Houstonia is a small genus of flowering plants in the madder family (Rubiaceae), tribe Spermacoceae of subfamily Rubioideae. Roughly twenty species are currently recognized within Houstonia proper, with a further five segregated into the closely related genus Stenaria. The genus is entirely North American, with species occurring from eastern Canada south through the United States and into northern and central Mexico.
The bluets are mostly diminutive plants — perennial or annual herbs with slender single or multiple stems and small opposite leaves joined by inter-petiolar stipules typical of the family. Some species are barely an inch tall in flower: Houstonia rosea, for instance, grows only about one inch high. Inflorescences may be open cymes or reduced to solitary flowers held above the foliage on thread-like pedicels.
The flowers themselves are the easiest field mark. Each has four sepals (often colloquially called "petals") and a salverform corolla — a narrow tube flaring into four spreading lobes — set above an inferior ovary. Colors range from sky blue and lavender to purple, rose, and white, often with a contrasting yellow eye in the throat. Bluets are distinguished from forget-me-nots (Myosotis) by their four-merous, rather than five-merous, perianth. Many species are heterostylous, producing two floral forms (pin and thrum) that promote outcrossing, while others are homostylous. The fruit is a roughly cordate or didymous capsule that splits to release many small seeds.
The genus has a long taxonomic history. It was published by Linnaeus in Species Plantarum (1753) and is most commonly attributed to "L." in horticultural literature, though Gronovius is also cited as a pre-Linnaean author in some checklists. Many species have at times been placed in Hedyotis or Oldenlandia; Terrell's 1996 revision restored the present circumscription that most modern floras follow.
Etymology
The genus name commemorates William Houstoun, an eighteenth-century British physician and botanist who collected plants in the Caribbean and Central America. The honorific was applied to this North American group of small Rubiaceae and has remained the accepted genus name through Linnaeus's Species Plantarum (1753) and into modern treatments.
Distribution
Houstonia is restricted to North America. Its range extends from eastern Canada south through almost the entire eastern and central United States, with additional centers of diversity in the southwestern US and across northern and central Mexico — including the states of Chihuahua, Coahuila, Nuevo León, Tamaulipas, San Luis Potosí, Hidalgo, Veracruz, and Puebla. In New England only three species occur (H. caerulea, H. longifolia, H. purpurea), while the arid southwestern US and northern Mexico hold a separate, more drought-adapted assemblage that includes H. acerosa, H. rubra, H. wrightii, and H. pusilla.
Ecology
Many bluets are heterostylous, bearing pin and thrum floral forms on different individuals to favor outcrossing; other species are homostylous. The salverform corolla with a narrow tube and broad limb is consistent with pollination by small bees, butterflies, and bee flies that probe the throat for nectar.
Taxonomy notes
Houstonia is placed in tribe Spermacoceae of subfamily Rubioideae within the Rubiaceae. The genus was published in Linnaeus's Species Plantarum (1753, p. 105) and is variously cited as "Houstonia L." or "Houstonia Gronov." — GBIF currently treats "Houstonia Gronov." as the accepted record and "Houstonia L." as doubtful. Edward E. Terrell's 1996 revision in Rubiaceae–Hedyotideae is the standard modern treatment; it segregated five species into Stenaria and is the basis for the circumscription used by Flora of North America, Native Plant Trust, and southwestern US floras. Hedyotis is closely related and many Houstonia names have at times been placed there.
History
The genus was formally described by Linnaeus in Species Plantarum (1753), with the name commemorating William Houstoun. The modern circumscription rests on E. E. Terrell's 1996 monograph "Revision of Houstonia (Rubiaceae–Hedyotideae)", supplemented by Church & Taylor (2005) on speciation and hybridization within the genus.
