2 Science of Plant Life 



plant fibers ; and wool and silk come from animals that feed 

 on plants. 



(3) The trees supply the lumber that is used for the con- 

 struction of most houses, and even when houses are built 

 of stone or brick, wood is employed in finishing the interiors 

 and in making the furniture. Wood is used also in the 

 manufacture of paper, and in ahnost countless other ways. 



(4) Most houses are heated in winter by the burning of 

 wood, coal, or gas. The energy used in the driving of nearly 

 all machinery is derived from wood, coal, petroleum, and 

 natural gas. When we burn wood, we release, in the form 

 of heat, the great store of energy which the tree obtained 

 from the sunlight during its lifetime. When we burn coal, 

 petroleum, or natural gas, we release energy which plants 

 accumulated from the sunHght of millions of years ago. 



(5) Certain small plants have other and quite different re- 

 lations to human beings, and their activities are of the greatest 

 consequence to man. These particular plants, the bacteria, 

 are so minute that they can be seen only by the use of the 

 microscope. Some of them take nitrogen from the air and 

 build it into compounds that enrich the soil. Others render 

 a useful service by breaking down the dead bodies and waste 

 materials of plants and animals and converting them into 

 substances that can be used by green plants in the making 

 of foods. Still other kinds of bacteria cause many of the 

 diseases to which plants and animals are subject, and it is 

 necessary for us to learn about them in order that we may 

 avoid or destroy them. 



We see, therefore, that plants contribute to the pleasure 

 of hfe ; that, directly or indirectly, they furnish us with food 

 and provide most of our clothing and shelter ; that they sup- 



