formed, appears curdlike. The flesh of D. cayenensis is usually 

 yellow and smoother than the flesh of D. rotundata. 



Aerial tubers are seldom produced but may be stimulated by 

 accidental or intentional girdling of the stem. 



Cytology 



Chromosome numbers of Dioscorea are based on multiples of 

 9 and 10, the latter being the most common (16). Miege has 

 postulated a third basic number of 12 (20). Odd numbers of 

 chromosomes deviating from these multiples have been reported 

 and indeed could partly account for the sterility of some species, 

 but such numbers are hard to verify. The chromosomes of 

 Dioscorea are particularly small, and determination methods, very 

 tedious. 



Chromosomes of the African species have been counted by 

 Miege (20) . Races of D. rotundata and D. cayenensis have either 

 36 or 54 chromosomes. These numbers correspond to tetraploid 

 and hexaploid numbers, respectively. In the Ivory Coast the 

 36-chromosome races are found in the north and the 54-chromo- 

 some types in the south. In the 36-chromosome races, meiosis is 

 normal, which suggests that the species is a very old tetraploid. 

 Occasional reproductive failures and sterilities in open pollinations 

 and controlled crosses could be associated with hexaploid numbers. 



The study of chromosome numbers has not been sufficiently 

 extensive to well characterize the two species. Wider study could 

 shed light on their origins and their relationships. However, this 

 field of study has been neglected in recent years. 



The production of seed by female plants is not common but 

 is enhanced by planting several varieties together. Seedlings have 

 been grown by a number of different investigators in what might 

 be described as preliminary breeding attempts. The seedlings 

 are quite tender and may require 2 years to produce a suitably 

 large tuber. Breaking of the normal dormancy period, which 

 inhibits germination during the dry season, may make possible 

 a 1-year seed-to-seed cycle. 



Efforts to breed D. rotundata have been begun by Sadik and 

 Okereke (27, 28) at the International Institute of Tropical Agri- 

 culture in Ibadan, Nigeria. Three generations of plants, about 

 40,000, have been raised from seeds produced by natural crossing. 

 These plants flower readily and cross with ease; moreover, they 

 exhibit a wide spectrum of genetic diversity in respect to disease 

 resistance, degree of flowering and fruiting, and many morpho- 

 logical features. The use of seeds has increased the percentage of 

 flowering offspring twofold over plants grown from tuber cut- 

 tings, and the percentage of female plants and the number of 

 flowers and fruits per plant have also increased. Of interest is 



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