CHAPTER XXII 



IRRITABILITY AND MOVEMENTS OF LIVING 

 PARTS OF PLANTS 



When plants or animals are influenced by a disturbance in the 

 world outside them, the effect produced is not so simple as is 

 the case when dead bodies are acted upon. We may illustrate 

 this statement by an example : Supposing we push a book along 

 a table, we notice two facts with reference to the behaviour of 

 the book, (i.) The amount of movement is exactly propor- 

 tionate to each push we give, (ii.) The book moves in the 

 direction of the push. We now consider an influence oper- 

 ating on a living organisih. Suppose a tiny particle of food to 

 enter the windpipe of a man, it may cause the man to cough so 

 violently that many of the muscles of his body are called into 

 play by the paroxysms. There is no proportion between the 

 insignificant disturbing external cause (the particle in the wind- 

 pipe) and the very great response. Again, if we place a bean- 

 seedling on moist earth in a dark place in such a manner that 

 its main root and main stem lie horizontal, when growth 

 begins, the end of the stem will bend up, but the tip of the 

 root will curve downwards. Thus the same disturbing cause 

 (gravity), acting in same direction on the root and stem, 

 occasions a different and even opposite response in the two 

 parts of the plant. This property of responding in a peculiar 

 manner to external clianges, and which is possessed only by 

 living plants and animals, is termed irritability. A dis- 

 turbing external influence which is able to call forth a 

 response is termed a stimulus. 



PERIODIC MOVEMENTS OF LEAVES AND FLOWERS. 

 At night-time many leaves place themselves in postures 

 different from those which they assume during the day, and 

 execute movements to attain their day-position and night-posi- 

 tion. These movements may be seen in the cotyledons of 



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