Anulocaulis is a small genus of flowering plants in the family Nyctaginaceae (the four-o'clock family), order Caryophyllales, commonly known as ringstems. The genus comprises five species of thickly-rooted perennial wildflowers native to North America, particularly the arid and semi-arid regions of the southwestern United States and northern Mexico.
The defining characteristic of ringstems — and the source of both their common and scientific names — is the presence of sticky, glutinous brown bands encircling the stem at each internode. These resinous rings are conspicuous and consistent across the genus, distinguishing them from related genera at a glance. Plants bear tubular flowers clustered at the tops of their stems, a floral form typical of the Nyctaginaceae family.
The species are sometimes treated as members of the closely related genus Boerhavia, also in Nyctaginaceae, reflecting ongoing taxonomic discussion about the boundaries of these genera. Recognized species include Anulocaulis annulatus (valley ringstem), A. eriosolenus (Big Bend ringstem), A. gypsogenus (gypsum ringstem), A. leiosolenus (southwestern ringstem), and A. reflexus (Texas ringstem).
Etymology
The genus name Anulocaulis derives from Latin anulus (ring) and caulis (stem), referring to the distinctive glutinous brown rings encircling the stem internodes. The common name "ringstems" is a direct English translation of the same anatomical feature.
Distribution
Ringstems are native to North America, with species concentrated in the arid and semi-arid southwestern United States. Individual species have more restricted ranges: A. eriosolenus is associated with the Big Bend region of Texas, A. gypsogenus with gypsum soils, and A. leiosolenus with the broader southwestern region.
Taxonomy Notes
Anulocaulis is placed in the family Nyctaginaceae (four-o'clock family), order Caryophyllales. The species are sometimes treated as members of genus Boerhavia, reflecting their close relationship within Nyctaginaceae. GBIF accepts Anulocaulis as a distinct genus with ACCEPTED status.
