58 GENERAL BIOLOGY 



the pi ant-eaters for food . For animals , unlike plants , 

 are quite unable to utilize, directly, the energy of 

 the sun's rays, and combine into sugars and starches 

 the water and CO 2 with which they are surrounded. 

 Nor can they, like plants, utilize the nitrogen as it 

 exists in simple combination. Nitrogen is an es- 

 sential element of protein, and protein an essential 

 of protoplasm, and without it the animal cannot 

 repair the wastes of katabolism. But this nitrogen 

 must be furnished to the animal already combined in 

 proteins. 



The metabolism of animals therefore begins at a 

 higher level than that of plants. Plants take in and 

 assimilate gases and liquids of very simple composi- 

 tion, whereas animals require liquid or solid food al- 

 ready organized as fats, carbohydrates, or proteins. 

 The latter kind of nutrition is sometimes referred to 

 as Holozoic, the former, which is characteristic of all 

 green (chlorophyll-bearing) plants, as Holophytic. 



Some groups of plants, however, show decided 

 exceptions to such a rule. The bacteria and the 

 fungi, for instance, lack chlorophyll and cannot 

 manufacture starch, but must depend on other or- 

 ganisms, either plant. or animal, for their food supply. 

 Such plants are called parasitic when they feed on 

 living tissue, or saprophytic when they subsist on dead 

 and decaying tissue. Even some of the higher 

 plants, such as the dodder, a relative of the morning 

 glory, have abandoned the independent manufacture 

 of their own food materials and live as parasites 

 on other plants. 



