Common names: Groundnut, Wild Bean, Potato-bean, American Potato Bean, Indian-potato (1, 4, 7).
Etymology: Apios is Greek for “pear”; the plant was named such because of its pear-shaped tubers. Americana means “American” (3, 7, 17).
Botanical synonyms: Apios tuberosa (4), Glycine apios (14).
FAMILY: Fabaceae (the Pea family). Fabaceae is also known as Leguminosae
Quick Notable Features:
¬ Purple-brown flowers in axillary racemes
¬ Papilionaceous (butterfly-like) corolla
¬ Stem and leaves are unarmed
¬ Pinnately compound leaves with 5-7 ovate to lanceolate leaflets per leaf, each leaflet with a round (not C-shaped) petiolule
Plant Height: Climbing individuals grow up to 5m long (4).
Subspecies/varieties recognized (11):
Apios americana f. cleistogama Fernald
Apios americana f. keihneri Oswald
Apios americana f. mcculloughii Oswald
Apios americana f. pilosa Steyerm.
Apios americana var. turrigera Fernald
Most Likely Confused with: Apios priceana, Wisteria frutescens and Wisteria sinensis, Amphicarpaea bracteata, Desmodium rotundifolium, Strophostyles helvola, as well as the species in the genera Lathyrus, Rubus, and Coronilla.
Habitat Preference: A. americana can grow in partial or no shade (5). It prefers low and moist woods, thickets, stream and riverbanks, ponds, marshes, meadows, and wet ravines (1, 4).
Geographic Distribution in Michigan: Located in 32 counties of the Lower Peninsula, nearly all in the southern half, and in Mackinac, Delta, and Menominee counties of the Upper Peninsula (1).
Known Elevational Distribution: Groundnut can be found cultivated up to elevations of 1000m (16).
Complete Geographic Distribution: Native to the eastern United and Canada. A. americana is found in every state east of and including Colorado, and in Quebec and Ontario in Canada (14).
Vegetative Plant Description: An herbaceous, perennial vine. Stems range from glabrous to lightly pubescent and bear glabrous to pubescent petioles, up to 1.8 cm long. A. americana can climb high, as well as grow over shrubs and herbs on the ground. Rhizomes are slim and have 2 or more tubers “in a series” (2). Tubers can be up to 6cm in diameter. The leaves bear deciduous stipules that are 4-7mm long. The leaves are alternate, pinnately compound, and bear 5-7 leaflets per leaf. The leaflets can be glabrous or pubescent and are ovate to lanceolate, with a rounded base and an acute tip. They range from 2-10cm in length, but are typically 4-6cm long and 1.5-4cm wide (1, 2, 4, 7, 8).
Climbing Mechanism: A. americana is an apical stem twiner (pers. obs., Robyn J. Burnham).
Flower Description: Flowers are borne in axillary racemes on peduncles. The racemes are dense and compact in the northern states and more elongated in the southern states. The peduncle is 2-5 cm long and bears pedicels that are 2-5 mm long. Flowers are perfect. They are 10-13mm long with a hemispheric calyx tube, 3-5mm in length. The 4 upper calyx lobes are short or non-existent, and the single lower lobe is usually half as long as the calyx tube. The corolla is papilionaceous, or “butterfly shaped”, with retuse petals (bearing a notch at the top). The banner petal has a whitish back and a reddish-brown front. Wings are purplish-brown, keel is “strongly curved” (1) and sometimes coiled. There are 10 diadelphous (1+9) stamens. The pistil is stipitate with a disk that surrounds the stipe. The ovary is 5.5-7 mm long and glabrous to tomentose with 7-16 ovules. The style is coiled and glabrous to bearded. The capitate stigma has a stigmatic membrane (1, 2, 4, 5, 7, 8, 18).
Flowering Time: The species flowers from July to September in the Great Plains (4).
Pollinator: In Colorado, honeybees were observed crawling between the banner and keel (12). Leafcutter bees were observed tripping the flower in Connecticut (19).
Fruit Type and Description: A straight or slightly curved legume, 5-11.5cm long and 4-6 mm wide (4,21). The tip is shortly-aristate to acuminate. The legume is glabrous to tomentose when young and can be glabrous or bear striga (sharp-pointed appressed bristles) when mature. The endocarp is silver to “off-white” (18). The dehiscent pods may have up to 12 seeds per pod (19).
