Microsteris is a monotypic genus endemic to western North America and southern South America. Microsteris has often been placed in the genus Phlox, which it is fact sister to. The generic affinities of Microsteris were initially obscure as discussed by Mason (1941). Mason (citing Greene) notes that Microsteris is similar to Phlox in perianth, androecium, and fruit and only differs in the presence of mucilaginous seeds and smaller corolla in Microsteris. However, based on wide infrageneric variability in seed mucilage and corolla size in other Polemoniaceae, Mason argues for subsuming Microsteris into Phlox. He cites the similarity of this genus to annual phloxes. Wherry (1943) disagreed with Mason’s conclusions in describing a new species of Phlox (P. grahami = P. longifolia) but noted that this new species was intermediate between Phlox and Microsteris in having linear leaves and a sparse inflorescence.
Microsteris is often misidentified as Collomia linearis. While the two are superficially similar and prefer similar habitats, C. linearis has alternate leaves and an herbaceous calyx.
Microsteris can be identified based on the following features:
- annual habit
- hyaline calyx membrane
- opposite, entire leaves
- Microsteris gracilis (Doug.) Greene
- M. gracilis (Doug.) Greene subsp. gracilis
- M. gracilis (Doug.) Greene subsp. humilior (Greene) H. Mason
These subspecies are distinguished primarily based on habit: erect in the type subspecies and spreading in subsp. humilior. Because of the variable nature of annual species, these differences are likely phenotypic. More study is needed to determine if there is a genetic basis for these morphological differences.
Mason, H.L. 1941. The taxonomic status of Microsteris Greene. Madrono. 6:122-127.
Wherry, E.T. 1943. Microsteris, Phlox, and an intermediate. Brittonia. 5: 60-63.