304 USEFUL AND HARMFUL PLANTS 
ample, there is the influence of forests upon water-supply, 
by which is meant their action as reservoirs feeding the 
streams gradually in spring, thereby avoiding floods, and 
at the same time keeping back plenty for the dry season. 
Then, too, there is the important action of plants in soil- 
making, and the purifying influence of vegetation upon air 
and water whereby they are made to serve better the needs 
of animal life. 
All these various relations of plants to the life of the world, 
and to our own lives in particular, are as profitable and 
attractive matters of study as any that have claimed our 
attention; and the student will do well to learn all he can — 
regarding them. It should be said, however, that many of 
these relations are best understood in the light of vegetable 
biology. Moreover, the student’s pursuit of economic botany 
cannot well proceed much farther than we have here at- 
tempted to go, without his first acquiring such an elementary 
knowledge of systematic botany as the following chapters may 
help him to gain. 