Dispersal Syndrome: A. americana can be diploid or triploid. Both types have similar morphologies and can reproduce asexually via tubers. Apparently only the diploid plants can reproduce sexually and bear seeds (19). Triploid plants may flower, but are without seeds. Anderson and Spackman hypothesized that, in Colorado, the tubers are dispersed by flooding and human activity (12). No mechanism for dispersal by seed has been found.
Distinguished by: The flower of Apios americana is purplish-brown and retuse, while Apios priceana (a species of the southern Midwest USA) has a whitish-green flower with a “spongy appendage” (8) protruding from the top of each flower. A. americana produces multiple tubers compared to only one in A. priceana. A. americana has 5-7 leaflets per leaf and is herbaceous while Wisteria species differ in their purple-blue flowers, woody stems, and 7-15 foliate leaves that are pubescent on both the top and bottom. Amphicarpaea bracteata, Desmodium rotundifolium and Strophostyles helvola are all trifoliate. A. bracteata has underground, 1-seeded legumes and cleistogamous flowers that can grow near or under the soil. It does not have the excurrent midvein that A. americana does. S. helvola bears whitish-purple flowers, as well as leaflets with 2-3 lobes. D. rotundifolium has round to ovate leaflets whereas A. americana has ovate to lanceolate leaflets. The terminal leaflet of Lathyrus species has been replaced with a tendril. This is not seen in A. americana. Rubus species can be distinguished by their armed branches and leaves. Coronilla can be distinguished by its umbellate inflorescence and many-foliate leaves.
Other members of the family in Michigan (number species): Cercis (1), Crotalaria (1), Melilotus (3), Trifolium (10), Medicago (3), Cytisus (1), Baptisia (3), Amphicarpaea (1), Phaseolus (2), Strophostyles (1), Desmodium (12), Psoralea (1), Lespedeza (8), Glycine (1), Caragana (1), Gymnocladus (1), Gleditsia (1), Wisteria (2), Amorpha (2), Robinia (3), Schrankia (1), Lupinus (3), Cassia (4), Pisum (1), Lathyrus (10), Vicia (9), Dalea (2), Anthyllis (1), Lotus (1), Coronilla (1), Apios (1), Tephrosia (1), Astragalus (3), Hedysarum (1) Source (1).
Ethnobotanical Uses: The tubers of A. americana are 17% protein by mass, edible, and prepared much like potatoes. Native Americans and early explorers ate them roasted or fried. The tubers can be harvested during the winter and possibly could have saved the settlers at Plymouth Rock from starvation during their first winter. Some studies have indicated that the tubers contain isoflavines, chemicals that are linked to a lesser occurrence of prostate and breast cancer. However, in some instances people have become ill after eating the tubers. The shoots, flowers and seeds are also edible (1, 4, 12, 20).
Phylogenetic Information: The genus Apios is a member of the subfamily Faboideae, also known as Papilionoideae, in the family Fabaceae. Faboideae is distinguished from the other subfamilies because of its bilaterally symmetrical flower, with the uppermost petal largest as well as outermost, and the two “basal petals” fused together (6). This looks similar to the wings of a butterfly. Fabaceae makes up the majority of the order Fabales, which is a member of the Rosid subclass. The Rosids are eudicots (9, 10).
Interesting Quotation or Other Interesting Factoids not inserted above:
-Apios americana shares a symbiotic relationship with Rhizobium soil bacteria (9). This bacteria fixes nitrogen that the plant can then use.
-This plant shows considerable variation in the amount of pubescence, raceme density, and leaflet size among individuals (4,5).
-A. americana can thrive in temperate as well as subtropical climates (4,5).
-Horses and other livestock also graze its leaves (4, 5).
-This species experiences low fruit and seed production in certain populations. This might be attributed to the many triploid individuals seen in the northern part of the geographic range of this plant. A study done in Connecticut showed that triploid plants did not produce fruit in the field (19).
-One form of the species, “f. cleistogama” has flowers that do not open at all, instead forming progeny by self-pollination (7).
