No. 595] SHORTER ARTICLES AND DISCUSSION 439 



dry parts of Mexico, in the portions of the United States which 

 border on Mexico, in eastern and central Brazil, and in portions 

 of the Andes countries. Taking two or three genera as examples, 

 we learn, for instance, that Cereus occurs in Mexico, and in the 

 Andes of Argentina and Brazil. Echinocactus extends from 

 the southwestern part of our country to Brazil and Chili Opuntia 

 is found in Mexico, Peru, Chili, in Central America and in the 

 southwestern portions, especially, of the United States. Although 

 certain species are outside of this range, as especially certain 

 opuntias, where the winters are exceedingly cold, all are subject 

 in summer, when active growth takes place, to conditions which 

 are in rather close accord. A glance at the summer climates of 

 these regions will, I think, establish this point. 



In the central part of Mexico, at Tehuacan, the annual rainfall 

 is about 15 i aches, most of which occurs in summer, and at 

 Pueblo, .70 miles distant, and at a higher altitude, where the 

 annual precipitation is more than twice that at Tehuacan, 72 per 

 cent, of the rain comes in the warm season. The Tehuacan region 

 has been characterized as being the richest of any known in cacti. 8 

 At Chihuahua, where the rainfall is 10.86 inches, the amount 

 falling in the summer season is also over 70 per cent. 



In the southwestern part of the United States, where the cacti 

 constitute a conspicuous portion of the flora, a relatively large 

 summer rainfall is also reported. At Tucson, for example, the 

 precipitation amounts to 11.74 inches annually, of which 54.7 per 

 cent, is received in July, August, and the first part of September. 



Turning now to South America, and without especial regard as 

 to the presence of cacti at the particular stations quoted, we find 

 that over a relatively large area, a large percentage of rainfall is 

 in the warm part of the year. For example, at Matto Grosso, 

 Brazil, the greatest rainfall is in December. From June to Au- 

 gust and generally for a month before and after this period, the 

 climate is usually dry. 4 



Along the east coast rain occurs from February to April, June 

 to September being drv. In the Cordilleras of Bolivia and Peru, 

 the rainy period is in December-March, and the climate is dry 

 from April to October. At La Paz, although rain may fall any 

 month of the year, December to February is regarded as being 

 the season of rain. 



a MacDougal, D. T., < ' Botanical Features of the North American Deserts," 

 Carnegie Inst. Wash. Pub. 99, 1908. 



*Hann, "Handbuch der Klimatologie, » Bd. II, 1910. 



