April 3, 2012

Phaseolus polystachios (L.) BSP. - Wild Bean


Phaseolus polystachios is the only member of Phaseolus genus native to Missouri. Most members of this genus are found in tropical climates, and in the United States are native only in the south and southwest. It is not, however, the only plant in Missouri known as the wild bean. See the entry on Strophostyles helvola in this book for information on a closely related species.


These small beans are kidney shaped with an indentation where the hilum is (See Figure. 1). They are plump rather than thin and flat. Overall they are shaped very much like a common bean (Phaseolus vulgaris) but smaller. Before carbonization, a small sample of wild beans from St. Louis county averaged 7.3 mm in length and 5.5 mm in width (See Table 1).

The surface of the seed coat is a very smooth and shiny black. There are often one or two dimples or depressions on the surface of the seed coat, but these are not consistent from bean to bean. Beneath the seed coat, the surfaces of the cotyledons are also smooth. The hilum is white and shaped like an elongated oval (about 3 mm long by 1 mm wide) with the micropyle clearly visible, sometimes even in profile, and a smaller but still distinct lens below it. The hilum is much larger and longer than the hilum of the common bean (P. vulgaris) and may be the easiest way to differentiate between the two (See Figure. 2).

Figure 2. P. polystachios hilum.

Archaeological Distribution

Phaseolus polystachios has been identified from very few archaeological sites in eastern North America. Fritz (1989) found some in remains from Copple Mound at the Spiro site in Oklahoma. Cutler and Blake (1976) also report P. polystachios from just one site, 3NW31, in Arkansas.

Fritz (1986: 175-177) also identified many P. polystachios beans and their pods in desiccated samples from Ozark rockshelters. Interestingly, the beans from her samples showed possible evidence of human influence. Normally, P. polystachios pods expel the beans by twisting when they are ripe. Many of the pods recovered in her samples, however, while clearly belonging to P. polystachios, were only slightly or not at all twisted. She suggest that pods that did not twist and expel the beans had a selective advantage when humans were doing the dispersal (Fritz 1986: 177).

Part of the reason for the low occurrence of P. polystachios in archaeological contexts may be because of what happens when the beans are carbonized. When I experimentally charred six beans, I found that P. polystachios beans tend to shrink about 9% when carbonized (See Table 1). The seed coat of the bean becomes very brittle and tends to flake off. The cotyledons split easily and the hilum falls off, usually attached with micropyle and lens (See Figure 3).

Figure 3. Charred P. polystachios.
If a reasonably complete bean is found, it could probably be identified fairly easily.  P. polystachios is smaller than P. vulgaris, which it most closely resembles. The long, oval hilums of P. polystachios beans, as discussed above, are much larger than those of P. vulgaris, which tend to be small and round. Smaller fragments, less than a full cotyledon, would be very hard to identify and could be easily confused with P. vulgaris or one of the Strophostyles species.

The Modern Plant and Its Distribution

Phaseolus polystachios, the only member of the Phaseolus genus native to Missouri, is a climbing, twining or trailing perennial (See Figure. 4). It flowers in July through September, and the seeds are not mature until late in the summer or early in the fall. The seeds are born in pods like common beans except they twist when mature to expel and disperse the beans. It commonly grows in ravines, at the base of bluffs, rocky open woods, and thickets. Occurring mostly in southern Missouri, its range extends from Florida and Texas to New Jersey, Ohio, and Illinois. From the east coast, its range extends west as far as Iowa and Nebraska (Steyermark 1963: 949-950).

Figure 4. P. polystachios plant (from Steyermark 1963:949).


Phaseolus polystachios beans can be cooked and eaten like common beans (Steyermark 1963: 950). Beans could be cooked and eaten as a vegetable, mashed into a paste, boiled alone or with some other food to produce soup, made into a bread, or dried and stored (Moerman 1998: 391). They are a protein-rich local food source and were probably exploited more before the introduction of the common bean to the area. As Fritz (1986: 177) pointed out, P. polystachios beans “are large and could have provided a good deal of food if their numbers were sufficient.”

There are, so far as I was able to determine, no documented uses of this plant for medicine or anything other than food. This is not surprising as there are no documented ethnographic medical uses for P. vulgaris either. Interestingly, however, Lewis and Elvin-Lewis (1977: 100, 362) indicate that P. vulgaris might be useful for delaying transplant rejections, and antitumor therapies, and that it has antifungal properties.

Table 1. Experimentally Charred Phaseolus polystachios (Size in mm)


Cutler, H. C., and L. W. Blake
    1976   Plants from Archaeological Sites East of the Rockies. Missouri Archaeological
            Society, 15 Switzler Hall, Univ. of Missouri in Columbia, Missouri 65201. Printed in
            microfiche L.C. No. 76-49422 (Correceted and updated version of the 1973 edition).

Fritz, G. J.
    1989   Evidence of Plant Use from Copple Mound at the Spiro Site. In Contributions to Spiro
            Archaeology: Mound Excavations and Regional Perspectives, edited by J. D. Rogers,
            D. G. Wyckoff, and D. A. Peterson, pp. 65-88.  Oklahoma Archaeological Survey.
            Studies in Oklahoma’s Past No. 16.
    1986   Prehistoric Ozark Agriculture: The University of Arkansas Rockshelter Collections.
            Ph. D. dissertation, Microfilm. Department of Anthropology, University of North Carolina,
            Chapel Hill.

Lewis, W. H., and M. P. F. Elvin-Lewis
    1977   Medical Botany. John Wiley & Sons, Inc., New York.

Moerman, D. E.
    1998   Native American Ethnobotany. Timber Press: Portland, OR.

Steyermark, J. A.
    1963   Flora of Missouri. Iowa State University Press: Ames, Iowa.

Written by: Kimberly Schaefer