No. 595] SHORTER ARTICLES AND DISCUSSION 437 



est to note, on the other hand, that deeply placed root-systems, 

 such as of Prosopis velutina, may have a relatively rapid growth 

 rate at relatively low temperatures. 2 In such a case it is quite 

 possible that the role played by root response to temperature in 

 species distribution may be less important, or, at any rate, differ- 

 ent from that played by the roots of the cacti, for example, to the 

 distribution of members of that family. 



We will now glance at the most striking conditions of soil tem- 

 perature as they obtain at the Desert Laboratory, where much of 

 the work here referred to has been carried on, before taking up a 

 resume of the response of the roots of the cacti to the temperature 

 of the soil and the relation this suggests to the general distribu- 

 tion of the family. 



Three series of soil thermographic records, which are now being 

 supplemented by others, have been kept at the Desert Laboratory. 

 These relate to three depths, namely, 15 cm., 30 cm., and about 

 2.6 m. Although the records cover a series of years, it will serve 

 the purpose in hand if we refer to those for the year 1910 only. 



The mean maxima and the mean minima temperatures for the 

 three depths will provide sufficient data for interesting com- 

 parisons. 



At the shallowest depth, 15 cm., the mean maxima temperatures 

 for midwinter and midsummer were 8.1° and 34° C, respectively. 

 The mean minima, for the same seasons, were 3.9° and 30.8° C. 

 At a depth of 30 cm. the maximal range was from 12.2° C, in 

 January, to 33° C, in July, and the minima temperatures, for the 

 same months, 10° and 32.2° C, respectively. It was observed that 

 from June to September, inclusive, the curve of the mean maxima 

 for this depth did not fall below 32.2° C. 



At a depth of 2.6 m., the mean maxima temperatures ranged 

 from 18.6° C, in January, to 27° C, in July. 



Upon comparing, in a general way, the mean maxima for the 

 different soil depths we see that the shallowest soil is the warmest 

 from April to August, inclusive; that in September and October 

 only the highest temperatures are found at a depth of 30 cm.; 

 and that in late winter-early spring the lowest level is also the 



The relation of the rate of root growth in Opuntia versicolor, 

 as representative of the cacti, to different soil temperatures indi- 

 cates interesting conditions and possibilities, and will be given in 

 the following paragraph : 



