THE ANIMAL IN THE WORLD 531 



of a mimosa causes distant leaflets to fold. That it can 

 execute movements may in many cases be seen under the 

 microscope, when it will be found to stream round the cell. 

 That it makes substances by chemical activity and secretes 

 them is illustrated by the long list of drugs and other 

 substances obtained from plants. That it grows and re- 

 produces need not be argued. 



For all this agreement in essentials, however, there are 



between animals and plants distinctions which 

 Differences are both far-reaching and obvious. We may 

 AnhnaJsand ta k e our start from familiar notions on the 

 Plants. subject. Any one who tried to state in words 



the ideas which he had unconsciously formed of 

 animals and plants would probably find them to be some- 

 what as follows : An animal is a being that moves and 

 feeds; a plant is a green thing that grows in the earth. 

 Let us examine these notions. It will be best to base our 

 analysis upon our definition of a plant. We find that the 

 information it implicitly contains is : (1) That the plant is 

 green, (2) that it does not swallow food, but draws nourish- 

 ment from the earth (the fact that it also obtains food from 

 the air is less generally known), (3) that it is fixed in one 

 place and does not move about — usually, indeed, does 

 not move at all. 



1. The green colour of plants is due to the presence of 

 chlorophyll '(p. 167). This is never found in animals, except 

 in certain cases where minute green plants live embedded 

 in the protoplasm of animal bodies, as in the green Hydra. 

 At the same time it must be remembered that certain 

 plants, such as the fungi, have no chlorophyll. 



2. More important than the mere presence of chlorophyll 

 is its function in the body, which is connected with the 

 nutrition of the plant. We have already seen (p. 167) 

 that this function is the obtaining of carbon from carbon 

 dioxide by means of the energy of the sun's rays, the carbon 

 being caused to combine with the elements of water to 

 form carbohydrates, and afterwards with nitrogen, sulphur, 

 and phosphorus obtained in inorganic salts, to form more 

 complex organic substances. From this peculiarity of 

 nutrition arise several other features peculiar to the life 

 of plants, (i) We have here the reason for the well-known 



