Apios americana Medik. (groundnut, American potato bean)

A. americana f. pilosa Steyerm.

A. americana var. turrigera Fernald

A. apios (L.) MacMill., an illegitimate name

A. tuberosa Moench

Pl. 388 j–l; Map 1711

Plants perennial herbs, frequently climbing on other vegetation, with white latex (the sap thus appearing milky), with long-creeping, subterranean rhizomes bearing few to many fleshy tubers, these 2–5 cm long, 1–2 cm wide, frequently appearing in chains. Stems 100–300 cm long or rarely longer, twining, forming loose mats or climbing, unarmed, glabrous to sparsely or occasionally densely pubescent with mostly short, fine hairs (but rarely with longer, more matted hairs). Leaves alternate, odd-pinnately compound with (3)5 or 7 leaflets, the petiole 1.5–7.0 cm long, glabrous or hairy, the hairs sometimes minute. Stipules 4–7 mm long, hairlike, shed early; stipels 1–2 mm long, inconspicuous, shed early. Leaflets 2–10 cm long, 2–6 cm wide, lanceolate to ovate, rounded at the base, angled or somewhat tapered to a sharply pointed tip, the margins entire, the surfaces glabrous or sparsely to moderately short-hairy, pinnately veined. Terminal leaflet with the stalk 15–30 mm long, symmetric at the base; lateral leaflets with the stalk 2–3 mm long, not or only very slightly asymmetric at the base. Inflorescences axillary racemes, these often relatively short and dense, the inflorescence stalk 2–5 cm long, hairy, the bracts 2–3 mm long, linear to lanceolate, shed early; bractlets 2–3 mm long, linear to narrowly lanceolate, closely subtending the flowers, shed early. Calyces irregularly 5-lobed, the tube 2.0–3.5 mm long, broadly bell-shaped to more or less hemispheric, glabrous or sparsely hairy, the lowest lobe up to 1/2 as long as the tube, broadly triangular, the other lobes narrower, very short or occasionally absent. Corollas papilionaceous, purple to brownish red, the banner usually pale purple to nearly white on the outer surface, 9.0–12.5 mm long, 10–14(–16) mm wide, the expanded portion broadly obovate, rounded to broadly notched at the tip, strongly concave and somewhat thickened apically (forming a small hoodlike structure), the wings 9.0–10.5 mm long, 4–5 mm wide, the expanded portion oblanceolate to obovate, downward angled and somewhat spreading, the keel 12–14 mm long, 2–4 mm wide, narrowly oblanceolate in outline and somewhat sickle-shaped, strongly curved upward, fused nearly to the rounded tip. Stamens 10, all of similar length, 9 of the filaments fused and 1 free, strongly curved, the fused portion 8–10 mm long, the free portion 1–3 mm long, the anthers small, attached near the base. Ovary 5–6 mm long, the style 1–3 mm long, relatively stout, strongly curved or coiled, the stigma capitate and terminal, covered by a roughened membrane (this ruptures and folds back during pollination). Fruits legumes, 4–6 cm long (to 10 cm elsewhere), 5–6 mm wide, linear, tapered (often asymmetrically so) at the tip, usually to a short beak, somewhat flattened, dehiscing by 2 valves, these green at maturity, twisting spirally after dehiscence, mostly 4- or 6-seeded. Seeds 4–5 mm long, 3.5–4.0 mm wide, broadly oblong-elliptic to oblong or occasionally nearly circular in outline, the surface dark brown, bluntly few-wrinkled, somewhat shiny. 2n=22 (33 elsewhere). July–September.

Scattered nearly throughout the state, but less common north of the Missouri River (eastern U.S. west to North Dakota, Colorado, and Texas; Canada). Bottomland forests, bottomland prairies, banks of streams, rivers, and spring branches, margins of lakes and sinkhole ponds, fens, and marshes; also old fields, gardens, ditches, fencerows, roadsides, and moist, open disturbed areas.

Apios americana has an extensive rhizome system with numerous tubers, spaced evenly along the rhizome like beads on a string. The tubers are close to the surface and each is capable of sprouting to form a new stem when severed. They were an important food source for the Native Americans and settlers (Moerman, 1998). Tubers could be gathered at any season, but were most commonly collected in the autumn, often from rodent caches. They are sweet and are best when boiled or roasted like potatoes. Eating raw tubers is not recommended because they may contain toxic substances, which seem to be destroyed by cooking. Attempts have been made to domesticate and improve groundnuts (Blackmon and Reynolds, 1986). Although more nutritious than potatoes, groundnuts take two or three years to mature a crop of tubers. The beans can be eaten like peas and contain up to 18% protein by dry weight, although seed set in nature is relatively low. The plants also are eaten by livestock.

The flowers of Apios have an interesting pollination mechanism (Bruneau and Anderson, 1988). The stamens, pistil, and keel are strongly curved and held under pressure by a pocket in the basal fold of the banner. When an insect visitor trips the keel from its notch the stamens and pistil are forced out of the keel, depositing pollen on the insect and exposing the sticky stigma. Each flower can be tripped only one time. Abundant nectar is produced at the base of the pistil, and the flowers produce a sweet scent. Bees are the only insects that have been observed and recorded visiting and tripping the flowers (Bruneau and Anderson, 1994), but Westerkamp and Paul (1993) suggested that the maroon flower color and scent might be attractive to flies. Fruit set is everywhere rather low, and practically nonexistent in the northern part of the range (Seabrook and Dionne, 1976). Two chromosomal races have been found in A. americana. Populations north of Connecticut are more or less sterile triploids, set very few fruits, reproduce primarily by vegetative means, and may have been spread into this area by humans. Populations from the southern part of the range are mostly diploid and set more fruit (Bruneau and Anderson, 1988).

Harvey (2009) reported the presence of extrafloral nectaries in A. americana in populations in Georgia. These nectar-producing glands are located at the base of each flower cluster in the inflorescence and serve to attract ants. Harvey noted a correlation between the absence of ants on some plants and an increase in herbivory of fruits by insects. He suggested that elimination or reduction in density of ants by insecticides and other agricultural practices might have a negative effect on the ability of A. americana to reproduce by seeds.

Botanists in southeastern Missouri should search for the only other North American member of the genus, Price’s groundnut, A. priceana B.L. Rob. This Federally Threatened species is found sporadically in portions of Kentucky, Tennessee, Mississippi, and Alabama (and historically in southern Illinois). It has a solitary tuber and no rhizomes, leaves with longer petioles (6.5–7.5 cm), large (2.0–2.5 cm long), greenish white to purplish green flowers, the banner with a fleshy apical knob, and longer fruits (12–18 cm) (M. Woods, 2005).