10 BOTANY. 



personal influence must be carefully eliminated, and the facts permitted to stand isolated and 

 independent, until, without compulsion, they shall crystallize into truths. 



The botanist going from the valley of the Mississippi to the Pacific coast, will be immediately 

 struck by certain general differences which he will perceive to exist between the vegetation of 

 the region he has left and that to which he has co.ne. The first feature in the aspect of the 

 botany of the west, which he will be likely to notice, is the paucity of arborescent and the 

 variety of annual plants. 



The forests he finds restricted, for the most part, to the sea coast and the mountain sides, and 

 exhibiting a great preponderance of coniferous over dicotyledonous trees. The forms of vegeta- 

 tion by which he is surrounded in these forests are among the most magnificent which the 

 world affords, and nearly all are new to him. He may traverse the country for weeks, perhaps 

 months, before he meets with a tree with which he has been familiar on the eastern side of the 

 continent, and when he finds such an one, it exhibits a growth and appearance so different from 

 that of the same tree at the east as to be not immediately recognized. The number of forest 

 trees, exclusive of shrubs, found growing north of San Francisco and south of the Columbia, 

 does not, probably, exceed fifty. These are distributed among the following genera: Pinus, 8 

 Abies, 5 ; Picea, 3 ; Sequoia, 2 ; Cupressus, 2 ; Thuja, 1 ; Libocedrus, 1 ; Larix, 1 ; Taxus, 1 

 Torreya, 1 ; Quercus, 5 ; Populus, 3 ; Salix, 5 ; Fraxinus, 2 ; Acer, 2 ; Almis, 1 ; Cornus, 1 

 Platanus, 1 ; Gastanea, 1 ; JEsculus, 1 ; Arbutus, 1 ; Oreodaphne, 1. Both in numbers of 

 individuals and in size, the coniferce, as has been mentioned, greatly preponderating. The 

 annual vegetation which covers the prairie country of the valleys also presents an assemblage of 

 forms quite new to the eastern botanist, and among them he will not fail to notice a greater 

 relative number of liliaceous plants than in any part of the eastern States. The different 

 mountain ranges he finds covered with vegetation which exhibits marked differences, and the 

 areas which lie between and eastward of the coast ranges and Sierra Nevada, have each a 

 flora so far peculiar to itself as to permit of its study in a degree apart from the others. 



In order to trace the connection which exists between the physical geography of the region 

 under consideration and the character and distribution of tbe plants which cover its surface, a 

 general idea of the topography,* the climate, and geological structure of the different districts 

 which it includes, is necessary. 



Climate. — As is generally known, the climate of the Pacific coast, as compared with that 

 of the Atlantic and the valley of the Mississippi, is much more equable, presenting no such 

 extremes of heat or cold as that of the east, while the isothermal lines, when traced westward, 

 are deflected to the north, striking the Pacific coast several degrees higher than the points 

 where they pass that of the Atlantic. This seems to be due, in a great degree, to the influence 

 of the prevalent westerly winds, which, constantly blowing in from the Pacific, assume the 

 uniformity of temperature of the surface over which they pass. In the valleys of California the 

 seasons are divided into wet and dry, rather than into cold and hot, while on the mountains 

 snow falls to a considerable depth, and the severity of winter is proportioned to the altitude of 

 the locality and its distance from the ocean. The summer temperature varies greatly in 

 different localities, being extreme in the valleys of the interior, while in the mountains and on 

 the coast a degree of heat is never suffered which is at all oppressive. 



The annual precipitation of moisture exhibits even greater local variation than the tern- 

 s' Xuk by Lieui. Abbot. — This subject is full)' treated in Chapter I of the General Report. 



