METABOLISM OF ORGANISMS 37 



3. Respiration and Excretion 



Of course, during life the Amoeba, like Pleurococcus, is con- 

 tinually breaking down its food and its own protoplasm by a proc- 

 ess of combustion which involves an intake of free oxygen and 

 the liberation of carbon dioxide and water. Nitrogenous wastes, 

 chiefly uric acid or urea, as well as inorganic salts are also ex- 

 creted. This interchange takes place over the entire surface of 

 the animal, aided by a contractile vacuole that expels fluid 

 from the cell. So the animal, like the plant, returns to its environ- 

 ment the elements in simple combinations which are devoid or 

 nearly devoid of energy. We have stated that green plants are 

 essentially constructive and animals destructive agents in nature. 

 It is apparent, of course, that green plants are both constructive 

 and destructive, but the constructive processes of green plants are 

 necessary and sufficient not only for themselves but for all living 

 things. 



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. For example urea, (NH^CO, 

 which still has a little energy left that the animal is unable to 

 extract, must be transformed into nitrates. 



Furthermore, since plants die, which are not consumed by ani- 

 mals, 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 colorless plants ; that is, plants 

 which, because chlorophyll is not present, lack the power of photo- 

 synthesis and so in most cases are dependent for food on more 

 complex substances than green plants demand, though not so 

 complex as animals require. 



C. Colorless Plants 



As representative of the diverse types of colorless plants which, 

 lacking chlorophyll or a functionally similar pigment, are without 

 the power of photosynthesis, we select the vast group known as 



