Proc. Amer. Acad. Arts 7: 378. 1868.
Herbs, annual. Stems ascending to erect, glabrous proximally, glandular-hairy distally. Leaves cauline, opposite; petiole present or absent; blade not fleshy, not leathery, margins entire, crenate, dentate, or serrate. Inflorescences terminal, racemes; bracts present. Pedicels present, glandular-hairy, sometimes glabrous or glabrate; bracteoles absent. Flowers bisexual; sepals 5, proximally connate, calyx ± bilaterally symmetric, subrotate to campanulate, lobes triangular to lanceolate; corolla white on tube and lobes proximally, blue to violet or lavender on lobes distally, with dark violet spots internally near bases of adaxial and lateral lobes, bilaterally symmetric, weakly bilabiate, subrotate, tube base gibbous adaxially, sometimes obscurely so, not spurred abaxially, throat not densely pilose internally, lobes 5, abaxial 3, middle lobe of abaxial lip not folded lengthwise, not enclosing stamens and style, adaxial 2; stamens 4, medially adnate to corolla, equal or ± didynamous, exserted, filaments glandular; staminode 1, minute; ovary 2-locular, placentation axile; stigma linear. Fruits capsules, dehiscence septicidal and loculicidal. Seeds 2 or 4, dark brown to black, reniform to ovoid, wings absent.
w North America.
Species 2 (2 in the flora).
Studies of vascular anatomy, organography, and gametophyte development (G. F. Schrock and B. F. Palser 1967) and molecular phylogenetic analyses (B. G. Baldwin et al. 2011) support the monophyly of Tonella and its sister-relationship to Collinsia.