538 



ANNUAL KEPORT SMITHSONIAN INSTITUTION, 1908. 



of Uruguay, and melon-sliaped Echinopses amid the snow and hail 

 of the lofty Bolivian plateau. It is interesting to note that the genus 

 Mamillaria, so well represented in Mexico, is practically absent from 

 Costa Rica and the countries adjacent to it, the genera representing 

 the family in that region being Cereus, Pereskia, Pereskiopsis, Nopa- 

 lea, and Opuntia. These genera also occur in the warm Huasteca 

 region of northern Veracruz and southern Tamaulipas, which is con- 

 nected with San Luis Potosi by the Mexican Central Railway. Melo- 

 cactus (or Cactus, as the genus is now called by Dr. J. N. Rose) is 

 essentially West Indian and Central American ; and the genus Rhip- 

 salis referred to above, also occurring in tropical America and the 



West Indies, extends to Africa, the island of 

 Mauritius, and even to Ceylon. 



Structure. 



The Cactacese have a woody axis more or 

 less pronounced, usually surrounded by pulpy 

 cellular tissue (parenchyma), in which water 

 is stored. The transpiring surface is much 

 reduced and the stomata are usually situated 

 in depressions or grooves in the leathery 

 cuticle of the stem. As an additional means 

 for checking transpiration the cell sap is 

 nearly always mucilaginous, and in some 

 genera there are latex ducts filled with 

 milky or gummy fluid. Certain species of 

 Echinocactus (pi. 8) are like great melons or barrels and are filled 

 with pulp of the consistency of watermelon rind.*^ On the other 

 hand, the stems of certain Pereslrias, Pereskiopses, and Opuntias 

 become quite hard and woody. The hard, woody, reticulated skele- 

 tons of several species of Opuntia are used for various purposes, 

 such as the manufacture of napkin rings, walking sticks, table and 

 chair legs, and even for veneering. In some parts of South America 

 and in Lower California, where other vegetation is lacking, the 

 stems of cardones, or columnar Cerei are used by the natives in 

 constructing their habitations, corrals for their animals, and even 

 in timbering mines, and of some species the stems are dried and 

 soaked in pitch for use as illuminating torches. Many species have 

 tuberous roots. One of these, Opuntia rnegarrJuza., collected by 

 Doctor Palmer at Alvarez, in the State of San Luis Potosi, is sold 



°' For an account of tlie transpiration, root systems, and structure of Carne- 

 giea gigantea, Echinocactus wislizeni, and Opuntia versicolor, see Cannon, W. A., 

 " Biological relations of certain cacti," Desert Botanical Laboratory publica- 

 tions No. 11, reprinted in the Amer. Naturalist, Vol. 40, January, 1906. 



