THE FOREST 33 



we have seen, transition types where one form of forest merges 

 into another. From north to south, from coast to mountain top, 

 the traveller finds the forests changing. 



Differences between the types are often profound. Some trop- 

 ical forests are so dense the explorer must literally carve his 

 path with axe and knife. On the other extreme forests in parts 

 of our Rocky Mountain states are so scattered one can drive a 

 car through them. In the Orient are the "dwarf forests," whose 

 tree tops come no higher than a tall man's head in California 

 and Australia are forests two and three hundred feet high. 



Some trees grow only in narrowly restricted regions like the 

 cypress which is found over a very small part of the United 

 States, or the sequoia which grows only on the coast of north- 

 ern California. Others are the wanderers of the tree kingdom 

 the willow and aspen. One finds them from coast to coast and 

 from the shadow of the Arctic Circle to far south of the snow 

 line. 



The number of different tree species varies enormously 

 throughout the world. Some regions possess many species, some 

 few. Over that great stretch of country covered by north Russia, 

 Sweden, and Norway, the forests contain only about half a 

 dozen tree species. In the hardwood forests of the East, one can 

 find ten times that many in an afternoon's walk. Tropical 

 forests have thousands of known species and perhaps hundreds 

 more not yet discovered. About eight hundred different tree 

 species grow in the United States. Some of them are practically 

 useless to man and are classed as forest weeds. It is one of the 

 objects of forestry to replace these weeds of the forest with 

 trees of greater value and usefulness. 



Some trees are more exacting than others. These we find 



