THE DICOTYLEDONES 1705 



ARISTOLOCHIALES 



The Aristolochiales are Archichlamydeae in which the flowers are 

 either monoecious or dioecious, cychc, and either actinomorphic or zygo- 

 morphic. The perianth consists of a single whorl and may be either peta- 

 loid or fleshy, the parts being either separate or joined together. The 

 stamens vary in number but are often a multiple of the number of perianth 

 segments; they mav be free or united with the style to form a central 

 column or gynostemium. The ovary is inferior, four or six locular with 

 axile placentation, or unilocular with parietal placentation. The ovules are 

 numerous and possess two integuments. The fruit is a berry or capsule. 

 The seed is provided with endosperm and sometimes with additional 

 perisperm. The embryo is small. 



The plants are either herbs or woody climbers with simple, alternate, 

 exstipulate leaves. Some species are parasites, in which the vegetative 

 structure is greatly reduced and may be even endophytic, while the flowers 

 are often very large and conspicuous. The members of this order live 

 chiefly in the tropics and are of particular interest on account of their 

 parasitic representatives and their method of pollination. 



According to Engler the order includes three families, Aristolochiaceae, 

 Rafliesiaceae and Hydnoraceae. Hutchinson also includes the family 

 Nepenthaceae, which Engler adds to the Sarraceniales. We shall follow 

 Engler's system in this respect. 



In the Aristolochiaceae the plants are mostly woody climbers, though 

 some are herbs. There are five genera distributed through the tropics 

 but extending into temperate regions (Fig. 1572). Asarum europaeiim is a 



Fig. 1572. — Aristolochia elei>ans. The perianth tube is 

 greatly elongated into a coloured lip. 



rare British perennial herb, while Aristolochia clematitis (Fig. 1573), a 

 native of central and southern Europe, is established as a garden escape 



