t; 2 ELEMENTARY ZOOLOGY. 



use it after it has been built up by other organisms into 

 complex substances like sugar and proteids. 



68. This relation of the living things to these two im- 

 portant elements in the food supply may be put more 

 directly from still another point of view. 



Green plants (organisms possessing chlorophyll) can, 

 under proper conditions of sunlight, take up the gaseous 

 carbon dioxide of the atmosphere, unite it with the hy- 

 drogen and oxygen in water, and build it up into starch 

 of its own. Using this starch, such a plant can take the 

 inorganic nitrogenous compounds of the soil (only in rare 

 instances the free nitrogen of the air) and build up pro- 

 toplasm or any other complex substance which it needs, 

 without any organic food at all. In other w r ords, such 

 plants are most nearly independent of other organisms. 



Fungi, or the non-green plants, can do the same, pro- 

 vided they are supplied with the starch or sugar. That 

 is to say, they can get their nitrogen from inorganic 

 sources, provided their carbon is supplied in an organic 

 form. This is the reason that the yeast plant, which is 

 a fungus, must be supplied with some starch or sugar 

 for its growth. These forms are found as parasites or 

 saprophytes on organic matter. 



Animals must have both their carbon and nitrogen in 

 organic form ; that is, in complex and unstable compounds, 

 like starches, fats, and flesh or its equivalent in vegetable 

 protein. Animals are thus seen to be most dependent 

 on organic sources of food ; green plants least dependent ; 

 and the fungi are intermediate between the others. 



69. Summary. 



i. Plants and animals agree in so many respects that 

 we must regard them as being fundamentally alike. The 



