MANUAL OF NATURAL HISTORY. 495 



The wood of the stem is Endogenous, the youngest 

 parts being in the centre, while, on the other hand, 

 the root exhibits concentric zones like an Exogen, 

 with a disposition to a radiated distribution of ves- 

 sels, and in some with a central pith ; the leaves, 

 also, are net-veined, and usually disarticulate with 

 the stem. Dictyogens are far from numerous, about 

 seventeen genera being at present enumerated, con- 

 taining upwards of two-hundred-and-sixty species. 

 They may be divided into two Sub-classes, according 

 to the unisexual or bisexual nature of the flower, 

 the two being united by the Smilaceoe, which con- 

 tain both forms of structure. Some of the families 

 are small, and imperfectly known, but two are of 

 considerable consequence, the one affording the 

 Yam, an important farinaceous article of diet in the 

 Tropics, and the other yielding the Sarsaparilla 

 plants, largely imported into England for medical 

 use. 



III. CLASS. DICTYOGENS (Dictyogense). 



Growth of stem endogenous ; root with the wood 

 arranged in solid concentric circles ; foliage broad, 

 net-veined, deciduous ; cotyledon single. 



I. SUB-CLASS BisEXUAL-DicTYOGENS (Monanthiae). 



Flowers perfect, each bearing male and female 



organs. 



1. FAMILY. Roxburgh-worts (Koxburghiaceae). 

 Shrubs, twining; roots tuberous; leaves 

 coriaceous ; flowers large, showy, solitary, 



