24 



UNITED STATES AND MEXICAN BOUNDARY. 



most 



mewliat stimulating property to the atmospl 



This odor is due not only to the ahundance of artemisiaS; hut to a great variety of plants, 

 especially those of the Labiate order, such as species of Salvia and Audihertia. Most of the 

 Compositce found here are more or less aromatic. In all cases the aroma seems to depend upon 

 a resinous exudation, which prohahly performs the office of checking evaporation, by closing 

 the pores of the leaves^ and thus enables the vegetation of these arid tracts to survive the long 



continued dry season. 



The effect of the peculiarities of the climate upon the vegetation of this region may be noticed 

 here. The singular alternations of wet and dryness^ heat and cold^ produce a confused blending 

 of seasons. Upon the immediate coast most of the annuals and those perennial plants having a 



m 



impediment Irom 



months the temperature rarely falls below the freezing point. When the Novemcer rains com- 



e falling vegetation puts on its spring aspect, every barren hill is soon clothed in green^ and 

 by midwinter flowers are blooming in profusion everywhere^ and many have even passed their 

 brief season and have gone to seed. At this time the larger trees and deciduous shrubs drop 

 their leaveS; and only resume them at the close of the rainy season. This seeming exception to 

 '^e general activity of vegetable life during the winter months is accounted for by the fact that 

 ilio class of plants alluded to is almost exclusively confined to the margins of streams^ hence 

 their growth depends less upon moisture from the atmosphere. As dryness advances, during 

 ihr^ months of May and June, all the evanescent forms are swept away, and the profusion of 

 spring flowers gives place to the scanty products of the arid summer, finally, these two yield 



nly confined to the courses of streams. 



3f seasons more like that of temperate 

 climates. Here there is a winter sufficiently cold to prevent and a summer sufficiently moist to 

 favor vegetable growth, which continues throughout the latter season. The scanty vegetation 



ma 



mountains 



mountains 



uncertain 



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As the waters recede, the inundated places are speedily covered by a very rapid growth. 



An enumeration of the plants observed, with precise and detailed accounts of the most 

 interesting: amonc: them, will be found in the accomnanvinor memoir bv Professor John Torrev. 



AGRICULTURAL CAPACITIES. 



We will conclude this general sketch by briefly considering the agricultural capacities of the 

 country, as indicated by its general geological features, its climate, and the natural botanical 



productions. 



Commencing at the coast, we notice a prominent wall of high tertiary stuff abutting on the 

 sea. This portion, which is thinly clad with verdure at any season, presents an uninviting 

 aspect. Hence it happens that to the traveller who views it from the sea it is forbidding in 

 the extreme. At a variable distance inland, however, where we find the line of settlements, 

 the rounded hills are covered with a deep rich loam, which in the snrino- nrnHiK-Ps a Invnrln.nt 



f 



margins of this section are also of the 



