52 FUNCTIONS OF LIVING THINGS 



fornia, stand out as notable examples. The large plants and ani- 

 mals are made up of more, not necessarily larger, cells. 



What Protoplasm can Do. — It responds to influences or stimu- 

 lation from without its own substance. Both plants and animals 

 are sensitive to touch or stimulation by light, heat or coid, certain 

 chemical substances, gravity, and electricity. Green plants turn 

 toward the source of light. Some animals are attracted to light 

 and others repelled by it; the earthworm is an example of the 

 latter. Protoplasm is thus said to be irritable. 



Protoplasrn has the power to contract and to move. Muscular 

 movement is a familiar instance of this power. Movement 

 may also take place in plants. Some plants fold up their leaves 

 at night ; others, like the sensitive plant, fold their leaflets when 

 touched. 



Protoplasm can form new limng matter out of food. To do this, 

 food materials must be absorbed into the cells of the living 

 organism. To make protoplasm, it is evident that the same chem- 

 ical elements must enter into the composition of the food sub- 

 stances as are found in living matter. The simplest plants and 

 animals have this wonderful power as certainly developed as the 

 most complex forms of life. 



Protoplasm, be it in plant or animal, breathes and throws off waste 

 materials. When a living thing does work oxygen unites with food 

 in the body ; the food is burned or oxidized and work is done by 

 means of the energy released from the food. The waste materials 

 are excreted or passed out. Plants and animals alike pass off the 

 carbon dioxide which results from the oxidation of food and of 

 parts of their own bodies. Animals eliminate wastes containing 

 nitrogen through the skin and the kidneys. 



Protoplasm can reproduce, that is, form other matter like itself. 

 New plants are constantly appearing to take the places of those 

 that die. The supply of living things upon the earth is not de- 

 creasing; reproduction is constantly taking place. In a general 

 way it is possible to say that plants and animals reproduce in a 

 very similar manner. 



The Importance of Reproduction. — Reproduction is the final 

 process that plants and animals are called upon to perform. 



