Introduction 13 



for the nutrition of all higher organisms. This 

 is accomplished in two ways; first, by the assimila- 

 tion of free nitrogen and the changing of certain 

 nitrogeneous constituents of the soil into forms that 

 can be used by the higher plants; and second, 

 through the decomposition of dead organic matter 

 which is thus put in such a form that it can again 

 be used by green plants. All organisms in the 

 course of their development give off waste products 

 which ultimately serve as food for other organisms. 

 Most of these waste products except CO 2 and water 

 are not directly available for the food of green 

 plants, but must first be acted upon by bacteria. 

 The bacteria, also, as has been indicated, are the 

 principal agents by which the dead tissues of plants 

 and animals are decomposed so that they are again 

 available as food for the higher plants. There 

 are, however, many undoubted plants such as the 

 fungi i.e., molds, mildews, rusts, mushrooms, etc. 

 as well as certain flowering plants that are desti- 

 tute of chlorophyll, like the dodder and Indian pipe, 

 which require organic food just as animals do. 



We may say, however, that the typical plant con- 

 tains chlorophyll and has the power of photosyn- 

 thesis; that is, can use the energy of sunlight for 

 the manufacture of organic compounds from CO 2 

 and water. Animals are not provided with chloro- 

 phyll and are entirely dependent upon organic food 

 for their existence. Animals are as a rule much 

 more mobile than are plants, and this is correlated 



