CHAPTER Il. 
LOCAL DISTRIBUTION OF SPECIES. 
TOPOGRAPHIC AND SOIL RELATIONS. 
In the first division of this paper the aim was to present a clear account 
of the natural associations of plants on and near the Laboratory domain, 
and to point out, wherever possible, certain obvious relations between 
local distribution and environmental factors. In this division, dealing 
more in detail with constituent species of the associations, the attempt to 
trace cause and effect is carried a step farther. Certain species have 
been carefully mapped and their habits have been more thoroughly 
studied with reference to differences of soil and aspect. The species 
selected for special study are characteristic in their respective places, and 
the maps show at a glance their remarkable definiteness of habitat 
preference. 
The mapping has been done by Mr. J. C. Blumer, and the account which 
follows is based on his field-notes, which have been placed at my disposal. 
Observations of soil and air temperatures on opposite sides of the gulch 
near the Laboratory, made by myself for a number of months, aid in 
analyzing the complex factors determined by aspect, or direction of slope. 
The discussion of soil conditions by Dr. Livingston brings to light impor- 
tant relations and is presented in a later section as a special contribution. 
Beginning with the species that have been mapped, a certain amount 
of detail which is indispensable will be presented. 
ENCELIA FARINOSA. 
On the Laboratory domain, and in its vicinity, the lower limit of this 
plant coincides closely with the 2,500-foot contour, but its local distri- 
bution is evidently determined by other factors than altitude. It is 
usually limited to steep slopes, and its lower limit is often sharply marked 
by a few degrees difference in gradient. But steep slopes are here char- 
acterized by a thin layer of residual adobe soil of high retentive power 
-for water, and such slopes, with thin layers of soil, point to rock-masses 
near the surface, either in the form of fissured or laminated bed-rock, 
or of detached bowlders, conditions which indicate the presence of a 
relatively high degree of moisture. On this point see Livingston (1906). 
Thus, the rain of January 2-3, 1908 (0.62 in.), penetrated down about 
5 inches at the Laboratory, but only 3 inches at the farm near by. This 
indicates greater penetrability of rocky soil of this character. 
This species also shows in its distribution a marked and definite rela- 
tion to aspect. Other things being equal, its density increases directly 
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