10 
BOTANY. 
personal influence must be carefully eliminated, and tlie 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 he 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 come. 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: Finns , 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 ; Alnus, 1; Cornus, 1 ; 
Platanus, 1; Castanea, 1 ; AEsculus, 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 diffei’ent 
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 the 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 tem- 
® Note by Lieut. Abbot .—This subject is fully treated in Chapter I of the General Report. 
