Habit and leaf form. Shrubs, or herbs; without essential oils. Autotrophic. Annual to perennial. Leaves basal and cauline. Plants usually with a basal concentration of leaves, or with neither basal nor terminal concentrations of leaves; to 1.5 m high. Helophytic, or mesophytic, or xerophytic. Leaves medium-sized; alternate; petiolate (at base), or subsessile to sessile (above); non-sheathing; not gland-dotted; simple; epulvinate. Leaf blades dorsiventral, or isobilateral; entire; flat; linear to lanceolate; linear, or ovate, or obovate, or elliptic; cross-venulate; auriculate at the base, or cordate, or cuneate at the base. Mature leaf blades adaxially pubescent; abaxially pubescent. Leaves without stipules. Leaf blade margins entire. Leaves without a persistent basal meristem. Leaf anatomy. Hairs present (tuberculate at base). Urticating hairs absent. Stem anatomy. 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 cymes. Inflorescences simple, or compound. The terminal inflorescence unit cymose (coiled at first). Inflorescences terminal; of 1-sided cymes or a panicle of cymes, with flowers arranged in 2 usually dense rows; not pseudanthial. Flowers sessile; bracteate; bracteolate; small to medium-sized; usually somewhat irregular to very irregular, or regular; when irregular, somewhat zygomorphic; 5 merous; cyclic; tetracyclic. Free hypanthium absent. Hypogynous disk present, or absent. Perianth with distinct calyx and corolla; 10; 2 -whorled; isomerous. Calyx present; 5; 1 -whorled; gamosepalous; lobed. Calyx lobes about the same length as the tube, or markedly longer than the tube. Calyx imbricate, or open in bud, or valvate. Degree of gamosepaly, maximum length joined/total calyx length 0.2–0.5. Calyx regular, or unequal but not bilabiate; neither appendaged nor spurred; persistent; accrescent. Calyx lobes ovate, or linear. Corolla present; 5; 1 -whorled; appendiculate (often, with a circle of 5 or 10 nectary scales or tufts of hairs or a membrane at the base), or not appendiculate; gamopetalous; lobed. Corolla lobes markedly shorter than the tube, or about the same length as the tube. Corolla imbricate, or contorted; funnel-shaped, or tubular; more or less regular, or bilabiate; hairy abaxially; hairy adaxially (around nectary scales), or glabrous adaxially; white, or yellow, or pink, or purple, or blue. Corolla lobes oblong (to square). Fertile stamens present. Androecial members definite in number. Androecium 5. Androecial members unbranched; adnate (to the corolla); all equal, or markedly unequal; free of one another; 1 -whorled. Androecium exclusively of fertile stamens. Stamens 5. Staminal insertion midway down the corolla tube, or in the throat of the corolla tube. Stamens inserted at markedly different levels, or all inserted at the same level; becoming exserted, or becoming exserted and remaining included (when the lowest 2 or 3 remaining included); isomerous with the perianth; oppositisepalous; filantherous (the filament much longer than the anther). Filaments filiform. Anthers dorsifixed to basifixed; dehiscing via longitudinal slits; introrse; tetrasporangiate; unappendaged. Gynoecium 2 carpelled. The pistil 4 celled. Carpels reduced in number relative to the perianth. Gynoecium syncarpous; synstylovarious to eu-syncarpous; superior. Ovary plurilocular (4-lobed); 2 locular (‘really’, but rarely ostensibly so), or 4 locular (ostensibly, via false septa). Locules secondarily divided by ‘false septa’. Gynoecium median; stylate. Styles 1; simple; from a depression at the top of the ovary; ‘gynobasic’; usually becoming exserted, or not becoming exserted. Stigmas 1; 1–2 - lobed; capitate. Placentation basal. Ovules differentiated; 2 per locule (i.e. per true locule), or 1 per locule (per cell, the gynoecium separating into one-ovuled portions); horizontal to ascending; epitropous; non-arillate; anatropous, or hemianatropous.
Fruit and seed features. Fruit aerial; non-fleshy; not hairy; not spinose; a schizocarp. Mericarps 4; comprising nutlets. Seeds endospermic, or non-endospermic. Cotyledons 2. Embryo straight, or curved.
Special features. Corolla tube straight, or curved.
Etymology. From the Greek for "viper", because the plant was supposed to cure a viper's bite.