Common name. Evening Primroses. Family Onagraceae.
Habit and leaf form. Herbs; bearing essential oils, or without essential oils. Annual, or biennial, or perennial. Leaves basal, or cauline. Plants with a basal concentration of leaves, or with neither basal nor terminal concentrations of leaves. Mesophytic. Leaves alternate; spiral; petiolate, or subsessile; non-sheathing; simple; epulvinate. Leaf blades dissected, or entire; elliptic, or oblong, or ovate, or obovate, or linear; when dissected, pinnatifid; pinnately veined; cross-venulate. Leaves without stipules. Leaf blade margins entire, or dentate. Leaves without a persistent basal meristem. Leaf anatomy. Hairs present, or absent; glandular hairs absent, or present.
Reproductive type, pollination. Fertile flowers hermaphrodite. Unisexual flowers absent. Plants hermaphrodite. Anemophilous, or entomophilous.
Inflorescence and flower features. Flowers solitary, or aggregated in ‘inflorescences’; in racemes, or in spikes. Inflorescences terminal, or axillary; inflorescence spicate or racemose or flowers solitary in axils of upper leaves. Flowers small to large; regular; 4 merous; cyclic; tricyclic, or tetracyclic, or pentacyclic. Free hypanthium present; extending beyond ovary; usually much elongated, slender, deciduous above the ovary after anthesis. Perianth with distinct calyx and corolla; 8; 2 -whorled; isomerous. Calyx present; 4; 1 -whorled; partially gamosepalous (connate in pairs at tips), or gamosepalous; blunt-lobed; lobes valvate; not persistent. Corolla present; 4; 1 -whorled; polypetalous; imbricate, or contorted; white, or yellow, or orange, or red, or pink, or purple. Petals clawed, or sessile. Androecium 4–8. Androecial members adnate, or free of the perianth; all equal, or markedly unequal (inner shorter than outer whorl); free of one another; 2 -whorled, or 1 -whorled. Androecium exclusively of fertile stamens. Stamens (4–)8 (in 2 whorls); all more or less similar in shape; reduced in number relative to the adjacent perianth, or isomerous with the perianth; alternisepalous and oppositisepalous. Anthers dorsifixed; versatile; dehiscing via longitudinal slits; introrse; tetrasporangiate. Pollen shed in aggregates, or shed as single grains; with viscin strands, or without viscin strands; if in aggregates, in tetrads. Gynoecium 4 carpelled. The pistil 4 celled. Carpels isomerous with the perianth. Gynoecium syncarpous; synstylovarious, or eu-syncarpous; inferior. Ovary plurilocular; 4 locular. Epigynous disk present. Gynoecium stylate. Styles 1; apical. Stigmas 1–4; usually 4 - lobed; sometimes capitate. Placentation axile, or parietal. Ovules 1–50 per locule (to ‘many’); pendulous, or ascending; non-arillate; anatropous.
Fruit and seed features. Fruit fleshy, or non-fleshy; dehiscent; a capsule. Capsules loculicidal and valvular (splitting into 4 valves). Fruit 20–100 seeded (many). Seeds non-endospermic; not conspicuously hairy. Cotyledons 2. Embryo straight.
Etymology. From the Greek oinothera; name of a plant which was used to flavour wine; and the Latin oenothera; name of a plant which when its juice was added to wine, induced sleep.