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24 ROOT HABITS OF DESERT PLANTS. 



The rainfall on each of the areas, in the vicinity of Tucson, is approxi- 

 mately the same, but the differences in the mutual relation of the areas, 

 the character of the soil, and its depth, as well as the surface configuration, 

 operate to make the water relations of the habitats very unlike. 



The flood-plains of the vSanta Cruz river and of West Wash, in addition 

 to the rains which fall on them directly, receive the run-off and seepage 

 waters from the bajada, and the bajada, in turn, those from Tumamoc Hill 

 or other higher elevations. In former years at flood-time the Santa Cruz 

 river overran its banks and covered the bottom lands, but now the river 

 has cut a deep channel and flooding from this cause never takes place. 

 The change is probably associated with the cutting of the mesquite forest, 

 which was once a rather heavy one, and the conversion of the bottoms 

 into ranches or waste lands, with second-growth mesquite. 



The water-retaining capacity of the soils of the river flood-plain is the 

 highest, below which should be placed the adobe of the bajada, at the north 

 base of the Hill, with the loamy sand of the flood-plain of West Wash as the 

 least. 



So far as is known from studies on the water content of the superficial 

 soils, the amount of water present during the dry autumn and winter on 

 the bajada north of Tumamoc and on the flood-plain by West Wash is 

 insufficient to be of direct benefit to the plants whose roots do not penetrate 

 deep, while at the same time the soils of Tumamoc Hill and of the river 

 flood-plain have moisture present in usable quantity. 



The period of optimum moisture content of the upper soil includes the 

 rainy period and a relatively short time after the beginning of the dry 

 season. The upper levels dry out soonest, and on the bajada were air-dry 

 three weeks after the close of the rains, while the soils on the river flood - 

 plain and Tumamoc Hill remained moist for a period exceeding six weeks. 

 This places the limit to the growing period of most annuals and the most 

 active vegetative period of all perennials without water storage capacity, 

 or deeply penetrating roots. 



The temperature of the soil has been observed on Tumamoc Hill only, 

 and chiefly at 15 cm. and 30 cm. The highest temperatures immediately 

 precede the summer rains. With the advent of the rains the temperature 

 falls quickly and continues to decHne until January-February, at a depth 

 of 15 cm., or March- April at the greater depth — 30 cm. The time of the 

 germination of the summer annuals and of greatest vegetative activity of 

 the perennials, therefore, is below the maximum heat, as that of the winter- 

 spring plants is above the minimum. 



The daily fluctuations of temperature for the two depths is unlike ; that 

 at the greater depth is about 2° F., while that at the lesser depth is about 

 8° F. The yearly range at the 15 cm. depth is about 34°, and that at 30 

 is about 44° F., with the maxima at 99° and 105° for the two depths 

 respectively. 



