ANGIOSPERMAE 



629 



have flattened branches (Fig. 197). Cereus (Fig. 631), Echinocactiis, with longitudinal 

 ridges on the stem ; Mammil- 

 laria has free projections (ma- 

 millae). The numerous groups 

 of spines on the shoots, ribs, or 

 separate mamillae correspond 

 to axillary shoots, the subtend- 

 ing leaves of which are re- 

 duced, while the leaves of the 

 expanded axis of the axillary 

 shoot are metamorphosed into 

 spines (Fig. 631). 



Cactaceae form a dominant 

 constituent of the vegetation 

 in the dry south-western re- 

 gions of the United States and 

 in Mexico. They are also widely 

 distributed in the West Indies 

 and South America. A similar 

 habit is found in some Euphor- 

 biaceae and Asclepiadaceae 

 living under corresponding 

 climatic conditions (cf. p. 174). 

 There are numerous epiphytic 

 Cactaceae, especially species of 

 Rhipsalis, Epiphyllum. and 

 Phyllocactus, which clothe the 

 branches of trees and atfect 

 the general aspect of the vege- 

 tation. Opuntia ficus indica 

 has become naturalised in the 

 Mediterranean region. The 

 fruits of this species and of 

 others of the genus are edible, 

 and the plants are cultivated 

 as fruit-trees. Some Cactaceae. 

 such as Anhalonium, contain 

 highly poisonous alkaloids and FIG. tV29. Saponaria officinalis ( nat. size). Porsoxocs. 

 saponin. The Cochineal in- 

 sect is grown upon species of Opuntia and Nopalea (N. coccinellifera). 



B. DlALYPETALAE 



Flowers with calyx and corolla. 



Order 1 1 . Polyearpieae 



Hermaphrodite, usually brightly coloured flowers, with an elongated 

 receptacle on which the free perianth segments, the stamens and the 

 apocarpous carpels are spirally arranged ; the carpels are indefinite in 



