Tubers and millets are crops domesticated mainly for starch food sources. The former group reserves starch in vegetative organs, such as tubers, corms or roots, but the latter does so in grains. This clear difference can be attributed to the difference in their places of origin, namely, the former mainly in the moist subtropics or tropics while the latter originated mainly in temperate or subtropical steppes or savannas. The so-called tuber plants include more than 1,000 species found both in wild and cultivated states. This attests to our long history of utilizing tubers as important staple food materials. In this article tuber crops are defined in the narrow sense as domesticated plants which reserve starch mostly in underground organs. The representative eleven tuber crops grown in wide areas of the world are listed in Table I. Tubers can be characterized by the following traits: (I) easy management for cultivation and harvesting, (2) difficulty in long-termed preservation, (3) high water content and (4) easy cooking. Tubers have played an important role in eating habits which is closely related to the crop rotation system in shifting cultivation and also in agricultural rituals. Among tuber crops, the potato is a very important food in the arctic-temperate regions of the world, sweet potato and cassava are highly valued in the tropics, and taro, yautia and the greater yam are grown together with Guinea yams in tropical West Africa.
The main cereals now cultivated extensively in the world include wheat, rice, maize and barley. It is usually considered that we are dependent on these four major cereals, but these cereals have become our staple food only quite recently. People have also grown many other cereals as staple food sources. Those cereals are mostly known as millets. They can be defined as a group of gramineous crops which have small grains and are cultivated mainly as summer crops in savanna-like environments of semiarid tropic or subtropic regions and in temperate monsoon regions. A great variety of millets are known in the world, and Table 2 gives the scientific name, tribe, common name and the place of origin for the twenty main millets. As evident from the table, these millets originated chiefly from Eurasia and Africa. The most important areas of domestication are East Asia, the Indian Subcontinent and the regions from the southern margin of the Sahara to the Ethiopian highlands in Africa. Different kinds of millets were domesticated in these two continents. Foxtail millet, common millet, Japanese barnyard millet, Indian barnyard millet and Job's tears are representatives of the millets which originated in Eurasia; sorghum, finger millet and pearl millet represent the African millets. Even in those areas where rice, wheat, maize or other major cereals are prevalently cultivated nowadays, history tells us that millets were rated highly in the past, playing traditionally an important role in our diet. This is because millets have a number of merits. First of all, millets can grow well even in those districts where soil, climate and other conditions are unfavorable. They are tolerant to excessively dry fields and infertile soils. Though their grains are small in size, their yield is stable. Secondly, when millets are bound and stored without threshing, they can be kept for a long time without much damage from insect pests. Because of this merit, they play the role of an emergency crop in years of bad harvest. Thirdly, many methods of using millets have been established as the materials of traditional staple foods. Fourthly, millets are used as the materials of making local drinks. This is closely related to the farming practice, dietary culture and agricultural rituals of rural communities. Finally, many millets have not attracted much attention by agronomists, and few attempts of modem breeding have been made. Thus, local landraces are still grown in many areas of the world.
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