362 SCIENCE IN AFRICA 



to a large extent by the Irish potato because the latter fetches a 

 higher market price locally, especially from Asiatics and Euro- 

 peans. Leakey (1934) has regarded this as an important factor in 

 the overstocking problem, especially in relation to sheep and goats 

 kept on cultivated land, because the vines of the sweet potato were 

 formerly so valuable as fodder during the dry season. 



Tams {Dioscorea spp.) are a staple food in many of the territories 

 with high rainfall, especially in countries bordering the Gulf of 

 Guinea. Their cultivation has been studied in detail by the 

 Nigerian department of agriculture, but little has been done to 

 improve the varieties. They are grown for distribution of propagat- 

 ing material, on native administration farms in Nigeria and the 

 Gold Coast, and at Njala in Sierra Leone. The air potato [Dioscorea 

 bulhifera) is another food plant which grows wild in West Africa 

 and is cultivated in some areas, for example in Sierra Leone. 



There are a number of minor crops, many of which are grown 

 only locally. Of these> the coco-yam [Colocasia antiquorum) is impor- 

 tant as food in some regions of damp climate, as in the cocoa-belt of 

 the Gold Coast, and the south-east of Nigeria and the Cameroons. 

 This plant is not related to the Dioscorea yams. It had long been 

 thought that crystals of Calcium oxalate, always present in the 

 roots, were injurious to health, but it has recently been shown in 

 Nigeria that the bulb contains toxic substances which are respon- 

 sible for this condition. 



BEVERAGES 



Coffee. All the cultivated species of coffee are African plants. 

 Coffea arabica from Abyssinia is best suited to high altitudes; C. 

 robusta and allied species are more tropical in origin and belong 

 to regions with a higher rainfall; they are indigenous in the southern 

 Sudan and in Uganda. C. liberica is indigenous to regions of high 

 rainfall in West Africa. C. stenophylla is indigenous in Sierra Leone 

 and neighbouring countries, and many of the wild coffees found 

 in East Africa are closely allied to this last species, if not identical 

 with it. 



In the case of coffee, selection of improved varieties appears to 

 be the principal requirement, but since there is a latent period of 

 about five years before bearing, results can only be achieved 



