VOL. III.] Forms of Trees. 3 



far the most common effect and especially refers to shrubs. Many 

 in.stances may be cited, but I will only mention one. Baccharis pibi- 

 laris, which grows everywhere oivthe 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 are common. I may, how- 

 ever, here especially recall the dwarf and scrubby Pi7ius 7nonticola 

 growing in the canons 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 ( Pimis in- 

 signis) and to the Monterey cypress ( Cupressiis 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 

 imequally affected. The foliage of the evergreens offers much more 

 resistance to the snow than do the bare limbs of trees and shrubs 



