722 



SCIENCE 



[N. S. Vol. XXX. No. 777 



of the larger pines, remains not always below 

 6,000 feet, nor the desert area below 4,500, 

 but, under favorable (or unfavorable!)' condi- 

 tions they may raise long tongues into the 

 upper zones. Thus near Paradise certain 

 points carry the former to a height of 8,700, 

 the latter to 6,350 feet, mainly dependent 

 upon aspect (slope exposure) modified by 

 gradient,' and secondarily upon character of 

 the rock and soil and other conditions. Also, 

 in fixing the lower limits at their first appear- 

 ance on residual slopes, we avoid the mere 

 fringes of species that normally belong to 

 higher zones, but follow the canyons and 

 watercourses dovTn and often far out into 

 desert and grassy plain. 



The second map, showing the vegetation as 

 governed by aspect, would have the appearance 

 of a veritable crazy-quilt in its patchwork of 

 many small areas of different color. By far 

 the larger part of the montane area is com- 

 posed of slopes facing either north, east, south 

 or west, or in intermediate directions. Given 

 the same altitude, locality, rock and soil, 

 each aspect supports a plant society differing 

 in some degree from those of other aspects. 

 The difference may consist in kind, number or 

 rela,tive proportion of species, or merely in 

 number, relative abundance, size and thrift 

 of individuals, usually two or more of these 

 combined. In view of many other influences 

 that may be at work, such as seepage, ex- 

 posure to local atmospheric currents, number 

 and size of boulders present, physical constitu- 

 tion and relative abundance of rock and soil, 

 presence of animals or insects, the greatest 

 caution is constantly necessary in attributing 

 the differences to the proper controlling cause. 

 However, it may be stated that, given other- 

 wise similar conditions, the more directly one 

 slope faces southward, and the other north- 

 ward, the greater is the difference between 

 their plant societies. 



The floral difference between two small con- 

 tiguous slopes of equal gradient and similar 

 limestone soil at 5,500 feet near Paradise,* 



'Spalding, Plant World, XI., p. 213, 1908. 



• See Merriam, N. A. Fauna No. 3, p. 27, PI. II. 



' Designated aa Slopes I and III in exsiccati dis- 

 tributed. 



directly facing each other, one north, the other 

 south, may serve as a simple example of the 

 influence of aspect: The north slope supports- 

 a dense, shrubby growth of Oeanothus greggii 

 A. Gray, interspersed with Cercocarpus hrevi- 

 florus A. Gray, Yiguiera helianthoides H. B., 

 K., and numerous smaller plants, from which 

 grasses are practically absent. The south slope 

 is entirely devoid of trees and shrubs, dotted 

 with Groton corymhulosus Engelm. and cov- 

 ered with grasses, among which no less than, 

 three species of Triodia are prominent. 



These slopes for the greater part are rather 

 sharply bounded by adjoining and opposing 

 slopes and canyon bottoms. But, unlike the- 

 more or less insensible transition between one 

 altitudinal zone and another, the tension lines 

 between adjoining plant societies follow these- 

 topographic boundaries, such as crests of 

 ridges, angles of V-shaped gullies, or sides of 

 canyon bottoms, and usually their degree of 

 definiteness is in direct proportion to th& 

 sharpness by which such boundaries are 

 marked. 



The third map would divide the mountains,, 

 without regard either to altitude or to aspect^ 

 into more or less irregular areas both large 

 and small, their number dependent upon the 

 degree of intensity employed, based upon the- 

 geologic origin and character of the rock andj 

 soil. The following main divisions may he- 

 made: 

 I. Transported Soils. 

 II. Eesidual Soils derived from 



1. Limestone. 



2. Recent eruptives. 



3. Other rocks. 



The small total area of transported soils i* 

 confined to outwash slopes, canyon bottoms,, 

 and small mountain parks. They support 

 floral elements either quite peculiar to them- 

 selves, or else derived from two or more resi- 

 dual societies. For the present are made only 

 the three divisions of residual soils that show^ 

 the greatest differences between their plant 

 societies as indicated by their trees and shrubs-. 

 Each division or group of societies is found to 

 be peculiar to a class of rocks and residual' 

 soils of its owa. Further, the so-called ten- 

 sion lines between these societies or groups- ofl 



