6 INTRODUCTION 



role in nature in that they are the chief sources, directly 

 or indirectly, of nearly all the energy (aside from that 

 generated by falling water and wind power) which man 

 uses in his operations. From them are derived all the 

 foods and all the fuels, besides a host of other useful 

 products. It is surely important, therefore, to under- 

 stand how they are able to catch and store up the energy 

 of the sunUght in foods and fuels. 



3. The Maintenance of Life in Animals and 

 Non-Green Plant Cells is wholly dependent on food 

 manufactured by green cells. This they use to build 

 their bodies and from it they derive the energy to do all 

 sorts of work. In liberating the energy stored up in foods 

 animals decompose them into the carbon dioxid and 

 water from which they were originally manufactured. 

 Some foods are used to build living protoplasm. This 

 very gradually breaks down in the course of its life activi- 

 ties and a portion containing the element nitrogen is 

 thrown off as waste and excreted from the body. Eventu- 

 ally it also returns to the soil to be used over again. 



4. Adjustment of Organisms to Their Environ- 

 ment is necessary to their continued existence. A plant 

 must be able to direct its roots into the soil and its leafy 

 stems into the air if they are to perform their proper 

 functions. In like manner, an animal must be able to 

 finrl and recognize suitable food in sufficient quantity, or 

 it starves. The very fact that they can do these things 

 shows that there is some sort of mechanism for securing 

 proper adjustment of organisms to the environment. It 

 will be shown that this depends on certain characteristics 

 possessed by all protoplasm, but which has become highly 

 specialized in certain organs of the higher plants and 

 animals. It may appear somewhat startling to the be- 

 ginning student to realize that his organs of special sense 

 (sight, taste, smell) and the system of muscles and bones 

 and joints, the organs of locomotion, are but the ultimate 

 developments of a food-getting mechanism and that they 



