Habit and leaf form. Glutinous-villous herbs. Annual. Leaves alternate, or opposite; petiolate; non-sheathing; simple. Leaf blades dorsiventral; cross-venulate. Leaves without stipules. Leaf anatomy. Hairs present; glandular hairs present. Stem anatomy. Secondary thickening developing from a conventional cambial ring.
Reproductive type, pollination. Fertile flowers hermaphrodite. Unisexual flowers absent. Plants hermaphrodite. Pollination mechanism conspicuously specialized (the two irritable stigmatic flaps close together when a pollinator touches them, preventing self-pollination by the visitor).
Inflorescence and flower features. Flowers aggregated in ‘inflorescences’; in racemes. The terminal inflorescence unit racemose. Inflorescences terminal. Flowers pedicellate; very irregular; zygomorphic. The floral asymmetry involving the perianth and involving the androecium. Flowers 5 merous; tetracyclic. Free hypanthium absent. Hypogynous disk present; intrastaminal. Perianth with distinct calyx and corolla; 10; 2 -whorled; isomerous. Calyx present; 5; 1 -whorled; gamosepalous; lobed; blunt-lobed; unequal but not bilabiate (cup-shaped, the upper side in P. louisianica up to twice as long as the lower side, and slit on the lower side to the base); with the median member posterior. Corolla present; 5; 1 -whorled; gamopetalous; imbricate; unequal but not bilabiate to bilabiate (the tube of P. louisianica topped by a widely spreading spotted limb divided into 2 upper lobes and 3 much longer lower lobes); cream to purple, or red to purple. Androecial members definite in number. Androecium 5. Androecial sequence not determinable. Androecial members adnate (epipetalous); markedly unequal; free of one another; 1 -whorled. Androecium including staminodes. Staminodes 1; representing the posterior median member (vestigial). Stamens 4; didynamous; reduced in number relative to the adjacent perianth; oppositisepalous; all alternating with the corolla members. Anthers connivent; dehiscing via longitudinal slits; tetrasporangiate. Gynoecium 2 carpelled. The pistil 1 celled. Carpels reduced in number relative to the perianth. Gynoecium syncarpous; synstylovarious to eu-syncarpous; superior. Ovary unilocular; 1 locular. Locules secondarily divided by ‘false septa’, or without ‘false septa’. Gynoecium median; stylate. Styles 1; attenuate from the ovary; apical. Stigmas 2. Placentation parietal. Ovules in the single cavity 3–100 (‘few to many’); anatropous.
Fruit and seed features. Fruit fleshy to non-fleshy (the fleshy exocarp caducous leaving the woody endocarp); dehiscent; a capsule. Capsules loculicidal. Dispersal by animals, associated with usually hooked or curved spurs which develop from the tip of the midrib of each carpel. Seeds scantily endospermic. Cotyledons 2. Embryo straight. Seedling. Germination phanerocotylar.
Additional characters Fruit rostrate (the horns as long as or longer than the main body).