PHYSIOLOGICAL CHARACTERISTICS 17 



that some of them at least merely facilitate the work of 

 the living machine by their presence. In a crude way, 

 one might suppose that some of these salts enable the 

 living machine to function in something the same way 

 that oil enables an automobile engine to function. In 

 the latter case, the oil does not constitute a part of the 

 mechanism but the engine will not run long without it. 

 The Physiological Characteristics of Protoplasm. 

 — It has already been mentioned that wherever proto- 

 plasm is found it always exhibits certain characteristics. 

 Of these we may enumerate six: 



1. It has the power of growth, waste, and repair. 



2. It has the power to utilize foods and to liberate 

 energy from them to do work. 



3. The energy thus liberated may be used to execute 

 forcible movements. 



4. The energy liberated may be used in carrying out 

 chemical reactions in the manufacture of other sub- 

 stances. 



5. It has the power of response to external stimuli. 



6. It has the power of automatic division and re- 

 production. 



Growth, Waste, and Repair. — Wherever living pro- 

 toplasm is found it is always in a state of change. The 

 protoplasm of the growing parts of plants and animals 

 is able to utilize foods to construct new protoplasm. 

 Protoplasm wherever it exists always wastes away under 

 its own activities. All protoplasm has the power to 

 utilize foods to repair this wastage. These topics need 

 little more than mention at the present time, inasmuch 

 as they form the subject matter of a subsequent chapter. 



The Power to Liberate Energy from Food. — Food 

 is used by protoplasm not only for growth and repair but 

 also as a source of energy. It will be shown later that 

 in the formation of food the energy of sunlight is locked 

 up in the chemical compounds of sugar and other similar 

 substances. These, when used as food by protoplasm, 



