CHAPTER I. 
PLANT ASSOCIATIONS AND HABITATS. 

For reasons already stated, the plant associations and habitats of 
Tumamoce Hill and the adjacent valley have been made the basis of the 
studies of desert plants reported in the following pages. ‘The topography 
of the tract with which we are immediately concerned is so intimately 
related to its plant life that it becomes necessary at the outset to refer 
to its salient features. These are (a) the Santa Cruz River and its flood- 
plain, (b) the long mesa-like slopes leading from this to the mountains, 
and (c) the Tumamoc Hills. Proceeding westward from the city of 
Tucson, we come first to the river, which forms the eastern boundary 
of the area studied. Beyond this, the flat flood-plain, at this point a 
mile or less in width, stretches to the edge of the slope, from which a 
little beyond rises the group of three hills to which has been given collec- 
tively the name of the highest one, Tumamoe, which attains a height of 
742 feet above the flood-plain. ; 
The cliffs and steep slopes of Tumamoc Hill are in turn succeeded on 
the west by the mesa-like slopes through which ‘‘the wash,” with its 
numerous branches, passes on its course to the flood-plain north of the 
Laboratory. The mountains farther to the west, seen in plate 2, belong 
to the Tucson Range, and are beyond the tract now under consideration. 
By observing the more striking features of the landscape the various 
habitats may be partly located. Further details are given in connection 
with the maps of distribution. 
It will be seen that within this area, with a radius of hardly more than 
a mile, there are habitats exceedingly diverse in character, inhabited by 
plants correspondingly widely different, and that these differences, more- 
over, are distinctly edaphic. It is evident at once that the water-relation, 
first of all as regards soil-water, is a determining factor, through which, 
under essentially identical atmospheric conditions, plants of the most 
widely different habits and requirements are growing almost side by side. 
The same thing is seen the world over in both temperate and tropical 
zones, but it is especially striking here from the close juxtaposition of 
extreme ecological types. 
The general features of the vegetation of Tumamoc Hill have been 
made familiar through papers of Lloyd (1904), MacDougal (1908), and 
others that have appeared within the past few years. Accordingly it is 
only necessary here to refer to them in the briefest manner by way of 
preparation for the study of plant associations which follows, and the 
analysis of the flora given in a later section. 
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