58 A PRIMER OF BIOLOGY 



means reduced to their ultimate constituents, are 

 also got rid of by the kidneys and sweat glands in 

 animals. Several of these bodies are highly nitroge- 

 nous, such as urea, uric acid, &c. In plants the solid 

 waste is stored either in parts of the body which are 

 periodically thrown off, e.g., leaves, bark, fruits, &c., 

 or is permanently retained in tissues, such as old 

 wood, otherwise of service only for mechanical pur- 

 poses (p. 88). It is unnecessary for our present 

 purpose to go further into these subjects since the 

 task before us is to endeavour to master the principles 

 of biology, not the details of the different physiological 

 processes. 



In Chapter VI an attempt was made to show 

 graphically the circulation of matter from the in- 

 organic world through the organic back once more 

 to the inorganic. We must now try to express 

 graphically the circulation of energy. 



We have seen already in Chapter VI (p. 48) that 

 solar energy is in part stored as potential energy 

 in the complex organic compounds formed by the 

 green organism during the process of photosynthesis, 

 and that these compounds are used as " food," that 

 is to say, as stores of potential energy, by the green 

 plant itself, or by animals which feed on other animals, 

 which feed, in turn, on green plants. In a word, we 

 learned that the ultimate source of all " food " of 

 non-green organisms is the green plant, and that we 

 ourselves are dependent for our nutriment in the long 

 run on the activities of chlorophyll. Further, we 

 see that the ultimate source of the matter of which 

 the body of the highest organism is composed is, 

 ultimately, the soil, the water, and the air ; and that, 

 in the process of tissue metabolism as the sum of 

 all these complex chemical changes is termed and 



