Arachis hypogaea L. (peanut, groundnut)

Pl. 389 e; Map 1712

Plants annual, with stout taproots. Stems 10–35 cm long, erect to loosely ascending or trailing with runners, sometimes branched at the lower nodes, unarmed, sparsely to densely pubescent with fine, yellow to tan, mostly spreading hairs 1–2 mm long. Leaves alternate, 8–12 cm long, even-pinnately compound, the petiole 3–6 cm long, the rachis 4–9 cm long. Stipules 20–30 mm long, 2–3 mm wide, narrowly lanceolate, fused to the petiole toward the base, long-tapered at the tip, persistent and conspicuous, the venation prominent; stipels absent. Leaflets 4 in 2 pairs, 3–5 cm long, 2–3 cm wide, obovate, bluntly angled at the base, rounded to bluntly pointed at the tip, sometimes shallowly notched, the margins entire, the upper surface glabrous, the undersurface glabrous to sparsely hairy, pinnately veined. Inflorescences axillary, solitary or more commonly few-flowered spikelike clusters, but these often appearing as solitary flowers because only 1 flower develops at a time, each with a pair of conspicuous basal bracts, these 10–12 mm long, lanceolate, sheathing, hairy toward the base; bractlets absent. Flowers sessile (becoming long-stalked as the fruits develop), but appearing stalked because of a slender hypanthium below the perianth, this 2–3 cm long, hairy. Calyces deeply 2-lipped, the upper lip 5–6 mm long, with 4 short toothlike lobes, the lower lip 5–7 mm long, unlobed. Corollas papilionaceous, orange or yellow, the banner 10–12 mm long, 8–9 mm wide, the expanded portion broadly ovate, rounded and usually shallowly notched at the tip, the wings 7–8 mm long, 3–4 mm wide, obovate, the keel 5–6 mm long, 1.0–1.5 mm wide, boat-shaped, tapered at the tip, sharply curved upward. Stamens 10 but usually 2 of these nonfunctional and with small capitate tips, the filaments all fused, the fused portion attached at the tip of the hypanthium, 3–4 mm long, the free portion 3–4 mm long (shorter in nonfunctional stamens), the anthers of fertile stamens 0.7–1.0 mm long, oblong, attached near the midpoint. Ovary 3–4 mm long, more or less ovoid, the style 23–35 mm long, hairy, the stigma minute, terminal. Fruits developing underground following development of a long stalk at the ovary base after flowering, this burying the developing fruit, loments, 2–4 cm long, 1.0–1.5 cm wide, oblong, circular in cross-section, constricted between the seeds, the surface light brown, with a coarse, fibrous network, (1)2(3)-seeded, indehiscent, not breaking up into segments. Seeds 8–12 mm long, 4–6 mm wide, ovoid with a thin, papery, reddish brown seed coat. 2n=40. July–September.

Introduced, uncommon in the Mississippi Lowlands Division (cultigen thought to have originated in Brazil, cultivated nearly worldwide; introduced sporadically in the southern U.S.). Roadsides and open disturbed areas.

Arachis hypogaea is distinguished from most other legumes (and most other plants) by the development of underground fruits, known as geocarpy. The flowers are self-fertile and sometimes cleistogamous. After fertilization occurs, the base of the ovary elongates forming a peg that bears the ovary underground. This requires hot summers and well-drained, friable, loamy or sandy soils. The seeds are very nutritious, with a protein content of 25–30% and an oil content of 45–50% (Sauer, 1993). The root nodules produce so much nitrogen that fertilization with inorganic nitrogen is seldom necessary. The cultivated peanut is unknown in the wild. It is an allotetraploid formed by the crossing of two wild diploid species, with subsequent chromosome doubling (Gregory et al., 1980; Smartt, 1990). Brazil was the primary center of peanut evolution and domestication. Peanuts were cultivated in South America as early as 2000 B.C., and later brought to the West Indies by the Arawaks. The Spanish introduced peanuts to Mexico and Africa, where they are an important crop with a large number of varieties. Peanuts are a major crop in Texas, Oklahoma, and several southeastern states, but are rarely planted in Missouri. The great American botanist, chemist, and educator, George Washington Carver (1864–1943), who was born in southwestern Missouri, was instrumental in promoting the uses of the peanut (Carver, 1916) and developed more than 300 commercial uses for the plant, its fruits, and its oil, revolutionizing agriculture in the southeastern United States as a result.