96 



CARNEGIE INSTITUTION OF WASHINGTON. 



The dififerences in rate of growth at the same temperature appears 

 in a striking fashion in the following summary of one experiment in 

 which both Prosopis and Fouquieria were growing in the same tube. 

 The interval is 12 hours. 



The temperature response of Opuntia versicolor is similar to that of 

 Fouquieria, at least for temperatures around 30° C. For example, in 

 one instance, at temperature ranging between 13.5° and 21.5° C, 

 growth of 0.5 mm. in 24 hours was observed, and in another instance 

 the growth was 2.5 mm. at the same temperature. With increase in 

 temperature, the growth-rate of the roots is accelerated, so that, for 

 instance, at 30° C. the growth in 24 hours was 12.8 mm., at 31° C. it 

 was 18.6 mm., and when the temperature was dropped to the earlier 

 low position the total growth for 24 hours fell to 3.5 mm. 



As a contrast to the temperature relation (as just given) of Opuntia 

 versicolor from the Tucson region, it is of interest to note that of Opuntia 

 sp. from an island in the Susquehanna River, where the natural tem- 

 perature relations are necessarily quite different from those of the 

 former region. When given temperature ranging between 13.5° and 

 21.5° C. for 24 hours, the Susquehanna cactus grew 8.4 mm. At 

 4-hour periods, and at the temperature of the soil named, the roots of 

 this Opuntia made the following growths : at 25° C, 2.1 mm. ; at 34° C, 

 2.4 mm.; at 38° C, 4.3 mm.; at 39° C, 2.7 mm. 



Methods used in Studying the Response of Roots to the Temperature of 

 the Soil, hy W. A. Cannon. 



In the investigations on the relation of root-growth to the tempera- 

 ture of the soil, two general methods have been employed. In the one 

 case, root-boxes have been used having one side of glass, sloping at an 

 appropriate angle, so that the growing roots should come into contact 

 with the glass, and the direction as well as the rate of their growth 

 could be easily observed. In a special form of root chamber a differen- 

 tial temperature effect was obtained by heating one end and cooling 

 the other. 



The second type of apparatus was one in which the temperature is 

 controlled automatically by a regulating device. This form consists 

 of several separate thermostats, built of metal and insulated, and so 

 arranged that the roots which are growing in glass tubes containing 

 soil are kept at any desired temperature for an indefinite time. In 



