PRACTICAL STUDY OF PLANT FORMATIONS. 641 



1131. According to Merriam. The most recent arrangement of vegeta- 

 tion regions as applied to North America are the life zones and areas of 

 Merriam, which are based on the climatic factors, temperature and moisture, 

 the influence of temperature being most important during the period of 

 growth and reproduction. These life zones and areas have been worked 

 out more largely with reference to animals, though plants have been con- 

 sidered also, and Merriam believes that the distribution of plants as well as 

 animals is limited by these life zones. For a discussion of them see Chap- 

 ter XLVIII ; see also fig. 492. 



I. LIFE ZONES AND AREAS OF NORTH AMERICA. 

 Regions. Zones. Areas. 



f Arctic, or 

 ; Arctic-alpine. 

 Boreal 1 Hudsonian. 



Austral. . 



Canadian. 



l Alleghanian. 

 Transition -j Arid transition. 



' Pacific coast transition. 



Upper Austral (Carolinian. 



( Upper Sonoran. 



Lower Austral J Austroriparian. 



I Lower Sonoran. 

 Semitropical Gulf strip. 



( Humid tropical. 

 Tro P lcal 1 Arid tropical. 



2. CHARACTERISTICS OF THE ZONES AND AREAS. 



1132. Briefly the limits and characteristic plants of the zones and areas 

 are as follows: 



BOREAL REGION. There are three natural belts. 



1. Arctic or Arctic-Alpine Zone. This is the northernmost belt and lies 

 beyond the limit of tree growth. The larger part is perpetually covered 

 with snow and ice. It is characterized by extensive fields of mosses, by 

 the arctic poppy, dwarf willows and various saxifrages, as well as other 

 arctic plants, and by some writers is called the hyperboreal region. 

 Within the United States the Arctic -Alpine zone is restricted to the area 

 above timber line on the summits of high mountains. It is inhabited by 

 arctic-alpine plants and animals, and is far too cold for agriculture. 



2. The Hudsonian Zone. This is a subarctic zone embracing the most 

 northern part of the great transcontinental coniferous forests a forest 

 of spruces, firs, birches, and aspens, stretching from Labrador to Alaska 

 in the region of Hudson's Bay. By its position it is sometimes called 



