Amphicarpaea bracteata (L.) Fernald (hog peanut)

A. bracteata var. comosa (L.) Fernald

A. monoica Nutt.

Glycine bracteata L.

Pl. 388 a–c; Map 1710

Plants annual, with a shallow taproot, frequently climbing on other vegetation. Stems (0.5–)1.0–2.0 m long, twining, loosely spreading or climbing, unarmed, glabrous to densely pubescent with pale yellow to brownish yellow downward-pointed to spreading hairs. Leaves alternate, pinnately trifoliate, the petiole 2–5 cm long, hairy. Stipules 3–5 mm long, membranous or sometimes papery with age, persistent, broadly lanceolate to ovate, angled or tapered to a sharply pointed tip, the venation prominent; stipels 1–2 mm long, inconspicuous, often shed early. Leaflets (1.5–)3.0–7.0(–10.0) cm long, (1–)2–6(–7) cm wide, ovate to rhombic, broadly angled or rounded at the base, angled to a bluntly or less commonly sharply pointed tip, the margins entire, the surfaces sparsely to moderately hairy, more or less pinnately veined (with a pair of well-developed lateral veins produced at the base, especially on lateral leaflets). Terminal leaflet with the stalk 5–20 mm long, symmetric at the base; lateral leaflets with the stalk 1–2 mm long, usually asymmetric at the base. Inflorescences of 2 or 3 kinds, with 3 types of flowers and fruits (aerial and open-flowering, aerial and cleistogamous, and subterranean and cleistogamous). Open flowers in short dense racemes from the axils of upper leaves, sometimes appearing clustered, the inflorescence stalk 1–2 cm long, hairy, the bracts 2–3 mm long, 2–3 mm wide, fan-shaped, strongly veined, broadly angled at the tip, the flower stalks 1–2 mm long. Cleistogamous flowers in short dense racemes from the median stem nodes or solitary or few in open racemelike inflorescences from the lower stem nodes (or sometimes from subterranean nodes of the rootstock), the stalk of such inflorescences 10–30 cm or more long, stoloniferous in appearance, the flowers developing at the soil surface or underground, subtended by 2 small bracts. Calyces of open flowers 5-lobed, the lobes much shorter than the tube, similar in length but the upper 2 lobes fused nearly to the tip, the tube 4–6 mm long, somewhat asymmetric (pouched) at base, sparsely to densely hairy, the lobes 1–3 mm long, triangular, tapered to a sharply pointed tip, usually sparsely hairy; calyces of cleistogamous flowers reduced, the lobes about as long as the tube, 1–2 mm long, not fully opening at maturity. Corollas papilionaceous (absent or highly reduced in cleistogamous flowers), glabrous, white, tinged with pale pink or lavender, the petals with a stalklike base; the banner 11–14 mm long, 5–6 mm wide, obovate, abruptly bent outward above the stalklike basal portion; the wings 10–13 mm long, 2–3 mm wide, oblong, the blade portion with a pair of small auricles at the base; the keel 10–12 mm long, 2.5–3.0 mm wide, oblanceolate and straight in outline, boat-shaped. Stamens 10 (only 2–5 short stamens in cleistogamous flowers), 9 of the filaments fused and 1 free (often all free in cleistogamous flowers), the fused portion 8–10 mm long, the free portion 1–2 mm long, curved, the anthers small, attached near the midpoint, all similar in size. Ovary 4–5 mm long, linear-elliptic, short-stalked, hairy, the style 6–7 mm long (short and strongly recurved in cleistogamous flowers), slender, curved, glabrous, the stigma small and terminal. Fruits of 3 kinds: those maturing from open flowers 20–40 mm long, 6–8 mm wide, short-stalked above the persistent calyx, pointed at both ends, strongly flattened, glabrous or sparsely to moderately hairy (sometimes only along the margins), papery or somewhat leathery in texture, with (1)2–4 seeds, explosively dehiscent, the valves coiling after dehiscence; those of the aerial cleistogamous flowers similar but shorter and 1-seeded; those of the subterranean cleistogamous flowers 5–13 mm long, ovoid to ellipsoid or nearly globose, not flattened, spongy in texture, indehiscent. Seeds 3–5 mm long, 2–3 mm wide, kidney-shaped, shallowly notched, the surfaces smooth, glabrous, reddish brown or mottled lighter and darker brown. 2n=20. August–September.

Scattered nearly throughout the state (eastern U.S. west to Montana and Texas; Canada; disjunct in southern Mexico). Banks of streams and rivers, fens, bottomland forests mesic upland forests, and bases of bluffs; also roadsides and shaded disturbed areas.

Amphicarpaea bracteata is variable in leaf size, stem robustness, and degree of hairiness, which has given rise to several named variants. Steyermark (1963) accepted two of these for Missouri. Plants growing in sunnier locations tend to be robust, with large leaves and dense, tawny hairs, and are sometimes recognized as var. comosa, whereas those in either sunny or shaded sites with smaller leaves and sparse pubescence have been called var. bracteata. These varieties were not accepted by B. L. Turner and Fearing (1964) because they often occur side by side, freely intergrade, and have no geographical correlation. However, preliminary comparative data from an allozyme study (Parker, 1996) indicated a genetic component to this morphological variation and morphometric analysis coupled with greenhouse and garden studies (Callahan, 1997) further suggested that the taxonomic distinctness of the two varieties should be studied in more detail. For the present, no attempt to distinguish varieties has been made during the present research.

Hog peanut has an interesting and somewhat complicated reproductive system. Individuals potentially produce more or less three kinds of inflorescences, although not always on the same plant. Open flowers with papilionaceous corollas are produced in short racemes from the upper nodes. These are pollinated primarily by bumblebees (Schnee and Waller, 1986). Reduced cleistogamous flowers are sometimes produced in short aerial racemes from the median nodes of some plants (a sort of intergradation into the next type). Both of these flower types produce flat, elongate, dehiscent legumes with two to four seeds. The seeds are dispersed ballistically when the valves suddenly dehisce and coil. Because of the height advantage, the seeds from open (chasmogamous) flowers are dispersed the greatest distance, possibly promoting cross fertilization (Trapp, 1988). Cleistogamous flowers also are sometimes produced from long thin runners that originate from the lowermost aboveground nodes of the plant. Thin shoots originating from these runners grow along the ground and into the soil to produce subterranean, cleistogamous flowers. Entirely subterranean cleistogamous flowers also can be produced from the underground nodes of the root system (Schnee and Waller, 1986). The fruits that develop from the subterranean flowers are large and fleshy with a relatively thin outer wall and a single fleshy seed. According to Steyermark (1963), these subterranean fruits are an important food source for mice and voles and were also gathered by Native Americans. He noted that when cooked, seasoned, and buttered they have a flavor similar to that of green beans. Production of the different flower and fruit types is correlated with the size of the plant and the relative amount of sun and shade.

Along some forest openings and margins large dense colonies of this species can develop in response to disturbance that appear to remain vegetative throughout the growing season. These thickets might be mistaken for the nonnative perennial Pueraria (kudzu), but Amphicarpaea is an annual (note the lack of dried stems from the previous season) that is much less noticeably hairy and has smaller leaves. Its population density usually declines within a few seasons. Steyermark (1963) also noted that vegetative specimens of Amphicarpaea might be confused with Strophostyles umbellata, but aside from the differences in flower and inflorescence morphology, the leaves of that species have a much shorter stalk at the base of the terminal leaflet and the stipules have fewer main veins (1–7 vs. 10–12).