DIFFERENCES BETWEEN ANIMALS AND PLANTS 23 



body is organised round the gut (alimentary canal), 

 which digests the organic food, i.e. changes it into 

 forms which can directly nourish the body. The 

 branching habit of plants, on the other hand, exposes 

 the greatest possible surface to the soil and to the air, 

 from which the plant directly absorbs the liquid and 

 gaseous inorganic substances which form its food. 



Plants are green or have green parts because of 

 the green colouring matter {chlorophyll ^) which enables 

 them to build up their bodies from simple inorganic 

 substances — or, in other words, to form organic 

 substances from inorganic. 



And the many other differences of structure and 

 organisation which distinguish the higher animals and 

 the higher plants are all related, directly or indirectly, 

 to the fundamental difference in their foods. 



There are, however, cases in which both animals 

 and plants depart, in one or other respect, from these 

 general characters. There are parasitic animals whichv 

 do not move about, but remain fixed on or in the 

 " host," and which consume only liquid food, though 

 this is always organic, and derived from the body of 

 the host. There are plants which are partly carnivorous, 

 with special organs which digest solid animal food 

 and absorb the products of digestion. There are 

 branching animals and compact plants. There are 

 plants which live largely on organic (liquid) food. 



Furthermore, there are simple minute microscopic 

 organisms living in water and other liquids which 

 have some animal and some plant characters, so that 

 biologists have sometimes considered them as animals 

 and sometimes as plants. These last are more or less 

 changed representatives of some of the organisms 



' " Leaf-green," from Greek x^apdg, green, and ^A\ov, a leaf. 



