338 ELEMENTS OF COMPARATIVE ZOOLOGY. 



Plants really have their organs of feeding and of respi- 

 ration in their roots and leaves, while animals as high as 

 the parasitic worms have no organs for taking food or for 

 respiration, the absorption of nourishment taking place 

 at any spot on the surface of the body. 



Several other tests have been suggested to separate ani- 

 mals from plants. Plants reproduce by seeds, by spores, 

 and by buds; animals by means of eggs. Plants take up 

 carbon dioxide and give off oxygen; animals use oxygen 

 and give off carbon dioxide. Plants take either liquid or 

 gaseous nourishment, while animals partake of solid food. 

 Plants may have a peculiar green coloring substance called 

 chlorophyl, lacking in animals. Plants produce a peculiar 

 chemical substance known as cellulose. These features 

 when accurately analyzed are all seen to have their excep- 

 tions. Many animals reproduce by budding, while the 

 sexual reproduction of animals and of plants is essentially 

 the same. Plants require oxygen as much as animals, and 

 it is only the green plants which give off oxygen ; a mush- 

 room or a toadstool takes up oxygen and gives off carbon 

 dioxide the same as does any animal. Quite a number of 

 animals possess chlorophyl, while it is lacking from many 

 plants; and cellulose is found even in the Tunicata. In 

 the matter of food the distinction is a little sharper. While 

 some animals like the parasitic worms take only nourish- 

 ment in solution, no plant takes solid nourishment. 



Yet, although we cannot frame a perfect definition which 

 will at once separate all animals from all plants, we prac- 

 tically have little difficulty in deciding in any given case 

 that is likely to arise in our every-day experience as to 

 whether the form in question shall be placed in the one 

 kingdom or in the other. 



The difficulty of framing a definition arises from the fact 



