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 . Of the plants studied 

 in connection with this paper which do not drop their leaves with change of 



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