No. 17] FLOWERING PLANTS AND FERNS 21 
Arctic, or Arctic-Alpine Zone. 
) Hudsonian Zone. 
Canadian Zone. 
Boreal Region— 
Alleghanian Area. 
| a ae (arid Transition Area. 
Pacifie Coast Transi- 
\ tion Area. 
{Upper Austral ) Carolinian Area. 
Austral Region— \ Jone. / Upper Sonoran 
Area 
Zone. Lower Sonoran Area. 
Lower Austral Austroriparian Area. 
Humid Tropical 
EP OWICAL CRON, fr 8, ele-acschecters 
Arid Tropical 
In the Boreal Region the Arctie or Arctic-Alpine Zone 
embraces the treeless tundra from the northern limit of forests 
to the pole, and the crests of the highest mountain ranges in 
the United States. The Hudsonian Zone embraces the great 
northern evergreen forests of spruce, fir, birch, and aspen, ex- 
tending from Labrador to Alaska; the Canadian Zone, the 
more southern forests of spruce, fir, balsam, hemlock, and pine, 
with some oaks and other deciduous trees. The Southern limit 
of this zone extends into the northernmost parts of the United 
States. 
The Austral and Tropical Regions are divided into an 
eastern and a western area presenting distinct differences due - 
to difference in rainfall. The Alleghanian Area of the Transi- 
tion Zone includes the great deciduous forest region of the 
