DEPARTMENT OF BOTANICAL RESEARCH. 89 



to develop in the winter rains. In this case it would point either to a 

 relatively warm winter soil or to a somewhat different temperature 

 response than that already found in the cacti of southern Arizona. 

 To test the validity of the latter possibility, a series of experiments 

 was planned in which the root-growth of Opuntia ramosissima was to 

 be compared with that of 0. versicolor, from the vicinity of the Desert 

 Laboratory, as a control. 



In the experiments each series was continued about 8 hours at soil- 

 temperatures varying between 19° and 31° C. as extremes. The range 

 of temperatures employed and the variation were probably about what 

 the species experience in their proper habitats. The following sum- 

 mary of the experiments can be given: 



Experiments with O'puntia versicolor: (1) With soil-temperatui'es ranging between 21° and 

 27° C, the root-growth, in 8 hours, was 2.1 mm. (2) With temperatures from 

 19° to 27.5° C, the root-growth was 2.3 mm. (3) The root-growth at temperatures 

 between 23° and 30° C. was 2.7 mm. 



Experunents with O. ramosissima: (1) The root-growi,h, in 8 hours, at soil-temperatures 

 between 19° and 27° C. was 3.3 mm. (2) At soil-temperatures between 19° and 

 27° C. the root-growth was 3.2 mm. (3) At temperatures ranging between 19° 

 and 25° C. the root-growth was 1.8 mm. (4) The root-growth at temperatures 

 between 23° and 31° C. was 3.4 mm. (5) At temperatures between 20° and 25.5° 

 C. the growth was 4.2 mm. 



These experiments, and others, indicate that the roots of Opuntia 

 ramosissima at soil-temperatures below "optimum" and above 19° C. 

 increase at a somewhat faster rate than do those of 0. versicolor at the 

 same temperatures and under the same conditions. The difference 

 in rate between the two species is approximately 33 per cent. 



Further studies on the reaction of the roots of 0. ramosissima to soil- 

 temperatures under 20° C. indicate that the minimum temperature 

 for efficient growth-rate is probably lower than in 0. versicolor. For 

 example, in 2-hour periods, and at soil-temperatures ranging from 12° 

 to 17° C, the roots of 0. ramosissima increased 0.2 and 0.5 mm. in length, 

 while those of 0. versicolor grew, in one case, 0.1 mm., and in another 

 not at all. 



It would appear, therefore, from these observations, that the roots 

 of Opuntia ramosissima from the Mohave Desert grow somewhat more 

 rapidly at parallel temperatures than do the roots of 0. versicolor; and, 

 especially, the roots of the Mohave species appear to have a fairly 

 active growth-rate at relatively low temperatures. It would not seem 

 impossible, therefore, that the roots of the species in the Mohave Desert 

 (1) grow during seasons of relatively low temperatures, and (2) that the 

 roots may penetrate the ground relatively deeplj''. 



