gGo SCIENCE IN AFRICA 



introductions from Asia are Dioscorea alata, the greater yam, and 

 Colocasia antiquorum, the eddoe or coco-yam. Early introductions 

 from America are (i) Manihot utilissima, the cassava or tapioca. 

 This is scattered throughout Africa wherever conditions are suit- 

 able for its cultivation, though it may miss areas where its poisonous 

 properties have prejudiced its use. It occurs in many varieties, 

 which are recognized and named by the people. (2) Ipomaea batatas, 

 the sweet potato. This occurs throughout tropical and subtropical 

 Africa in many recognized and named varieties. (3) Xanthosoma 

 sagittifolium. It is uncertain when this was introduced and its 

 close resemblance to C. antiquorum may account for this un- 

 certainty. It is a matter of interest to note how these introduced 

 root crops have been adopted by the people and treated as their 

 own, and also how varieties have been recognized and evolved. 



Cassava {Alanihotutilissima), the root of which, when ground, gives 

 the West African food known as garri, is a staple carbohydrate 

 food almost equal in importance to cereals. Selection work and 

 observation trials have been carried out in Nyasaland, Tanganyika, 

 Uganda, and the West African territories. In Nyasaland, for 

 example, the department of agriculture are distributing a type 

 which is a heavy yielder and a quick grower. The plants suffer 

 from a virus disease known as cassava mosaic. In the West this is 

 thought to have been introduced and to be spreading inland, being 

 carried by an Aleurodid fly {see Chapter X) . Much attention has 

 been paid to selecting resistant strains, but here, as with the millets 

 and guinea corns, there are many local varieties, probably both 

 of the cassava and the disease organism, and strains which are 

 apparently resistant to mosaic in one area may not be so elsewhere. 

 Satisfactory resistant types have been produced in Nigeria at 

 Ibadan, in the Gold Coast at Kumasi, in Sierra Leone at Njala, 

 where the infestation near the coast is in the neighbourhood of 

 90 per cent, and in Uganda. The progeny of these are being dis- 

 tributed to local farmers with reasonable success. In Uganda, for 

 example, the resistant type of cassava is now important as a food 

 reserve in those areas where famines were a feature of the earlier 

 years of British administration. It is noteworthy that when resis- 

 tant strains from Nigeria were tried at Amani, they succumbed to 

 the East African mosaic disease. 



