CHAPTER V 

 THE BUILDING OF THE LIVING MACHINE 



The Living Machine, the living body of plant or 

 animal, consists of protoplasm the organized units of 

 which are called cells (see Chap. II). 



The Composition of Protoplasm. — Microscopic 

 analysis and the methods of the physical chemist show 

 that protoplasm consists of some substances too coarse 

 to pass through a porcelain filter, though going through 

 others less fine. Such substances are called colloids, or 

 are said to be in the colloidal condition or state. The 

 analytical chemist shows that protoplasm is made up, 

 in addition to water, of proteins of various sorts, and that 

 it may enclose a great variety of substances in solution 

 or otherwise, carbohydrates, fats, and oils, salts and other 

 electrolytes. These simple and complex substances, 

 water, salts, oils, and proteins, are built into a colloidal 

 system or systems. Food is used, therefore, to build and 

 to maintain this complex system. But the food must 

 first be made. 



Nitrogen. — This element, forming four-fifths of the 

 earth's atmosphere, is used by most plants and by all 

 animals only in compounds. Most plants absorb these 

 as nitrates from the soil. In certain instances ammonium 

 compounds and cyanamid can be used as sources of nitro- 

 gen, and a few species of carnivorous plants are known 

 which capture or entrap small insects and devour or con- 

 sume them to secure nitrogen. Nitrates of calcium and 

 potassium are the common salts used by plants as sources 

 of nitrogen. Nitrogen is a constituent element of pro- 

 tein, combined with carbon, hydrogen, and oxygen to 



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