THE PLANT COVERING OF THE EARTH 229 



Those types of plant associations in which trees are the 

 most conspicuous part are known as forests. There are other 

 types in which the grasses are most important, and they are 

 known as grasslands or meadows. 



247. Value of wild plants. The natural plant covering of 

 the earth's surface is of very great value to mankind. Possibly 

 the forests are the most valuable natural plant associations. 



FIG. 112. Plant associations on the margin of a pond 



Note the rushes in the water, the low-shrub and tall-shrub associations beyond, 

 and the tree association in the background 



They supply us with wood, which we use for numberless pur- 

 poses. If it were not for our forests we should be deprived of 

 the use of wood for all these purposes, and it is difficult to see 

 how we should meet these needs. It is of greatest importance 

 to our nation that the wood of our forests should be used 

 in such a way as to prevent waste and destruction (figs. 113 

 and 114). The forest is also of much importance in pre- 

 venting the erosion of the soil of steep hillsides, as we have 



