LIFE ZONES 459 



"middle plants," of neither very wet nor very 



dry ground. 



B. TRANSITION. This is the most difficult to de- Transition 

 fine, because it is in fact a meeting place of the boreal 

 and austral (southern) elements. Here will be found 

 biological tension lines, where northern and southern, 

 mountain and plain, organisms press outward from 

 their center of distribution, and meet one another. 

 The Transition may be divided into three areas : 



(a) Alleghanian Area. The humid Transition of the 



country east of the hundredth meridian. It is 

 especially prominent in Minnesota, Wiscon- 

 sin, Michigan, New York, Pennsylvania, On- 

 tario, New England, and the Alleghany region. 

 It is a region of mixed forests : chestnut, 

 walnut, oak, beech, maple, etc. The decid- 

 uous fruit trees are highly successful : apples, 

 pears, plums, etc. It is a region of hops and 

 potatoes also. 



(b) Coloradian or Arid Transition Area. This oc- 



cupies large parts of Colorado, Utah, New 

 Mexico, Wyoming, Nevada, and the North- 

 west, and is for the greater part rather barren 

 when not irrigated. It is characterized es- 

 pecially by the yellow pine and the so-called 

 sagebrush (Artemisia}. Under irrigation it 



* 



NOTE ON ZONE MAP. The Arctic or Arctic-alpine Zone, extending northward 

 beyond the limit of trees and on mountains above timber line, is not shown 

 on the map, nor are the subdivisions of the Transition Zone and the Sonoran 



Area. 



The diagonally shaded line marks the eastern border of the Great Plains and 

 divides the Austral Region into an eastern, more humid portion and a western, more 

 arid portion. East of the line the divisions are known respectively as the Alleghanian, 

 Carolinian, and Austroriparian Areas. The western portions of the same Zones are 

 known as the Transition, Upper Sonoran, and Lower Sonoran Areas. The Middle 

 Sonoran is not distinguished from the Lower Sonoran on the map. 



