PLANTS AND ANIMALS 33 



more oxidation than reduction, much more cleavage than 

 synthesis, but some of the energy made available by the 

 decomposition is turned to account to accomplish a cer- 

 tain amount of reconstruction. 



Food. It is now apparent that what we mean by a food 

 is generally something which can be oxidized to yield 

 energy for the support of the activities of the living state. 

 In other words, most food is fuel. But this is not a suf- 

 ficiently inclusive definition for the food of animals or 

 plants. We wish to reckon water as a food and we cannot 

 regard it as a fuel. There is the same difficulty with 

 mineral salts. We ought to regard as food any supply 

 that ministers to growth or repair as well as to the evolu- 

 tion of energy. Water is then a food because it makes 

 good an unavoidable loss which the body suffers. 



The proteins occupy an interesting position among 

 foods because they are necessary for the construction of 

 new living matter and at the same time they are available 

 as a source of energy. The owner of a house might have 

 a quantity of lumber brought to his premises with a 

 view to having an addition built. If he then abandoned 

 his plan he might saw up the boards and beams and use 

 them for fuel. So we supply ourselves with proteins 

 which are especially adapted to constructive uses and 

 yet we consign most of them to our vital fires. The 

 comparison is not wholly just; it conveys an impression 

 of wastefulness and improvidence which we should 

 scarcely hold to be valid in our own case or in that of 

 animals, in general. 



Ought we to class oxygen among foods? This has 

 sometimes been done, but a good reason can be given for 

 not admitting it to such rank. A food should either be 

 a fuel or a means of repair. Now oxygen is an agent of 

 destruction, and even though the decomposition is a 

 highly characteristic part of life it is well to keep oxygen 

 in a class by itself. We do not regard the draft which 

 keeps the fire going in a stove as resembling the coal 

 which we must also furnish. To think of the oxygen 



