally show us that light and heat 

 may act on a green flowering plant in two different manners. 

 In the first place, light and heat serve to keep the plant healthy, 

 so to speak, and thus enable it to respond to stimuli. A green 

 plant immured in darkness for a considerable time or kept in a 

 cold place becomes ill and loses its irritability. Thus we may 

 say that heat and light exert a tonic influence on plants. In the 

 second place, when the plant is in a tonic (healthy) condition 

 sudden changes in temperature or light act as stimuli and may 

 cause movements. Thus light and heat also exercise stimulat- 

 ing influences on the plant. 



