34 HUMAN PHYSIOLOGY 



rather than the fuel as the repository of energy waiting 

 to be set free would be an unusual and confusing mental 

 practice. We shall therefore place oxygen in contrast 

 with food instead of extending the term food to include 

 this other requisite of the living state. 



Summary. Animals and plants alike, though in un- 

 equal degree, are engaged while they live in turning the 

 latent or potential energy of organic compounds (fuels) 

 into heat. Mechanical work is done by them also and 

 is a manifestation of the same great process. This 

 oxidative release of energy which is coextensive with life 

 we call respiration. It would soon terminate because 

 of the exhaustion of the fuel supply of the world if a 

 compensating process were not going on. This is photo- 

 synthesis, the manufacture of fresh stores of organic 

 matter by the higher plants. It cannot occur unless 

 there is a supply of energy to drive it, and the ultimate 

 origin of this energy is in the sun. 



It is to be borne in mind that it is not only animals for 

 whose ravages the higher plants have to compensate. 

 The lower plants are destructive in their general tend- 

 ency. The fires which men build and those which run 

 riot in the forests or over the prairies destroy immense 

 quantities of fuel which can be recovered only by the 

 pigmented plants and in the light. It seems an unequal 

 struggle on the part of the leaves. Yet for ages the 

 balance has been fairly maintained. It is a question 

 whether human extravagance has now begun to disturb 

 it. Many think that this is the case and pleas for the 

 conservation of coal and the extension of woodland areas 

 have become familiar. Whenever the power of falling 

 water is substituted for that of steam we are sparing 

 organic fuel, though we are still employing the energy of 

 the sun. 



The Work of Bacteria. The statement has been made 

 that colorless plants act more or less like animals as 

 judged by the chief chemical effects which they produce. 

 It has also been said that they destroy the dead remains 



