PLANTS: Annual or perennial herbs. LEAVES: opposite (in AZ), with inter-petiolar stipules. INFLORESCENCE: cymose or solitary. FLOWERS: het-erostylous (distylous) or homostylous; calyces 4-lobed; corollas 4-lobed, salverform, funnelform, or tubular; stamens 4; stigmas bifid. FRUITS: biloculate capsules, 1/4-3/4 inferior. SEEDS: 2-45 per capsule, crateriform, i.e., subglobose with interior hilar cavity lacking hilar ridge, or slightly to somewhat compressed, with linear or oblong hilar ridge in a saucer- or cup-shaped concavity. NOTES: 20 spp.; U.S., Can., Mex. (for William Houstoun, British physician). REFERENCES: Terrell, Edward E. 1995 Rubiaceae. Houstonia. J. Ariz. - Nev. Acad. Sci. 29(l): 36.