No. 595] SHORTER ARTICLES AND DISCUSSION 441 



opuntia is commoon. At Mysore, according to Hann, 81 per cent, 

 of the rainfall is from May to October. At Lahore the prickly- 

 pear is not so abundant as further south, but it occurs escaped, 

 nevertheless. Here the July- August rains comprise 55 per cent, 

 of the total annual precipitation. 



In South America the Commission examined naturalized opun- 

 tias in portions of Brazil and Argentina chiefly. An important 

 prickly-pear region is northwestern Argentina, where native as 

 well as introduced species of cacti occur in abundance. At Salta 

 there is as good as no rain in the cold season, between May and 

 September. At Tucuman, 69 per cent, of the rainfall takes place 

 between December and March, inclusive (Hann), and at Cata- 

 •marca, between November and March, inclusive, 81 per cent, of 

 the total annual precipitation occurs. 



Without pursuing this phase of the matter further, it would 

 appear, in short, that in regions where cacti are abundant, either 

 native or introduced, rains occur during the warm season. It is 

 not intended to discuss in this place the actual amount of rain- 

 fall which falling in the warm season makes the presence of a 

 cactus flora possible. It is well known, however, that the amount 

 of precipitation in regions where cacti occur is extremely unlike, 

 and that it may vary from season to season in any one region. 

 This last, in fact, is one of the leading characteristics of an arid, 

 or semi-arid region. So far as regards the precipitation differ- ' 

 ences in separate regions frequented by cacti, it is interesting to 

 note that at Rockhampton, Queensland, it is 40.09 inches, 9 while 

 at Phoenix, Arizona, it is 7.06 inches, 10 and that in the former 

 region 20 inches occurs in the warm season, while the amount of 

 summer precipitation at Phoenix is between 0.9 and 2.1 inches, 

 as means of the extremes. 11 



In the Mohave the annual rainfall is 4.97 inches, 12 about two 

 inches less than the mean precipitation for Phoenix. In the Mo- 

 have, however, 86 per cent, of the rainfall is in winter, which 

 greatly emphasizes the differences in summer aridity of these 

 regions, and points to a probable reason why cacti are almost 



»" Results of Bainfall Observations made in Queensland," H. A. Hunt, 



""Botanical Features of North American Deserts," D. T. MacDougal, 



11 ''Climatology of the United States," A. J. Henry, U. S. Dep. Ag. Bull. 

 1906. 



