278 FORESTS AND TREES OF NORTH AMERICA, 



near the limits of perpetual snow, the Esquimmix vegeta.tion is almost 

 precisely copied. But, on the other hand, the lower plains present 

 every shade of succession, from the continuous forests of the Apa- 

 lachian, through the rich prairies of the Illinois, to the barren deserts 

 of the Camanche region. All these characteristics occur, however, in 

 comparatively narrow belts surrounding isolated peaks or ranges,, and 

 the species of trees met with are nearly all distinct from those of the 

 eastern provinces. 



Another distinctive character is in the fact that this province receives 

 its rains from the west, (except, perhaps, some of the most eastern 

 mountain slopes and those of Arizona,) and the supply of moisture 

 is in direct proportion to the vicinity of any region to the Pacific, and 

 the obstacles between it and that reservoir. Thus the Sierra Nevada 

 cuts off almost all the rain from Utah, the little that reaches its eastern 

 part being from local evaporation and what is intercepted by the lofty 

 central ranges from the higher currents of tlie atmosphere. 



It thus happens that no constant elevation and no similar exposure 

 has always the same amount of forest or other vegetation ; local cir- 

 cumstances make every range of mountains and every valley dilEfer 

 somewhat from those around it. 



But, as a general fact, we find that those regions towards the no7ih 

 are the best supplied with moisture, and therefore best wooded — ex- 

 actly contrary to the character of those regions which receive their 

 moisture from the Gulf of Mexico. 



Near the Mexican boundary we enter the belt of rainless regions 

 described by Lieutenant Maury ; and here the supply is indeed preca- 

 rious, though apparently more adequate to vegetation than in the 

 Utah region. 



Although I have enumerated a long list of trees as first appearing 

 on this line, it is in reality, for the most part, a treeless belt. Scat- 

 tered individuals of numerous species occur, often limited to one nar- 

 row locality, as if merely outlines of more extensive forests in Mexico, 

 or of what were once more extensive here, and have been destroyed 

 by drought. Lieutenant Ives found great tracts of some of the more 

 common trees thus standing dry and dead, as if killed within a recent 

 period ; but this is not the place to discuss these apparent changes in 

 the climate of the country. 



The higher San Francisco and other ranges seem, however, to 

 receive a better supply of moisture from the upper strata of the air, 

 while their more impervious rocks probably retain it, and their cool 

 summits condense around them enough moisture for the leaves of trees. 

 I may remark here, that it would seem as if trees, rising high above 

 the surface of the ground and expanding a vast evaporating surface of 

 leaves to the air, require a greater degree of moisture in the air than 

 herbaceous plants. They cannot, like the herbs of all arid regions, 

 dry up and die down to their roots^ to spring again with the wet sea- 

 son ; they must retain vitality throughout or die. This constitutes a 

 real physiological distinction between trees and herbs. The shrubs 

 which live in those arid regions, presenting less evaporating surface, 

 and having larger rootstocks in proportion, withstand droughts. 



6. The (Jalifornian region stands alone, unless combined with the 



