13. Nuttallanthus D.A. Sutton

Plants annual, with small taproots, terrestrial. Stems of 2 kinds: a rosette of few to several vegetative stems (sometimes withered or absent at flowering), these 1–6 cm long (sometimes elongating to 10 cm with age), mostly prostrate (sometimes with ascending tips), usually unbranched; and 1 or few fertile stems, these 10–40(–60) cm long, erect or strongly ascending, unbranched or with ascending branches toward the tip; both types glabrous (sometimes sparsely glandular-hairy in the inflorescence). Leaves simple, unlobed and entire, glabrous, with at most a midvein apparent, of 2 kinds: those of the vegetative stems mostly opposite or in whorls of 3, sessile or occasionally short-petiolate, the blades 0.2–1.0 cm long, linear to oblanceolate or spatulate; those of the fertile stems mostly alternate (except sometimes the lowermost), sessile, 0.5–3.0 cm long, 0.5–2.0 mm wide, linear or threadlike. Inflorescences terminal, slender racemes, these sometimes relatively short and dense at the start of flowering but later usually becoming elongated, the flower stalks 1–6(–9) mm long at flowering, not becoming elongated at fruiting, each subtended by a linear to lanceolate bract; bractlets absent. Flowers perfect. Calyces deeply 5-lobed nearly to the base, the lobes slightly unequal in length, linear or narrowly lanceolate to lanceolate or elliptic-lanceolate, sharply pointed at the tip, glabrous. Corollas 8–22 mm long (including the spur), bilabiate, 5-lobed, glabrous or sparsely hairy toward the base of the lower lip, the tube shorter than to about as long as the lobes, pale blue to light purplish blue, sometimes with a paler or white throat (rarely entirely white elsewhere), the tube with a curved, slender spur 2–11 mm long at the base (this positioned between the lower 2 calyx lobes), the throat partially closed by the strongly convex base of the lower lip, the upper lip relatively short, more or less straight, flat, the lobes erect or angled slightly backward, the lower lip with the lobes spreading and somewhat curved. Fertile stamens 4, the filaments of 2 lengths, not exserted, somewhat curved inward toward their tips, the anther sacs spreading; staminodes absent. Style 1, not exserted, the stigma capitate, unlobed. Fruits capsules, oblong-ovoid to broadly oblong-ovoid, glabrous, the 2 locules equal in size, each dehiscent from the tip by 3–5 teeth. Seeds numerous, minute, ovate to rectangular, more or less trapezoidal, or triangular in profile, 4–7-angled in cross-section, the surface black to dark gray, variously ridged and/or with small tubercles, the tubercles sometimes poorly developed and the seed then nearly smooth (except under high magnification). Four species, North America, South America.

Sutton’s (1988) segregation of the New World Nuttallanthus from the otherwise Old World genus Linaria has become accepted by most botanists. The flowers of the two genera differ in the shapes and relative lengths of the lips, as well as their spurs, and the seeds of the two genera are markedly different (prismatic vs. flattened in cross-section). The two Missouri species are closely related and have been treated as components of a single species by some botanists. They are occasionally difficult to distinguish and sometimes have been recorded as growing in mixed populations.