82 CARNEGIE INSTITUTION OF WASHINGTON. 



when the soil temperatures are integrated. For example, the summa- 

 tion of soil temperatures, at a depth of 15 cm., for January 1910, and 

 for July of the same year, holds the relation to each other of 1:7. 



In the warmer months, July to September, the soil temperature at 

 a depth of 15 cm. ranges between 21.5° and 27.0° C. during times of 

 storms, and between 27.0° and 32.5° C. in fair weather. The soil is 

 rapidly, and greatly, cooled by the summer rains, as is graphically indi- 

 cated by figure 5, p. 95. At a depth of 30 cm. the summer temperature 

 is somewhat less than at the lesser depth, and the daily fluctuations 

 are relatively slight. At depths greater than 30 cm. these two features 

 are accentuated. 



During the colder months, from October to April, inclusive, the soil 

 temperatures at 15 cm. and below are, with brief exceptions, under 

 21.5° C. and frequently attain to 10° C, and even less. The daily 

 fluctuations at a depth of 30 cm., in winter as in summer, are less than 

 at the lesser depth. Although but few measurements of temperature 

 at a depth greater than 30 cm. have been made, it is known that it is 

 less variable from day to day, and season to season, than at the more 

 shallow depths, and also that at all times it is relatively low. 



Such being an outline of the soil-temperature throughout the year, 

 it appears, from what has been learned in regard to the response of the 

 roots of the perennials to temperature, that the roots of Prosopis and 

 of Fouquieria and Opuntia must needs take an unlike course in develop- 

 ment. Probably at no time of the year is the temperature of the soil 

 at the depth attained by the roots of Prosopis so low as to inhibit root- 

 growth or (especially in spring, summer, and autumn) so low as to be 

 unsuited to an effective rate of growth of the roots. Thus, low tem- 

 perature is not, in the case of Prosopis, sl limiting factor in root develop- 

 ment. So far, on the other hand, as concerns Fouquieria and Opuntia, 

 it appears that the soil temperatures, taken together with the impossibly 

 dry condition of the upper soil in the arid fore-summer, sharply limit 

 the period of root growth to the latter half of summer and September, 

 and to other seasons, especially spring, when the upper soil is moistened 

 by rains, and it is also so warm as to allow an effective growth-rate of 

 the roots of these species. Thus, as regards the period of root growth 

 in Fouquieria and Opuntia, low temperature is possibly the most impor- 

 tant factor by which it is limited ; and, as regards the development in 

 the soil, the same factor is probably the definitive one, inasmuch as, 

 within certain limits, the soil temperature falls with depth beneath the 

 surface. For example, with temperatures at depths greater than 40 cm. , 

 too low to allow effective root growth, such growth is confined to the 

 upper soil, and a superficial type of root-system results, such as char- 

 acterizes Fouquieria and Opuntia. 



