2 INTRODUCTION TO BOTANY 



marketing coal, or by any kind of manufacture in which the 

 motive power is obtained by the aid of coal, oil, or gas used 

 as fuel all these people depend upon the products of the 

 plant life that was abundant on the earth or in the seas 

 during past ages. The air which we breathe is purified partly 

 through processes by means of which green plants live. It is 



FIG. 1. Sparse vegetation as seen upon the sand dunes along the shore 

 of Lake Michigan 



The wind-blown sand presents a poor living place for plants, although along the 



upper beach a fringe of trees and other plants has become established. In the 



view here shown the sand has been blown into definite ripples reaching down 



almost to the water of the lake 



evident, then, that human life could not continue for a single 

 week without the aid of substances produced by the life and 

 growth of plants. 



2. Plant production. The domesticated animals that are so 

 extensively used to help man in his work, and those that he 

 grows for food or for market, could not be cared for if it were 

 not possible to have plants or plant products for their food. 



