L56 OUTLINE OF PLANT GEOGRAPHY 



are the ( Jacti, which reach their maximum development in Mexico, 

 as do most of the genera found in the states adjacent to Mexico. 

 Al'oui Tucson, in southern Arizona, the desert flora is a rich one, 

 embracing many species of Cacti, Fouquiera, Agave, Ephedra, 

 Larrea, mesquit (Prosopis), and others. Most of the species of 

 Yucca belong to this north Mexican region, as well as the species 

 of Agave, of which the century plant is sufficiently familiar. It 

 has been thought ] that these peculiarly American forms originated 

 in northern Mexico, and have migrated from this centre both 

 north and south. 



Of the more than 1,000 species of Cacti the greater number are 

 Mexican, and the same is true of Agave. Of the latter, 140 species 

 are found in Mexico, while only 24 are known elsewhere. 



The Mexican plateau region differs from that of the Pacific Slope 

 in a different distribution of rain, having a marked rainy season 

 in summer. This has an evident effect on the distribution of certain 

 plants. The Cacti, especially, are adapted to utilizing the summer 

 >howers, and are rare, or wanting in those parts of California 

 where summer rains are absent. This difference in rainfall is 

 probably largely responsible for the absence or scantiness, of 

 certain Arizona species in southern California. 



South of the dry Mexican plateau the rainfall is heavier, and the 

 mountains are clothed with forests much like those of the adjacent 

 L'nited States. Evergreen oaks, pines, and other northern trees, 

 are the principal constituents of the north Mexican mountain 

 forests. 



1 Harshberger, loc. cit., pp. 298-300. 



