THE DICOTYLEDONES 1737 



orthotropous basal ovule. The fruit is a single, three- or two-winged nutlet. 

 The seeds contain copious endosperm and often an eccentric embryo. 



The Polygonaceae contain about forty genera and some 750 species, 

 which live mostly in the north temperate zone. Among the more common 

 genera which occur in Britain may be mentioned Riimex (Dock) (Fig. 1613) 

 and Polygonum (Knotgrass). Several are of economic importance, e.g., 

 Rheum rhaponticum (Rhubarb) (Fig. 16 14) and Fagopyrum esnilentum 

 (Buckwheat). Muehlenbeckia platyclados, a native of the Solomon Islands, 

 has its stem and branches flattened in the form of cladodes. 



Some of the genera rely upon wind for pollination, but Rhetitn and 

 Polygonum are entomophilous, nectar being secreted at the base of the 

 stamens. They are rendered conspicuous by the number of flowers in the 

 inflorescence (Fig. 161 5), and by the red and white colours of the sepals. 



Fig. 161 5. — Polygonum sieboldii. Inflorescence. 



Anatomically the family is noteworthy on account of the occurrence 

 of inversely orientated medullary bundles, especially in species of Rumex 

 and Rheum. 



The Piperales, as mentioned above, are closely related to the Poly- 

 gonales. We shall not consider the order in detail but since they show 

 several interesting features we may briefly describe them here. They are 



