240 BARTRAM: BOTANIZING IN SOUTHERN ARIZONA 
of shrubs or rocks wherever a bit of moisture and shade was 
obtainable. Along the dry arroyos leading off through the 
mountains Anislotus brachycarpus and A. trispermus were just 
opening their first flowers, and on the dry flanking talus slopes 
the omnipresent Encelia farinosa in full bloom was temporarily 
forgotten in the pleasure incident to finding a patch of Polygala 
macradenia with inconspicuous little purple flowers barely 
visible even at close range. 
similar association of species, including Dryopetalon, 
Anemone and Thysanocarpus, was observed on the shaded side 
of a steep ridge at Robles Pass, where scattered pockets of soil 
had been formed by the disintegrating mosses and lichens, and 
no doubt a more thorough survey of the region would disclose 
many similar localities; but the complete isolation of these 
stations, one from another, is the one factor that seems to be 
repeatedly emphasized. 
Theorizing is always interesting, often productive, but some- 
times misleading, yet in the light of these facts one can hardly 
reject the inference that at some comparatively recent time the 
forms of plant life, not fitted to thrive in the arid gravel and 
rock formations of the desert as we find it today, were more 
widely distributed than they are now, and by progressive changes 
have been isolated by the rising tide of desert vegetation. 
Such a condition is plain in outline but difficult to translate 
into terms that are applicable when the available data are so 
meagre. Inequality in the distribution of rainfall, the processes 
of erosion and disintegration and variations in climatic conditions 
have been some of the contributing causes to the spread and 
contraction of the various plant communities as we now know 
them, but to what extent these influences have operated and in 
what manner the distribution has been modified by complicated 
cross currents of action and reaction are queries that may be 
more satisfactorily answered at some future time, when our 
knowledge of the subject will be more intimate and compre- 
hensive than it is now. 
The following list of species, collected between December 
23d, 1919, and January 26th, 1920, may be considered as 
fairly representative of the region within a radus of fifteen 
miles from Tucson but is necessarily incomplete. Many of the 
plants listed were in flower and nearly all of them provided with 
