DEPARTMENT OF BOTANICAL RESEARCH. 87 



Pinalenos were found many plants common to the Rockj^ Mountains 

 of northern New Mexico, but not in the Santa Catalinas. Some of the 

 latter species are found in the Pinaleno range only at elevations greater 

 than any in the Santa Catahnas, but the great majority are found at 

 elevations which do exist in the latter range. Owing to the similarity 

 of climatic and other environmental conditions in the two mountains, 

 the absence of these species from the Santa Catahnas would appear to 

 be due to causes other than those of the physical environment. It will 

 at least be possible to test the abiUty of these species to survive when 

 an extension of their ranges is attempted. 



Distribution of the Cacti with reference to the Role Played hy the Relation of 

 Root Response to Temperature, by W. A. Cannon. 



Studies on the reaction of the roots of the cacti to certain environ- 

 mental features, particularly the temperature of the soil, indicate that 

 the root-temperature relation maj' be of special importance among the 

 complex factors which determine the geographical distribution of the 

 family. 



Numerous garden cultures and experiments, carried on at the Desert 

 Laboratory and the Coastal Laboratory, show that the species of 

 cactus from the Tucson region require a relatively high temperature 

 for an effective growth-rate of the roots. For example, the average 

 hourly increase in length of the roots of Opuniia versicolor has been found 

 to be about 0.3 mm. at 20° C, 0.6 mm. at 30° C, and LO mm. at 34° C. 

 At a temperature of 16° C, however, a growth-rate of only 0.07 mm. 

 was observed. 



The period in which the soil-moisture and temperature is suitable 

 for the active growth of the roots of Opmitia versicolor and other cacti, 

 in the vicinity of the Desert Laboratory, is restricted to about six 

 weeks of each year. This is limited both by soil-moisture and soil- 

 temperature. At a depth of 15 cm. the mean maximum temperatures 

 for midwinter and midsummer of a typical year were 8.1° and 34° C, 

 respectively. The mean maxima at a depth of 30 cm. were 12.2° and 

 33° C. for January and July. The course of soil-moisture, as shown by 

 numerous studies, is such that there are two moist and two dry periods 

 each year. The moist periods are in winter and in summer. In the 

 intervening seasons, particularly in the fore-summer, the soil at a depth 

 of 15 to 30 cm., or that occupied by the roots of the cacti, carries an 

 insufficient amount of moisture for root absorption. From these envi- 

 ronmental conditions it happens, therefore, that in winter low soil- 

 temperatures prevent root-growth, and in the earlier part of summer 

 the soil is not sufficiently moist for the growth of roots. In short, it 

 is not until the coming of the rains of midsummer that in the Tucson 

 region the roots of the cacti experience conditions favorable for their 

 growth. Root-growth of the cacti ceases with the coming of autumn 



