30 PLANT LIFE OF ALABAMA. 



mean temperature during a brief period covering the hottest part of 

 the year. 



These boundaries were found to conform with the limits of dis- 

 tribution of species as arrived at by field observations. Differences 

 in moisture, the factor next in importance to heat in the 'distribution 

 of life on this continent most potent in its influence from east to 

 west determine the distribution of animals and plants within dis- 

 tinct areas of these life zones. 



Based on these principles Dr. Merriam recognizes in the North 

 American continent three primary divisions or regions Boreal, Aus- 

 tral, and Tropical and seven transcontinental zones. His classifica- 

 tion is as follows: 



(Arctic or Arctic- Alpine Zone. 



BOREAL REGION < Hudsonian Zone. 



I Canadian Zone. 



(Alleghenian Area. 

 Arid Transition Area. 

 Pacific Coast Transition Area. 



Upper Astral ^ (K^o^W. 



AUSTRAL REGION 



Ttnrrnw / Humid Tropical. 



---\AridTropical. 



BOREAL REGION. 



This, as seen in the table, presents three belts or natural subdivi- 

 sions. 



ARCTIC OR ARCTIC-ALPINE ZONE. 



This division, the first of the three, extends from the limit of tree 

 growth to the polar lands. The greatest portion is covered with eternal 

 snow and ice, and farther south it includes the summits of the moun- 

 tains reaching above the line of arboreal growth, subject to the same 

 conditions. This zone is recognized by some writers as the hyper- 

 boreal region, and as the realm of mosses and saxifrages. 



HUD8ONIAN ZONE. 



The second, namely, the Hudsonian or subarctic zone, embraces the 

 most northern part of the American forest. Its spruces, firs, and 

 most of its birches and aspens extend from the shores of Labrador to 

 Alaska. This zone is spoken of as the Northern Forest or Lacustrian 

 Forest of the North. 



CANADIAN ZONE. 



This is the third subdivision, or the second of the northern belts 

 of cone-bearing trees. In this zone pines, spruces, firs, and hemlocks 

 outnumber the deciduous forest growth. In the southern and warmer 

 parts of this belt the cultivation of the hardier cereals oats, rye, 

 barley is possible, and potatoes and other root crops succeed. 



