534 RELATION TO ENVIRONMENT. 



trees are known which have lived several thousand years. It 

 is said there is in Kent, England, a tree of the genus Taxus, 3000 

 years old ; also that there are now living on the slopes of Mt. ^Etna 

 chestnut trees from which Homer might have gathered nuts; 

 in southern Mexico there is an old cypress tree (Taxodium) 

 believed to be about 6000 years old, and in the Cape Verde 

 Islands an Adansonia of similar age. Another account states 

 that this old cypress in Mexico is about 2500 years old. It is 

 difficult to get accurate data concerning trees of such age, but 

 in the case of the Big trees of California (Sequoia washing- 

 toniana) data have been obtained by counting the annual rings 

 of a number of trees which shows their age to range up to 4000 

 years. 



II. Boreal Forests. 



The Boreal forests of North America form transcontinental 

 belts and-according to biothermal lines are in two zones. 



1013. Forests of the Hudsonian zone. These are the north- 

 ernmost of the forests, extending from Labrador to Alaska, 

 reaching the farthest north of tree growth, and extending south- 

 ward on the upper timbered slopes of the mountain ranges of 

 the United States and Mexico. The trees are mainly spruces and 

 firs, with here and there colonies and mixtures of birches and 

 aspens. The conifers, especially the spruces, firs, and balsams, 

 are better adapted to growing at low temperatures than other 

 trees, and this is why we find them pushing so far into the cold 

 of the north or that of high mountain altitudes. The leaves 

 are small, comparatively thick, and with a thick cuticle which 

 retards transpiration. This is necessary on account of the 

 high winds, which increase the loss of water, and the cold soil, 

 which retards absorption. The conical form of the tree dis- 

 tributes the weight of snow more evenly, so that the danger of 

 breaking is lessened, while the tops offer less resistance to the 

 wind, which is more severe in these regions. The higher mountain 

 peaks in the northern Alleghanian and in the Rocky Mountains 



