440 



BOTANY 



represented in the West by several species of Mentzelia, with showy flowers, 

 resembling those of the Cacti, to which they are probably related. 



Of the exotic families, the Begoniacese comprise many beautiful species culti- 

 vated in our gardens and greenhouses. The curious Papaya ( Carica Papaya) , 

 belonging to the Caricacese, is familiar to every traveller in the Tropics. 



Order XI. Opuntiales 



A single very characteristic family, Cactaceae, whose members are almost 

 exclusively American, represents the order Opuntiales. A few forms e. g. Rhip- 

 salis are epiphytes, but much the greater number are inhabitants of the arid 

 regions of the southern United States and Mexico. In these regions the Cacti 



///'. 



FIQ. 434. Mammillaria macromeris. (After BAILEY.J 



are the most striking members of the native flora. Their massive, leafless, and 

 spiny stems are perfectly adapted to their peculiar environment. The flowers 

 (Fig. 434) are very uniform in structure, and make the separation into genera 

 extremely difficult. 



Some of the smaller species of Prickly-pear (Opuntia) extend well northward, 

 but it is in the hot arid regions of the Southwest that they reach their greatest 

 development. In Arizona the Giant-cactus ( Cereus giganteus) reaches a height 

 of twenty metres or more. 



Order XII. Myrtiflorae 



The Myrtiflorse are largely tropical in their distribution, and in the warmer 

 parts of the world are represented by many species, some of which are gigantic 

 trees, like the Australian Gum-trees (Eucalyptus). 



