CHAPTER XXVIII 



IRRITABILITY; POLARITY AND CORRELATION 



A sensitive plant has no business growing in a public highway. 



— S. Coulter. 



Heredity and Environment. — The growth and development 

 of a plant are determined by two separate sets of factors. First, 

 there are those of heredity, which the plant inherits with the sperm 

 and egg, if it is the product of sexual reproduction, or which it 

 obtains from the one parent if produced asexually. Secondly, 

 are the factors which are not hereditary but which make up what 

 is called the environment of the plant. Although this term is 

 generally used to include only those factors which are external 

 to the plant, if we include everything which is not inherited as 

 environmental, then we must also speak of the internal environ- 

 mental factors as well. 



Stimuli and Responses. — Organisms are very sensitive to 

 changes in the environment. Changes in temperature, moisture, 

 pressure, etc., soon make themselves felt, since the protoplasm is 

 sensitive to all these environmental factors and to changes in 

 them. Environmental factors which cause the plant to alter its 

 growth and development are called stimuli, among the more com- 

 mon of which are light, water, heat, etc., the effects of which 

 have already been discussed. The change which the plant makes 

 in order to adjust itself to the different environment is called the 

 response. The response is not always visible since it may be an 

 internal change which cannot be detected from the exterior. The 

 sight of food may cause an animal to move toward the food or it 

 may merely cause the animal to desire the food. Both are re- 

 sponses but only the former is visible. In the case of the desire 

 there have been rearrangements within the nervous system, but it 

 may happen that no external sign indicates these changes. The re- 

 sponse may also be purely mechanical as when a gust of wind blows 

 the branches of a plant, but these mechanical responses are not the 

 ones usually meant when stimuli and responses are discussed. 



This power which organisms have to adjust themselves to 



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