Literature and websites used:
- Voss, E. G. 1985. Michigan Flora Part II: Dicots. Ann Arbor, Michigan, USA: Cranbrook Institute of Science.
- Gleason, H.A. 1963. The New Britton and Brown Illustrated Flora of the Northeastern United States and Adjacent Canada. New York, New York, USA: Hafner Publishing Co., Inc.
- Bailey, H.L. 1963. How Plants Get Their Names. New York, New York, USA: Dover Publications, Inc.
- McGregor R.L. 1986. Flora of the Great Plains. Lawrence, Kansas, USA: The University Press of Kansas.
- Plants For A Future, 1996-2008. http://www.pfaf.org/database/plants.php?Apios+americana
- Judd, W.S., C.S. Campbell, E.A. Kellogg and P.F. Stevens. 1999. Plant Systematics: A Phylogenetic Approach. Sunderland, Massachusetts, USA: Sinauer Associates, Inc.
- Fernald, M. L. 1950. Gray’s Manual of Botany, 8th ed. New York, USA: American Book Company.
- Gleason, H. A. 1963. Illustrated Flora of the Northeastern United States and Adjacent Canada, Volume 2. New York, New York, USA: Hafner Publishing Company, Inc.
- Zomlefer, W.B. 1994. Guide to Flowering Plant Families. Chapel Hill, North Carolina, USA: The University of North Carolina.
- Stevens, P.F. Angiosperm Phylogeny Website. Version 9, June 2008. http://www.mobot.org/MOBOT/research/APweb/.
- The International Plant Names Index. http://www.ipni.org/index.html.
- Anderson D.G. and S. Spackman. 2001. Inventory and Status Report of American Ground Nut (Apios Americana Medicus) in Colorado. Colorado Natural Heritage Program: 2001 Documents and Reports. http://www.cnhp.colostate.edu/reports.html.
- Weatherbee, E.E. and J.G. Bruce. 1982. Edible Wild Plants A Guide to Collecting and Cooking. 2nd ed. Ann Arbor, Michigan, USA: Weatherbee and Bruce.
- USDA, NRCS. 2008. The PLANTS Database. http://plants.usda.gov/java/profile?symbol=APAM (Jan. 26th 2009).
- Hoshikawa K., and Juliarni. 1994. The Growth of Apios (Apios americana Medikus), a New Crop, under field Conditions. Faculty of Agriculture, Tohoku University.
- Elzebroeck, A.T.G. and K. Wind, eds. 2008. Guide to cultivated plants. Wallingford, UK: CAB International Publishing.
- Allen, O.N. and E.K. Allen. 1981. The Leguminosae, a Source Book of Characteristics, Uses, and Nodulation. 1st ed. Madison, Wisconsin, USA: University of Wisconsin Press.
- Woods, M. 2005. A Revision of the North American species of Apios (Fabaceae). Castanea 70: 85-100.
- Bruneau, A. and G.T. Anderson. 1988. Reproductive biology of diploid and triploid Apiosamericana (Leguminosae). American Journal of Botany. 74 (12): 1876-1883.
- Dean, T. 2007. Stalking the Wild groundnut. Orion. Nov/Dec 2007. http://www.orionmagazine.org/index.php/articles/article/458/
- Britton, N.L. & H.A. Brown 1970. An Illustrated Flora of the Northern United States and Canada: Volume II. New York, NY: Dover Publications, Inc.
Image Credits (all used with permission):
- Image of flower and plant together © 2006 Louis-M. Landry downloaded from calphotos
- Image of the leaf close-up is from Robyn J. Burnham
- Image of flower close-up © 2006 Louis-M. Landry downloaded from calphotos
- Image of seeds courtesy Steve Hurst @ USDA-NRCS PLANTS Database
- Species distribution map, derived from the Michigan Flora Online.
PRIMARY AUTHOR: Dave Morris, with editing by John Bradtke, ReBecca Sonday, Cristine V. Santanna, and Robyn J. Burnham
© Robyn J. Burnham, University of Michigan
For additional information on Michigan Plant Diversity web pages please contact Robyn J. Burnham via email: rburnham“at”umich.edu