2040 A TEXTBOOK OF THEORETICAL BOTANY 



three carpels forming a trilocular or rarely a unilocular ovary, with two 

 anatropous ovules in each loculus. The fruit is a capsule, or occasionally a 

 berry, containing seeds with horny endosperm. 



The family contains some nine genera and over 200 species. The chief 

 genera are Dioscorea, Testudinaria and Tamils. 



The genus Dioscorea contains the bulk of the species (Fig. 1979). 

 D. pyrenaica is the only European species. The genus has twining annual 

 stems arising from tubers. These tubers vary in their morphology in the 

 ditferent species. In D. batatas, the tuber is derived from the hypocotyl 

 and is variously interpreted as a rhizome or a root. In D. simiata the tuber 

 originates from the internodes above the cotyledon, while in D. zillosa it is 

 a rhizome. The tubers are known collectively as Yams and contain an 

 abundance of starch. They are largely cultivated in tropical countries as a 

 source of food. In cultivation they are propagated by eyes in the same way 

 as potatoes and serve much the same function in diet. Axillary tubers are 

 also found on the main stem. 



The genus Testudinaria (Fig. 1980) contains two species which grow in 

 South Africa. The tuber is an enormous swelling of the first internode of 



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Fig. 1980. — Testudinaria ele- 

 p/iantipes. Stem tuber \\ ith 

 leafy shoot developed. 



the stem, which projects above the ground and is covered by a thick layer 

 of cork. Annual, long, thin climbing stems arise, which bear large leaves 

 and small flowers. T. elephantipes (Elephant's Foot) is the better known of 

 the two, and is the source of Hottentot Bread. 



