342 - Part III. Chapter 5. 
DRUDE’) gives the following useful summary of the vegetation-regions of 
North America transcribed in the original German. 
Gletscherwald- und er von Alaska mit arktischer Glacialflora. 
ie Kanadische 
Waldregion des N REN Seengebietes. 
Columbische Küstenwaldregion. 
Die Wald- und Hochgebirgsregion der nördlichen Rocky Mountains. 
Nördliche Waldprairieregion. 
Die ee ren on. 
Steppen- und Salzwüstenregion der Rocky Mountains. 
Kulforntiche Niederungs-Bergwald- und Hochgebirgsregion. 
. Sommergrüne Laubwaldregion des Mississippibeckens. 
. Die immergrüne Vegetationsregion der südatlantischen Staaten. 
. Steppen- und Wüstenregion von Arizona. 
. SEBPBAHAL ‚Vegetalopäregion von Texas und Nordmexico. 
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. Mexicanische subtropische Bergwaldregion (Tierra templada). 
17. Mexicanische Hochgebirgsregion (Tierra fria). 
18. Mexicanische subtropische Steppenregion. 
. Die tropische und Vegetationsregion von Nicaragua und Costa Rica. 
Antillen- und Bahamaregionen. 
a) Die dürre Croton- und Cacteenregion. 
b) Die antillanische Tropenregion. 
c) Die antillanische Bergwaldregion. 
d) Die antillanische Hochgebirgsregion. 
DB - 
[e Exe) 
MERRIAM 1898 states that the northward distribution of terrestrial animals 
and plants is governed by the sum of the positive temperature for the entire 
season of growth and reproduction, and that the southward distribution is 
governed by the mean temperature of a brief period during the hottest part 
of the year. Applying these principles, Merriam divides North America into 
three primary transcontinental regions (better to be named «zones»), viz., the 
Boreal, the Austral and the Tropical. 
The distinetive temperature of the three boreal zones (Arctic, Hudsonian and Canadian) are 
not positively known, but the southern limit of the boreal, ‘as a whole, is marked by the iso- 
herm of 18°C. (64.4° F.) for the six hottest consecutive weeks of the summer. It seems pro” 
bable that the res temperature of the southern boundaries of the Hudsonian and Arctic zones 
are respectively 14° C. (57.2° F.) and 10°C. (50° F.) for the same period. Transition zone 
species require a total quantity of heat of at least 5.500° C, (10,000° F.), but cannot endure a 
summer temperature the mean of which for the six hottest consecutive weeks exceeds 22°C. 
(71.6° F.). The northern boundary of the transition zone is marked by the isotherm showing 
a sum of normal positive MRPEIICESR of 5.500° C. (10.000° F.), while its southern boundary is 
coineident with the isotherm of 22°C. (71.6° F.) for the six hottest consecutive weeks. The 
transition zone comprises according to Merriam three principal en an eastern, OF 
Alleghanian humid area; a western arid area, and a Pacific coast humid are 
1) DRUDE, Oscar: Handbuch der Pflanzengeographie, 1890; cfr. Atlas der Pflanzengeogr-, 
UOROETRDRN, p- 40. : 
