CHAPTER XXVII 

 NUTRITION OF ANIMALS 



275. Plants and animals. In a previous discussion we have 

 seen that plants which have chlorophyll can manufacture 

 foods. We have seen also that they afterwards use some of 

 these foods in their own nutrition, or that some of the surplus 

 food may be stored in the root, stem, leaf, or fruit. There is 

 a very important relation between the animal kingdom and 

 the food material in plants. Chlorophyll is not found in ani- 

 mals (except possibly in a very few simple animals), and they 

 are dependent upon green plants for the results of chlorophyll 

 work ; that is, green plants supply the food materials for all 

 the animal kingdom. 



Animals may secure their food directly or indirectly from 

 plants. Some animals may devour the green leaves of plants ; 

 others may eat plant food that has been stored in stems and 

 roots : others may eat the rich grains or fruits that have been 

 made by plants; still others may devour animals that have 

 eaten plant food ; or two or more series of animals living upon 

 one another may intervene before plants appear as the real 

 source of animal food. 



276. Processes of animal nutrition. In but few cases can 

 even the larger animals introduce their food into the interior 

 of their bodies without breaking it up more or less. This proc- 

 ess of breaking up the food, known as mastication, usually 

 involves the addition to the food of liquids which, while mak- 

 ing chemical changes, also make swallowing easier. Another 

 process, digestion, helps further in reducing the food to a liquid 

 condition. Liquid food may be taken through the walls of the 

 digestive system by the process known as absorption. It may 



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