REQUIREMENTS FOR HEAT AND MOISTURE. 29 



slope, on the coutrarj-, lias almost no trees, because 

 its rainfall is very slight, and those which do grow 

 there are small and stunted in conii)arison with the 

 giants on the west. (See PI. XI Y.) Again, certain 

 trees, like the Bald Cypress and the River Birch, grow 

 only in very moist land; others, like the Mesquite and 

 the Piny on or Xut Pine, only on the driest soils; while 



Fig. 26. — Dense forest in a region of great rainfall. Olympic Peninsula, Wash- 



ington. 



still others, like the Bed Cedar and the Bed Fir, 

 seem to adapt themselves to almost any degree of 

 moisture, and are found on very wet and very dry 

 soils alike. In this way the different demands for 

 moisture often separate the kinds of trees which grow 

 in the bottom of a valley from those along its slopes, 

 or even those in the gullies of hillsides from those on 



