VOL. Il. ] Forms of Trees. 3 
far the most common effect and especially refers to shrubs. Many 
instances may be cited, but I will only mention one. Baccharis pilu- 
Zaris, which grows everywhere on the coast around San Francisco, 
clings typically to the soil and sandhills where exposed to the wind, 
while on the north side of Tamalpais, where the shelter is perfect 
and even in the oak scrub of Golden Gate Park, it assumes an erect 
form. So different is the outward appearance between these two 
forms, that the former has been described as a distinct species, 
B. consanguinea. : 
Similarly on nearly all our high mountain tops we meet with scrub- 
pines growing in the crevices and clinging to the rocks like real 
coverlets of verdure. But the same species may be found further 
down in the elevated valleys growing erect with sloping branches 
and undivided trunks. Such instances arecommon. I may, how- 
ever, here especially recall the dwarf and scrubby Pinus monticola 
growing in the cafions on the slope of Mount Dana, while further 
down splendid specimens are crowding the sheltered meadows. 
As an instance, again, where the effects of the wind have been 
partly permanent we may point to the Monterey pine ( Pinus in- 
signis) and to the Monterey cypress ( Cupressus macrocarpa). Ma- 
ture specimens of these varieties assume always horizontal crowns, 
even when growing inland, and only during their earlier growth do 
’ they show a tendency to grow erect like most species of pines or 
coniferous trees generally. We may presume that if the evolution 
of a species is accompanied by this continued wind force, the latter 
will to a great degree mould the outward form of the species. If 
again the evolution of a species takes place under various condi- 
tions of wind and calm, the form of the species will be variable ac- 
cording to exposure. 
The.effect of the wind while apparent everywhere and while 
found in every climate and in every country is, however, not the 
most powerful agent in shaping the forms of trees and shrubs. The 
‘snow which part of the year covers vast territories, often to a depth 
of thirty or more feet, has a great influence upon the forms of all 
plants which are exposed to it for a longer or shorter time. 
As the effects of the snow depend chiefly upon the resistance to 
pressure, it will be seen that evergreen and deciduous trees must be 
unequally affected. The foliage of the evergreens offers much more 
resistance to the snow than do the bare limbs of trees and shrubs — 
