This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol3", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Much-branched annual herbs, with entire leaves, all but the floral ones opposite, the small flowers solitary or in pairs in the upper axils. Calyx mostly campanulate, 5-cleft, scarious between the lobes. Corolla salverform, with a slender tube and a 5-lobed limb. Stamens short. Ovary 3-celled. Capsule at length distending and rupturing the calyx-tube. Seeds few and large, mucilaginous when wetted, but not emitting spiral tubes. [Greek, small Steris.]
1863. M. micrantha Greene, Pittonia 3: 303. 1898.
Annual, pubescent, at length corymbosely much branched, 2'-6' high, the branches ascending. Lower and basal leaves oblong to spatulate, obtuse, commonly opposite and nearly sessile, the upper linear or lanceolate, sessile, 1/2'-1' long, 1"-2" wide, opposite or alternate, entire; cymes 1-5-flowered; calyx-lobes linear-subulate, usually longer than the tube; corolla about 4" long, the tube yellowish, narrow, equalling or slightly longer than the calyx, the lobes purple or violet, short; ovules 2 or 3 in each cavity; capsule oblong, obtuse, as long as or longer than the calyx-tube.
In dry or moist soil, western Nebraska to Wyoming, Colorado, New Mexico, Arizona and California. In our first edition referred to the northwestern M. gracilis (Dougl.) Greene. April-Aug.