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 
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Fic. 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 
VAOSAAS 
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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 fer their food. 
