THE TOPOGRAPHY OF THE CHLOROPYHLL APPARATUS 
IN DESERT PLANTS. 
ENVIRONMENTAL CONDITIONS IN RELATION TO STRUCTURE. 
The plants which come under observation in this paper are fairly repre- 
sentative of the perennials of the Tucson region (2000 to 3000 feet), with 
an average rainfall of 12 inches, and occur in a comparatively wide range 
of habitats. These include the bottom-lands of the Santa Cruz River which 
are bordering upon the Laboratory domain; low desert mountains, Tumamoc 
Hill (a portion of the Laboratory domain); the lower slopes and washes of 
Tumamoc Hill; dry, low ridges, the so-called aerial mountain-deltas, which 
lead eastward from the main range of the Tucson Mountains; the broad 
and gently rolling mesa or table-land; and, finally, the bed of the Santa 
Cruz River and certain contiguous irrigating channels and roadside ditches. 
In these habitats a relatively large range of environmental conditions are 
encountered. In them there is a wide variety of soils, of drainage condi- 
tions, and of exposure to light and to air-currents. The bottom-lands are 
characteristically deep and are made up largely of a clay or loam, with 
strata of sand some distance beneath the surface. Toward the sides of the 
bottom-lands the top-soil becomes more or less sandy or gravelly, with the 
coarser material on the slopes immediately above and leading out of the 
bottoms. Then comes either the mesa with its thin layer of top-soil and a 
nearly impervious hardpan underlying it, or the lower slopes of the desert 
mountains, with coarse rock and bowlders and bed-rock, clayish soil, more 
perfect drainage, and various exposures. 
The water-table of the river-bottom lies from 6 to 12 m. from the surface 
of the soil; that of the mesa is frequently 25 m. and deeper beneath the 
surface. The location of the reservoirs of water on the mountain have not 
been determined, but are possibly connected with-the fissures and the 
pockets in the rocks. 
Very curiously the leaf-habit of these desert forms, and even their general 
xerophytic character, are not consistently associated with the character of 
the habitat. This will be apparent from a few examples. The evergreen 
habit is not correlated with the conditions of water-supply, or at least with 
the only sure water-supply—that of the river-bottoms. Ofthe plants studied 
in connection with this paper which do not drop their leaves with change of 
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