Herbs, annual. Stems erect, ascending, or sprawling, filiform, twining branches absent, glabrous or hairy. Leaves cauline, opposite proximally, alternate distally; petiole present proximally, absent distally; blade not fleshy, not leathery, margins entire. Inflorescences axillary, flowers solitary; bracts absent. Pedicels present, elongate, twining; bracteoles absent. Flowers bisexual, cleistogamous and chasmogamous; sepals 5, basally connate, calyx slightly bilaterally symmetric, cupulate, lobes lanceolate; corolla purple or yellow, bilaterally symmetric, bilabiate and personate, tubular, tube base gibbous abaxially, not spurred, lobes 5, abaxial 3, adaxial 2; stamens 4, basally adnate to corolla, didynamous, filaments glabrous or sparsely glandular-hairy distally, pollen sacs 2 per filament; staminode 1, minute; ovary 2-locular, placentation axile; stigma punctiform or subcapitate. Fruits capsules, locules equal, dehiscence poricidal. Seeds 20–100, black, ovoid to subglobular, wings absent. x = 15.
sw United States, nw Mexico.
Species 2 (2 in the flora).
Neogaerrhinum is closely related to the Sairocarpus species that have filiform, prehensile branches; in Neogaerrhinum, pedicels are elongate and twining, and fruits are symmetric and usually drooping.
In both species of Neogaerrhinum, the cleistogamous flowers have white corollas and are smaller than the chasmogamous flowers. Only the chasmogamous flowers are used in the key and descriptions below.