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: 
Hudsonian Zone. 
Canadian Zone. 
Arctic or Arctic-Alpine Zone. 
BoreaL REGION...----- 
Alleghenian Area. 
Transition Zone....-------- Arid Transition Area. 
Pacific Coast Transition Area. 
Carolinian Area. 
Upper Sonoran Area. 
Austroriparian Area. 
Lower Sonoran Area. 
Humid Tropical. 
Arid Tropical. 
AusTRAL REGION .--.---- Upper Austral Zone ..------ 
Lower Austral Zone ....---- 
TROPICAT REGION eecjecceee cs ok 2 silewee veceeeeeee 
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. 
HUDSONIAN 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. 
