CHAPTER V 



PHYSIOLOGY 



THE life processes and reactions of a living entity form 

 the special study of Physiology, whether it be of plants 

 or animals. These life processes and reactions among 

 plants are not nearly so obvious as they are in animals, 

 but many of them are strikingly similar in the two 

 classes of creatures. The most fundamental differ- 

 ence between plants and animals is in their methods 

 of feeding. The plant is constructive, and works up 

 for itself the simplest elements into food, while animals 

 are ultimately destructive and, in using these same 

 elements, destroy their combination, and leave them 

 in a form which is useless for food until they are once 

 more worked up by plants. All the carbohydrates, 

 the starches, and sugars, and all the nitrogen compounds, 

 the proteids, are ultimately provided for the whole 

 animal world by the plant world. The study of nutri- 

 tion, then, is one of the important branches of physio- 

 logical work, but it is not by any means the only one. 

 The breathing, drinking, and moving of plants must 

 also be studied, and their appreciation of and reaction 

 to light, heat, and gravitation. The sum total of all 

 these reactions and responses results in what we call, 

 simply, growth. And this " growth " is expressed by 

 the stretching, enlargement, or alteration in shape of 

 the organs and their increase in numbers according to 

 certain rules and rhythms, which are also studied by 



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