Distribution and Movements of Desert 
: Plants. 
INTRODUCTION. 
In the study which has led to the present paper it was deemed best to 
establish at the outset certain definite limits within which more exact and 
prolonged observational and experimental work could be undertaken, 
and beyond which, as opportunity offered, comparative studies could 
from time to time be made. Naturally the domain of the Desert Lab- 
oratory was chosen for the limited area within which the greater part of 
the work has been done, and the following pages are, first of all, a report 
of this work. The broader relations, which involve comparative studies 
in other regions and under different conditions are, as yet, owing to insuf- 
ficient data, less clearly established; but they have received such atten- 
tion as it has thus far been possible to give. 
Tumamoc Hill (plate 1), on the northern slope of which the Desert 
Botanical Laboratory of the Carnegie Institution of Washington is situ- 
ated, is an outlying peak close to the eastern slope of the Tucson Range, 
which latter extends from the neighborhood of the old mission of San 
Xavier del Bac, south of Tucson, in a general northwesterly direction 
upwards of 25 miles, parallel to the valley of the Santa Cruz River. Its 
geographical position and topographical features are alike favorable for 
an investigation of factors by which the distribution and present asso- 
ciations of plants here represented have been determined. 
Standing, as it does, on the border-land between the plateau region of 
central and eastern Arizona, on the one hand, and the great desert plain 
that stretches westward to the Colorado River and its delta, on the other; 
connected, moreover, by the valley of the Santa Cruz and by broken 
ranges of mountains and hills with the highlands of northern Mexico, 
there mingle within the narrow limits of this one hill and the adjacent 
valleys many plants represented in the widely different floras of Cali- 
fornia, New Mexico, and Texas; and along the north and south pathway 
in which it stands may be noted the northern limit of various Sonoran 
species, as they drop out one by one. Its altitude, slightly upwards of 
3,000 feet, gives at this latitude, in connection with its topographical 
features and variety of soils, an ensemble of conditions under which plants 
of very different habits occur together; well-marked mesophytes, for 
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