Lecture 4. Tubers and roots

Summary

Tubers and roots refer to the underground parts of plants, some of which are used as food. By definition, tubers are underground stems or roots that live over from season to season, and are swollen with food reserves. Root crops refer to edible roots or swollen stems, most of which are annual. Although generally low in protein and fat, many tubers (such as arrowroot, sweet potato, yam, potato and others), are rich in carbohydrates. They provide between two and six percent of the caloric intake in many countries. Many tubers and roots have medicinal properties as well.

Examples:

Species of several families yield edible tubers and root crops. These include:

• Dioscoreaceae, a small family of only 5 genera and 625 species of mostly pantropical distribution. They are perennial herbaceous or shrubby climbers with well-developed tubers or rhizomes, though a few are dwarf shrubs. The family is best-known in economic botany as the source of edible yams in the genus Dioscorea (most widely collected and cultivated in West Africa, but also found in South America, the Caribbean region and South East Asia). Some 60 species are cultivated or collected as subsistence or emergency foods, with five species accounting for most of the world production. The Dioscoreaceae is also a source of steroidal sapogenins (such as diosgenin), used in producing oral contraceptives.

• Convolvuceae, a small family of cosmopolitan distribution with 50 genera and about 1800 species found equally in temperate and tropical regions, in a wide range of habitats. They are herbaceous and woody plants, often climbers, always climbing to the right. The are used as ornamentals (such as Ipomoea, the morning glory) and food plants (Ipomoea batatas, sweet potato); the family also contains some important weeds. Ipomoea batatas (called sweet potato) is not known in a wild state. Cultivated from an early date in Mexico, Central and South America and the West Indies, and it was transported to Polynesia and New Zealand. Much later, Columbus introduced it to Spain, and then carried by Spaniards and the Portuguese to Asia and Africa, where it has become an important crop. There are now hundreds of varieties around the world, ranging in color from white to yellow, orange, red and even purple.

• Araceae, a medium sized family with some 110 genera and about 2000 species, is distributed primarily in the tropics, with a few representatives in the temperate zone. Several species yield edible swollen stems, and others are ornamentals. Members of the family are typically herbaceous plants with great variation in vegetative habit: many are climbers or epiphytes, but others are herbs with underground tubers or rhizomes. Important edible species include Colocasia esculenta, which originated in South East Asia, and was probably cultivated before rice. The tuber goes by various names: taro in the Pacific Islands, eddoe or dasheen in the West Indies, and old cocoyam in West Africa. It must be sufficiently cooked (by baking, roasting or boiling) to avoid irritation by calcium oxalate crystals. The new cocoyam (also called tannia or yautia) is Xanthosoma sagittifolium. It is a native of the New World and was cultivated in both tropical America and the West Indies in pre-Columbian times. It is now cultivated in West Africa and other regions as well. The new cocoyam can be told from taro by the leaves: the leaf stalk is attached to the edge of the leaf in the former, while it emerges near the center of the leaf in the latter. Both produce corms (a bulbous, swollen, underground stem with leaf scales and adventitious roots) with about 25% starch.

• Other families that give roots crops are Apiaceae (Daucus, Pastinaca), Asteraceae (Scorzonera, Tragopogon) and Euphorbiaceae (Manihot).

References:

Dounias, E. 1993. Perception and use of wild yams by the baka hunter-gathers in South Cameroon. Pages 621 – 632 in Hladik, C.M., A. Hladik, O. Linares, H. Pagezy, A. Semple and M. Hadley, editors, Tropical Forests, People and Food. Biocultural Interactions and Applications to Development. Volume 13, Man and the Biosphere Series. Paris, UNESCO.

Hladik, A. and E. Dounias. 1993. Wild yams of the African forest as potential food resources. Pages 163 – 176 in Hladik, C.M., A. Hladik, O. Linares, H. Pagezy, A. Semple and M. Hadley, editors, Tropical Forests, People and Food. Biocultural Interactions and Applications to Development. Volume 13, Man and the Biosphere Series. Paris, UNESCO.

Salick, J., N. Cellinese and S. Knapp. 1997. Indigenous diversity of cassava: generation, maintenance, use and loss among the Amuesha, Peruvian Upper Amazon. Economic Botany 51:6-19.

Exercises:

Free listing of tubers and roots: Split into small groups, and make a list of familiar tubers and roots that are used for food. Record the list in a table, indicating the common name of the tuber or root, its scientific name and botanical family, and its uses and geographical origin. Create a master list of all the roots and tubers named by participants in the exercise.

Preference ranking of tubers and roots: Using the master free list, ask each participant to rank his or her most favorite to least favorite tubers.

Questions for discussion:

Are there some regions and peoples of the world which are more dependent on roots and tubers than on grains? If so, what distinguishes these regions and peoples? Can you give any examples from Thailand?

Perspective for discussion:

"Plants with tubers hidden below ground in the rain forest merit renewed attention as potential foods for forest people, both past and present. The only visible starch-rich foods in tropical forest capable of being a staple resource are sago palms, and – for only a few weeks of the year’s cycle – the seeds of certain trees, especially Leguminosae.

Most hunter-gatherers presently living in tropical forest obtain their staple starch foods (cultivated tubers, plantain fruits, rice, etc.) through exchange with agriculturalists, but the role that wild tubers might have played in the past as staple starches remains a mystery.

Admittedly, assessing the production of spontaneous plants in a tropical forest is technically difficult, because individuals tend to be unevenly distributed and production is frequently unpredictable on a seasonal basis. However, assessing the production of tubers in even more difficult because they are often deeply buried in the forest floor and difficult to locate. The first survey of yam density and production in tropical humid forest was conducted in Gabon and the Central African Republic, thanks to the expertise of Aka pygmies who showed the research team how to locate and identify otherwise hidden yams."

From Hladik and Dounias. 1993. Wild yams of the African forest as potential food resources. Pages163-164.

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