The tropical world, within which most of the less developed countries (Ides) lie, includes many different ecosystems, with widely differing food production systems. Although in the Ides as in the temperate world grains are the most important staple foods, providing the main calorific bases of most diets, in many parts, especially of the humid tropics, root crops are of equal or even greater local importance. The most important of these are cassava, yams, cocoyams, sweet potatoes and white potatoes ]Coursey and Haynes 1970], whose total production is now around 185 mn tons per annum, thus providing the basic food for between 400 and 500 mn people across the tropical world.

They are very largely produced from smallscale, subsistence-level production systems, rather than from large-scale field operations and the technologies employed both in their production and their post-harvest utilisation are usually simple and founded on long-established traditional practice. Under humid tropical ecosystems, root crops are often far more productive than grains, whether in terms of production of available energy per hectare per year ide Vries et al 1967] or, especially in the case of cassava, in terms of labour input required to provide a given amount of food ]Coursey and Haynes 1970]. This last factor is of major importance to the small farmer of the tropics, who has little or no access to mechanical aids, nor often even to draught animals. Table 1 gives global production statistics for the major tropical root crops. Cassava production has increased rapidly in recent decades, as has white potato: yam and taro production has remained virtually static, while sweet potatoes appear to be declining, except in certain areas.