438 



THE AMERICAN NATURALIST 



[Vol. L 



Very many experimental cultures, of various kinds, made both 

 at the Desert Laboratory and the Coastal Laboratory, have shown 

 that the growth rate of the roots of Opuntia, within limits, varies 

 directly with the temperature. It is relatively slow at 20° C, 

 and most rapid at 34° C. The hourly increase in length of the 

 roots at 20° C. is about 0.3 mm., and at 30° C. it is approximately 

 twice this. Above 34° C, the rate falls off rapidly and ceases 

 at about 42.5° C. Below 20° C, the growth rate is very slow, as, 

 for example, at a temperature of about 16° C. an increase in 

 length of a perfectly normal root was found to be only 1 mm. in 

 14 hours. The maximum rate, taking place at about 34° C, is 

 about 1 mm. an hour. 



Referring back now to the soil temperatures, it will be seen 

 that the roots of this species are exposed to optimum conditions in 

 July and August only, although the soil temperatures for one 

 month before and one month following this period, at a depth of 

 30 cm., or less, is also high enough for an effective growth rate. 

 The soil temperatures, at this depth, in the other months, and at 

 the lowest level throughout the year, are not sufficiently high for 

 the best root activity. However this may be, we find, in short, 

 that suitable soil temperatures obtain at the depths occupied by 

 the roots of the cacti during four months of the year. But it does 

 not follow that root growth goes on throughout this period for 

 the reason that the foresummer is arid and the shallow soils are 

 impossibly dry, having less than 10 per cent, of moisture. Active 

 root growth of the cacti, in fact, commences with the coming of 

 the summer rainy season, about the middle of July. It is ended 

 by the cooling of the soil in early autumn. The length of the 

 active growing season of the roots of the cacti, therefore, does not 

 usually exceed six or eight weeks. 



It is in the response of the roots to the temperature and moisture 

 conditions, as just sketched, that lies the crux of the suggestion 

 offered in this paper, namely, that conditions being otherwise 

 favorable, the cacti, which are shallowly rooted, occur in such 

 regions as have the superficial soils moist at the same time they 

 are suitably warm, and they are wanting where such soil condi- 

 tions fail. 



With the reaction of the roots of the cacti to temperature in 

 mind, it will be instructive to examine briefly the leading climatic 

 features, so far as they affect the case in point, of the regions in 

 which the cacti form a conspicuous portion of the vegetation. 



According to Engler and Prantl, the cacti occur mainly in the 



