88 CARNEGIE INSTITUTION OF WASHINGTON. 



either because of excessive dryness of the soil or because of its low 

 temperature. 



From the observations on root-growth and root-relations of the 

 cacti as summarized in the foregoing paragraphs it appears that the 

 presence of the cacti in the Tucson region is in large part to be attributed 

 to the occurrence of rains at a season when the soil is also warm. These 

 observations, also, make it possible to suggest that a similar root-soil 

 relation may obtain among the cacti of other regions, explaining on the 

 one hand their presence in such regions, and suggesting on the other the 

 causes for their sparseness, or absence, in yet other regions where they 

 might be expected to occur. 



The cacti occur mainly in the southwestern United States, in the 

 uplands of Mexico, and in Central and South America. A feature of 

 the climate of the regions inhabited by the cacti is the relativelj^ scant 

 rainfall, which also is periodic. Whatever may be the character of the 

 winter climate, that of the summer season is marked by more or less 

 precipitation. Thus, at Tucson, for example, about 54 per cent of the 

 annual rainfall is in summer. At Tehuacan, on the IV'Iexican plateau, 

 which has been characterized as the richest region known in cacti, 72 

 per cent of the precipitation is in summer. In portions of Central 

 and South America where the cacti occur, rains constitute a feature of 

 the climate of the warm season. We may therefore make the general- 

 ization that in regions inhabited by the cacti their presence is in large 

 part to be related to the coincidence of precipitation and high tempera- 

 ture, by which favorable growth conditions, particularly of the roots, 

 are insured. 



In regions where the rainfall either is relatively light or wanting in 

 summer, and cacti occur to a hmited extent, it is not impossible that 

 the summer rainfall is not the minimum for the species, or that the 

 species are adjusted to a lower temperature relation than that found in 

 the cacti in southern Arizona. The latter alternative prevails with 

 Opuntia ramosissima of the Mohave Desert, as indicated in another 

 section, as well as that of certain extra-regional cacti, not here reported. 



Rate of Root-Growth of Opuntia ramosissima and its Possible Ecological 

 Significance, by W. A. Cannon. 



Opuntia ramosissivia is native in the Mohave Desert, An important 

 characteristic of the climate of this desert is its low annual rainfall, 

 only 14 per cent of the total occurring in summer ; and it has been 

 shown in another section that a relatively low summer rainfall does not 

 favor active root-growth. The fact that cacti occur in the Mohave 

 Desert, therefore, suggests special conditions. Among the possible 

 factors which might operate to bring about the survival of cacti in the 

 Mohave, one only need receive attention here, namely, the possibility 

 that the root-soil temperature response is such as permits this species 



