Arachis L.

Scientific Description

Family Papilionaceae. Aeschynomeneae.

Habit and leaf form. Low herbs; resinous, or not resinous. Perennial; plants with neither basal nor terminal concentrations of leaves; to 0.2–0.5 m high. Mesophytic, or xerophytic. Leaves small to large; alternate; spiral; ‘herbaceous’, or leathery; not imbricate; petiolate. Petioles wingless. Leaves not gland-dotted; compound; pulvinate; pinnate; paripinnate. Leaflets 4; not stipellate; pulvinate, or epulvinate; flat, or folded; without lateral lobes. Leaf blades dorsiventral. Leaves with stipules. Stipules intrapetiolar. Stipules more or less peltate, or not peltate. Stipules adnate to the petiole (to its lower part); concrescent (sheathing the stem, the leaf born in their sinus); persistent. Leaf blade margins entire. Leaves without a persistent basal meristem. Stem anatomy. Nodes tri-lacunar, or penta-lacunar. Secondary thickening developing from a conventional cambial ring.

Reproductive type, pollination. Fertile flowers hermaphrodite. Unisexual flowers absent. Plants hermaphrodite. Entomophilous.

Inflorescence and flower features. Flowers aggregated in ‘inflorescences’; in spikes (these sessile). Inflorescences axillary; dense, axillary sessile spikes. Flowers pedicellate (the pedicel subsequently lengthening to deposit the fruit below ground); bracteate (1–3 foliolate and stipule-like primary, and 2– or 3–lobed secondary); (1 or 2) bracteolate (the bracteoles translucent); small to medium-sized; very irregular; zygomorphic. The floral asymmetry involving the perianth and involving the androecium. Flowers papilionaceous (imbricate-descending, with the posterior petal outside and forming the ‘standard’); basically 5 merous. Floral receptacle with neither androphore nor gynophore. Free hypanthium present (longer than broad). Perianth with distinct calyx and corolla; 10; 2 -whorled; isomerous. Calyx present; 5; 1 -whorled; partially gamosepalous, or gamosepalous (the four upper lobes connate for about half their length, the anterior one free); (five) lobed (membranous); imbricate, or valvate; exceeding the corolla; bilabiate; persistent; non-accrescent; with the median member anterior. Corolla present; 5; 1 -whorled; appendiculate (the wings and keel auriculate and spurred). Standard not appendaged. Corolla partially gamopetalous. 2 of the petals joined (the two ventral petals connivent to form the ‘keel’). The joined petals of the papilionate corolla anterior. The wings of the corolla free from the keel (oblong); laterally spurred. Standard ‘normally’ developed; emarginate; not sericeous. Keel conspicuously exceeded by the wings to conspicuously exceeding the wings; long-acuminate and beaked (and incurved); neither coiled nor spiralled; not bent and beaked. Corolla imbricate (descending); plain, or with contrasting markings; yellow; deciduous. Petals shortly clawed. Androecial members definite in number. Androecium 9, or 10. Androecial sequence determinable, or not determinable. Androecial members free of the perianth; markedly unequal (the filaments alternately long and short); coherent; 1 - adelphous (the vexillary member sometimes missing), or 2 - adelphous (then 9+1). The staminal tube free from the keel petals. Androecial members 1 -whorled. Androecium exclusively of fertile stamens. Stamens 9, or 10; all more or less similar in shape; diplostemonous; both opposite and alternating with the corolla members. Anthers separate from one another, or connivent; dimorphic (alternately longer and sub-basifixed, shorter and versatile); dorsifixed, or dorsifixed and basifixed; versatile, or versatile and non-versatile; dehiscing via pores, or dehiscing via longitudinal slits; latrorse, or introrse; tetrasporangiate. Pollen shed as single grains. Gynoecium 1 carpelled. The pistil 1 celled. Carpels reduced in number relative to the perianth. Gynoecium monomerous; of one carpel; superior. Carpel stylate; apically stigmatic. Style curved. Style glabrous. Stigmatic tissue terminal. Carpel 2–3 ovuled. Placentation marginal (along the ventral suture). Gynoecium median (the placenta posterior, on the ventral suture). Ovary sessile. Ovules pendulous to ascending; biseriate; arillate, or non-arillate; anatropous, or campylotropous to amphitropous, or hemianatropous.

Fruit and seed features. Fruit subterranean; 3–7 mm long; sessile; non-fleshy; hairy to not hairy; not spinose. The fruiting carpel indehiscent; a legume. Pods globose to much elongated; not triangular; straight; beaked (via the persistent torus elongating, reflexing and becoming rigid-pointed, forming a hook); terete; irregularly constricted to not constricted between the seeds; not transversely septate; wingless. Loment segments rounded or cuboid. Fruit 1 celled; elastically dehiscent, or passively dehiscent; 1–3 seeded. Seeds ovate to oblong; endospermic, or non-endospermic; not mucous; not compressed; small to medium sized; non-arillate. Cotyledons 2; accumbent. Embryo curved, or bent. Testa non-operculate. Micropyle zigzag, or not zigzag. Seedling. Germination phanerocotylar.

Physiology, biochemistry. Nitrogen-fixing root nodules present. Aluminium accumulation not found. Photosynthetic pathway: C₃.

Geography, cytology, number of species. Adventive. 2n=20, 40. A genus of 22 species; 1 species in Western Australia (cultivated commercial peanut).