the production of monoecious plants, which could provide self- 

 pollination capabilities for breeding. In addition, a number of 

 plants have been dwarf or semidwarf, ranging in height from 

 30 to 70 centimeters. These could be important in that such plants 

 do not require staking. 



Characteristics of importance in breeding include tuber shape 

 and size, disease resistance, and eating and pounding quality. 



CULTIVARS 



No collection was ever assembled that was sufficiently ex- 

 tensive to make possible an estimate of the number of cultivars 

 of D. rotundata in existence. Unlike the examples of D. alata and 

 D. esculenta, the sexual fertility of D. rotundata and D. cayenensis 

 suggests that new seedlings are constantly arising. Although 

 cultural practices are such that one might not expect the establish- 

 ment and selection of new seedlings, the variation in cultivars 

 throughout West Africa suggests that this indeed has occurred. 

 Normally only a few cultivars are favored in a particular environ- 

 ment, although many more are recognized. Each cultivar occupies 

 a particular niche according to its special characteristics that 

 make it in demand. The intense localization of cultivars, so 

 characteristic of West Africa, adds confusion to the picture. It is 

 the authors' opinion that the number of varieties throughout 

 West Africa is between 500 and 2,500. 



The characteristics already mentioned distinguishing the two 

 species are also varietal distinguishing characteristics. A complete 

 list of characteristics observed in studying over 200 accessions of 

 the two species is given in table 2. 



The particular characteristics that are of value in a select 

 cultivar include yield at first harvest, a complex of components. 

 While dependent on many factors, good yielding ability is gen- 

 erally easily recognized by the farmer, and certain cultivars are 

 maintained for their yield, even though other characteristics may 

 be suboptimal. After yield, probably the most important character- 

 istic is the suitability of the tuber for fufu, the pounded yam 

 product so important to West Africans. In a particular region 

 the suitability of local cultivars for this use is well known. 



Often not recognized is the different susceptibility of varieties 

 to virus diseases. Viruses, when present, drastically reduce pro- 

 duction, especially when transmitted through the seed tuber to a 

 new generation of plants. It is highly likely, however, that either 

 consciously or unconsciously the more diseased tubers are elimi- 

 nated during the normal course of culture in two-harvest systems. 

 (See "Harvest and Yield.") When two harvests are made, the 

 regrowth of the tuber is severely limited in the case of badly 



(Continued on page 15.) 



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