1. Amsonia Walter (blue star)

Plants perennial herbs, often somewhat woody at the base. Stems usually several, erect or ascending, unbranched or few-branched toward the tip, often becoming more branched after flowering, usually hollow at maturity. Leaves alternate, sometimes appearing subopposite or nearly whorled at some nodes, sessile or short-petiolate, those of the lowermost nodes usually reduced and scalelike. Leaf blades linear to elliptic or ovate, narrowed or tapered to a sharply pointed tip, the margins sometimes curled under, sometimes minutely hairy along the margins. Stipules absent. Inflorescences terminal or occasionally subterminal, branched loose clusters of few to many flowers. Calyx lobes triangular to linear-triangular, glabrous. Corollas trumpet-shaped, glabrous or hairy on the outer surface, lacking appendages, light blue, the throat slightly swollen and hairy within, the lobes abruptly spreading, overlapping, lanceolate. Stamens attached near the top of the corolla tube, the anthers incurved but free from and positioned above the stigma, arrowhead-shaped, with a pair of prominent triangular basal lobes. Nectar glands absent. Style elongate, the stigma capitate, encircled by a small, cuplike wing above the midpoint. Fruits slender, straight or curved, erect to pendulous, glabrous. Seeds numerous, cylindrical with broadly rounded to more commonly truncate ends, lacking a tuft of hairs. Five to 20 species, southern U.S. and adjacent Mexico, eastern Asia.

Amsonia is one of the few wholly temperate genera in the family. Within the Apocynaceae, the genus is distinguished by the occurrence of free, unappendaged anthers, glabrous seeds, sinistrorse (left-overlapping) corolla lobes, and alternate leaves. It may be related to Vinca and Catharanthus (Endress and Bruyns, 2000), or perhaps to Haplophyton, a small genus of 3 species in the southwestern United States and Mexico that also has alternate leaves (Woodson, 1928; Rosatti, 1989). Within the genus Amsonia are two major groups, one in the southeastern United States and eastern Asia (where a single species occurs), and the other in the southwestern United States and northern Mexico. Within these groups the species are highly variable and circumscription of species is difficult. In the southeastern United States, there are 2–8 taxa, but it is not clear whether or at what taxonomic level some of them should be recognized.

Several species of Amsonia, including the three species in Missouri, are cultivated as ornamentals in gardens.