EXOGENOUS SERIES-BROADLEAF IVOODS. 69 



leaves of the several birches differ but little, but the decided 

 colors exhibited by their barks give names and serve to distin- 

 guish the species. Nine of the twenty-four known species of 

 birch occur in North America ; six are trees and the others low 

 shrubs. Betula is said to be derived from bitumen. 



