CHAPTER V 

 THE LIVING STUFF 



33. Plants and animals. We have spoken of plants and 

 animals as " living things " ; yet animals seem to most people 

 to differ from plants about as much as they do from non-living 

 things. Plants are just as much alive as animals are. They 

 are just like animals in those very points that distinguish 

 living things from non-living. That is to say, they are capa- 

 ble of growth, they are capable of movement, and they are 

 irritable, or sensitive to various kinds of disturbances — just as 

 animals are. And each organism originates from some other 

 organism. 



Yet it is true that there are differences between plants and 

 animals. In the matter of growth, plants are even better 

 growers than animals, taking both classes as a whole. This 

 means that generally a ten-pound plant can grow into a twenty- 

 pound plant more quickly than a ten-pound animal can grow 

 into a twenty-pound animal. But there are great variations in 

 the rate of growth among animals as well as among plants. 

 We may also say that in general the plants use up a larger 

 share of their total income for growth, while animals use up 

 a larger proportion of their income as fuel ; that is, more of 

 an animal's income is oxidized, releasing energy in the form 

 of heat or of motion. 



In the matter of sensitiveness, also, the animals seem, in 

 general, to be ahead of the plants, although we shall find 

 that there are some extremely sensitive plants and some 

 extremely unresponsive animals. 



It is difficult to see why, in spite of all the differences, 

 plants and animals should still be so much alike. How is it 



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