METABOLISM OF ANIMALS 43 



2. Respiration and Excretion 



Of course, during life, the animal, like the green plant, is 

 continually breaking down its food and its own protoplasm 

 by a process of combustion which involves an intake of free 

 oxygen and the liberation of carbon dioxide and water. 

 Nitrogenous wastes, chiefly UREA, as well as inorganic salts, 

 are also excreted. So the animal, like the plant, returns to 

 its environment the elements in simple combinations which 

 are devoid or nearly devoid of energy. We have stated that 

 green plants are essentially constructive and animals es- 

 sentially destructive agents in nature. It is now apparent 

 that green plants are both constructive and destructive, while 

 animals are essentially destructive. 



A little consideration of the income and outgo of green 

 plants and animals will show that, although the animals are 

 dependent on the plants for their complex foodstuffs, they do 

 not return, for example, the nitrogen to the outer world in 

 a form simple enough to be available for green plants. In 

 other words the urea, (NH 2 ) 2 CO, which still has a little 

 energy left which the animal is unable to extract, must be 

 transformed into nitrates. 



Furthermore, since plants die, which are not consumed by 

 animals, and animals die, which are not devoured by other 

 animals, large stores of matter and energy are locked up in the 

 complex compounds of their dead tissues. Clearly, there must 

 be some way of completing the cycle of the elements, for if 

 there were not, life, as we know it, could not have continued 

 long on the Earth. This gap is filled by the so-called COLOJI-^ 

 LESS PLANTS, that is plants which, because chlorophyll is not 

 present, lack the power of photosynthesis and so in most 

 cases are dependent for food on more complex substances 

 than green plants demand, though not so complex as animals 

 require. 



