138 ^ GENERAL PHYSIOLOGY 



the case of the compound organism, eating and drinking constitute 

 merely an extrinsic part of the process ; whatever is thus brought 

 to a single organ, the stomach, is for the good of each one of the 

 many millions of cells that constitute the body. If the life of the 

 body is to be maintained, all cells must take in certain food-sub- 

 stances. The following consideration must, therefore, cover two 

 points first, the nature of the substances that every cell needs in 

 order to maintain its life, and, second, the mode of ingestion of 

 those substances. 



1. Food-stuffs 



All living matter is continually undergoing decomposition and, 

 hence, must take in substances that contain all the chemical 

 elements of which it is constructed. 



While it is a vital phenomenon of every cell to take in food- 

 stuffs, the latter differ in kind with every form of cell. But 

 in spite of all specific differences in the substances that each form 

 of cell requires for its life, all organisms may be classified into a 

 few large groups, within each of which a general agreement in the 

 kind of nutrition prevails. 



A fundamental difference in the nutrition of plants and of animals 

 was discovered early. All green plants take up from the earth 

 and air simple inorganic materials from which to construct their 

 living substance ; on the other hand, all animals without excep- 

 tion, in order to be able to maintain life, require highly complex 

 organic compounds. 



This fact is easily confirmed. In order to prove that animals 

 cannot exist without organic food, it is only necessary to perform 

 suitable feeding experiments. When fed with purely inorganic 

 matters, such as water, salts, etc., even when these contain all the 

 chemical elements of living substance in the correct proportion, 

 animals always die after a longer or shorter time. On the other 

 hand, it can be shown that plants live solely at the expense 

 of inorganic substances, by allowing them to grow in so-called 

 nutrient solutions, which possess in the form of inorganic salts 

 the chemical elements that are necessary to the formation of 

 living substance. Such a nutrient solution, which contains in 

 soluble compounds the elements N, H, O, S, P, Cl, K, Na, Mg, Ca, Fe, 

 i.e., with the exception of carbon, all organic elements, is composed, 

 according to Sachs ('82), as follows : 



Water 1,000 c.c. 



Potassium nitrate 1 gr. 



Sodium chloride 0*5 



Calcium sulphate 0'5 



Magnesium sulphate 0'5 



Calcium phosphate 0'5 



Ferrous sulphate 0'005 



