16 MISCELLANEOUS PAPERS. 



CLIMATIC REQUIREMENTS OF PRICKLY PEARS. 



Prickly pears and other cacti are apparently inseparably connected 

 in the pubhc mind with drought and heat, but this conception of the 

 requirements for their best development is far from perfect. Our 

 driest deserts produce none of these plants in economic quantities, 

 and the same is true of our hottest regions. Rather than say they 

 are adapted to conditions of extreme heat and drought, it should be 

 said that they thrive best in a region which has an equable tempera- 

 ture and a considerable rainfall periodically distributed. There is 

 certainly no region in the world where these plants grow naturally in 

 such profusion as they do upon the plateau of Mexico, but this is not a 

 hot country; neither is it excessively dry. It is very dry during a 

 large part of the year. It is a desert as compared with eastern Texas, 

 for instance, but it has a considerable rainfall during an average year. 

 The rain falls mostly in the summer, and then the country looks like 

 anything but a desert. The average rainfall at Zacatecas for the 

 past ten years, as stated by Mr. Albert L. de Lautreppe, who has 

 made a special study of the weather records of that city in connection 

 with a business venture, is 31^ inches, but the average for the seasons 

 from January to April and from October to December is only five- 

 eighths of an inch to 2| inches, while the average for the other months 

 of the year is 3^ to 7J inches a month. June, July, and August are 

 the rainy months, having an average rainfall of 4^ to 7 J inches each 

 for the past ten years. 



While many species appear to be able to withstand high tempera- 

 tures, they develop naturally in the greatest profusion where the heat 

 is not excessive. The plateau of Mexico is a region with compara- 

 tively equable climate. Some species thrive under extremes of 

 heat. Opuntia lindheimeri is at home in the lower Rio Grande Valley 

 of Texas and Chihuahua, and the closely related Opuniia engelmanni 

 and Opuntia engelmanni cy chides thrive in southern Arizona, where 

 the mercury often reaches 111° F. On the other hand, there are 

 species which grow where the winter temperatures go to at least -40° 

 F., but the plants are small and of no economic importance in them- 

 selves except as they may be used to give a hardy character to more 

 valuable species. The valuable species of the Mexican liighlands 

 thrive where the temperature falls to 14° F. in very rare instances. 

 Usually the freezing point is only rarely reached here. During the 

 past ^vinter (1905-6) the mercury dropped at the city of Zacatecas 

 to 14° F., and many of the more delicate spineless forms, as well 

 as the natives, were badly injured. No pear was killed outright, but 

 the branches were frozen down for four or more joints. These rotted 

 and dropped off, but the old trunks survived. Opuntia lindheimeri, 

 the common species of southern Texas, has been injured very severely 

 within the memory of the present generation. It suffered some injury 



102—1 



