The Dioscoreales are primarily a tropical group, with a few species in Europe and temperate America. Most are vines or climbing plants, but some are herbs as well. The Dioscoreales include 600 species of Dioscorea, and about 35 species in other genera. The group is not especially large, but the genus Dioscorea is common in wet tropical regions around the world. In many countries, it is cultivated for its starchy tubers, sometimes called air potatoes or Chinese yams. Another member of the order, the large herb Tacca, is a common horticultural oddity favored by florists -- it is one of the few plants with blackish flowers, as shown at right.
A common characteristic of most Dioscoraleans is a thick, starchy underground stem called a tuber. The tuber provides an energy store for these plants, whose leaves and above-ground portions die back each year. The stored food allows the plants to begin growth again when conditions are favorable. The tubers of Dioscorea are full of starch and nutrients, and so are boiled and eaten in many cultures. These provide the primary source of carbohydrate for about 70 million people in more than a dozen countries in tropical Africa, as well as for peoples in southern Asia and parts of the Americas. The plants are propogated by cutting up the tubers, rather than from seed, as there are not usually many seeds produced.
The Dioscoreales are among those basal
monocot
groups which do not fit the textbook description of a monocot. In particular,
the leaves have a reticulated venation reminiscent of the
paleoherb dicots,
and may be heart-shaped or highly lobed.
For this reason, the Dioscoreales have at times been considered the most
"primitive" of monocots, but although they may retain many characters found
in early monocots, they now appear to be more closely related to the
Liliales.
The picture at left shows the leaves and fruits of Dioscorea;
click on the image to view more of it.
The fossil record of the Dioscoreales appears to extend back into the Upper Cretaceous (Maastrichtian). Broad heart-shaped leaves with reticulated venation are known from the Ripley Formation of the southeastern United States, and these are quite similar to some species of Dioscorea. The identity of these leaves is not entirely certain, however, since the leaves of Smilax in the Liliales are very similar. Only with further work, or with the discovery of preserved flowers, will the identity of these fossils be determined.
The Dioscoreales may be divided into the following families:
For more information about the Dioscoreales, try the Dioscoreaceae and Taccaceae Resources Lists at Texas A&M, or the DELTA descriptions for the Taccaceae, Trichopodaceae, and the Dioscoreaceae
Texas A&M Botany has a collection of images of the Dioscoreaceae and Taccaceae. You can also view images of Dioscoreaceae at University of Wisconsin. View images of Malagasy Taccaceae at the Missouri Botanical Garden
Sources:
C. P. Dahglian. 1981. A review of the fossil record of monocotyledons.
Botanical Review 47(4):517-555.
R. M. T. Dahlgren, H. T. Clifford, and P. F. Yeo. 1985. The Families of the Monocotyledons, 520 pp. Springer-Verlag, Berlin.
C. A. Segnou, et al. 1992. Studies on the reproductive biology of white yam (Dioscorea rotundata Poir.). Euphytica 64:197-203.
