140 GENERAL PHYSIOLOGY 



animals exist, such as carnivora, which require only animal food- 

 stuffs, especially flesh ; but, if the source of their food be 

 sought, it is always found ultimately in herbivora, and the 

 latter cannot live without plant-food. Thus, the carnivora depend 

 ultimately upon the existence of plants. Without plants all 

 animals would die, for plants alone are able to manufacture from 

 inorganic substances the carbohydrate, the fat and the proteid that 

 animals require for their existence. The old philosophy of nature, 

 prevalent at the beginning of the present century, was, hence, hot 

 entirely incorrect when in this sense it termed the whole animal 

 world parasites of the plants. 



For a long time it was believed that this difference in the 

 nutrition of animals and plants is an absolute one, that all living 

 cells, as regards their metabolism, can be divided simply into 

 animal- and plant-cells. But it has been found that the difference 

 exists only within certain limits, viz., only so far as animal-cells 

 and green, i.e., chlorophyll-containing, plant-cells are concerned, for 

 those constituents of the plant-cell in which carbonic acid is 

 received and elaborated are exclusively the green chlorophyll- 

 bodies. There are plants without chlorophyll — e.g., the fungi — which 

 in their metabolism form to some extent a transition between 

 animals and green plants. 



The fungi do not have the power of the chlorophyll-containing 

 plants to extract carbon from the carbonic acid of the atmospheric 

 air ; in order to satisfy their need of carbon they require, like 

 animals, organic substances, such as proteid, carbohj'drate, etc. 

 On the other hand, the fungi behave like plants in so far as the}' 

 satisfy their need of nitrogen from the inorganic salts of the earth, 

 while animals obtain their requisite nitrogen only from proteids 

 and their derivatives. These facts follow from experiments mth 

 nutrient solutions, in which fungi do not grow when no organic 

 material is at their disposal ; if, however, besides nitrogenous salts, 

 sugar be added to such a solution, they grow vigorously. 

 Thus, the fungi constitute a group of organisms which, as regards 

 their metabolism, combine half animal and half plant characters. 

 But still other relations occur in nature ; for among micro-organ- 

 isms numerous entirely similar transition-forms occur, and the 

 more the very peculiar life-relations of these microscopic beings, 

 especially the Bacteria, are investigated, the more it appears that 

 in this group of lowest organisms the metabolic relations in 

 general are not so sharply differentiated as in the higher organised 

 animals and plants. Thus, very recently the clever investigator, 

 Winogradsky ('90), has discovered Bacteria that live in the earth 

 and construct their living substance entirely from inorganic 

 material, chiefly ammonium carbonate and certain mineral sub- 

 stances. These remarkable nitrogen-bacteria (Nitromonas), there- 

 fore, although they possess no chlorophyll, behave exactly like 



