LAMIACEAE

LAMIACEAE View in CoL View at ENA

TRICHOSTEMA : Trichostema coeruleum , a new name for the species currently known as Trichostema brachiatum or Isanthus brachiatus , and its placement in a new section, Isanthus

Primary authors: R. Kevan Schoonover McClelland and Alan S.Weakley

A detailed review of the published names in the genus Trichostema ( Lamiaceae ) uncovered inconsistencies with applications of species names. Linnaeus (1753) named two species of Trichostema , T. dichotomum L. and T. brachiatum L. These two species have been assumed to represent the two most widely distributed species of the genus in eastern North America for the past 220 years, although Trichostema brachiatum has sometimes been placed in the genus Isanthus Michaux (1803) , based on its different flower morphology (as I. brachiatus (L.) Britton, Sterns, & Poggenb.; Poggenburg et al. 1888). The lectotype of Trichostema brachiatum was designated in 2001 ( Jarvis et al. 2001) and has since been determined to be an illustration of Trichostema dichotomum ( McClelland 2022) . Based on the lectotype, it is clear that Trichostema brachiatum / Isanthus brachiatus is conspecific with Trichostema dichotomum . Given that T. dichotomum and T. brachiatum were published simultaneously and their synonymy has not been established before, we choose to place T. brachiatum in synonymy under T. dichotomum given that T. dichotomum is the generitype species. Furthermore, because of the new synonymy, a new combination is needed for the plant originally published as Isanthus coeruleus Michaux (1803) in the genus Trichostema . Appropriate synonymy for T. dichotomum , T. brachiatum , and I. coeruleus Michaux (1803) is provided in addition to a new combination for I. coeruleus in the genus Trichostema . Furthermore, a monotypic section is erected to hold this species, based on our current understanding of the phylogeny of Trichostema .

Background

Trichostema Gronov. ( Linnaeus 1753) View in CoL is a genus in the mint family ( Lamiaceae View in CoL , Ajugoideae) endemic to North America. Its two most widespread eastern species, T. dichotomum View in CoL L. and T. brachiatum View in CoL L., were named in Linnaeus’ first edition of Species Plantarum (volume 2, 1753). The only character separating these two species in Species Plantarum is the staminal characters staminibus longissimus exsertis for T. dichotomum View in CoL and staminibus brevibus inclusis for T. brachiatum View in CoL . Linnaeus’ later works ( Linnaeus 1759, 1763, 1767, 1770, 1774) and encyclopedic and check-list style works modeled after Linnaeus ( Miller 1768; Forster 1771; Houttuyn 1778; Reichard 1780; Christmann 1781; Gmelin 1792; Persoon 1797; Willdenow 1801) consistently included and described both species. However, three floras compiled using specimens as evidence of presence or absence ( Gronovius 1762; Walter 1788; Michaux 1803) included only T. dichotomum View in CoL , not T. brachiatum View in CoL . This suggests that despite the extensive travels and collecting by these authors or their contributors, they were unable to find plants ascribable T. brachiatum View in CoL in the eastern Unites States.

The identity of (what was meant by) T. brachiatum View in CoL changed over the next 20 years following Michaux’s (1803) flora and his publication of the new genus Isanthus View in CoL and its one species I. coeruleus Michaux. Initially View in CoL , T. dichotomum View in CoL , T. brachiatum View in CoL , and I. coeruleus View in CoL were all considered separate species ( Lamarck & Poiret 1805; Lamarck & Poiret 1808; Pursh 1814).However, Muhlenberg (1813) and Nuttall (1818) treated T.brachiatum View in CoL as a synonym under I. coeruleus View in CoL . Subsequent authors continued to treat T. dichotomum View in CoL and I. coeruleus View in CoL as separate species without including T. brachiatum View in CoL in any synonymy ( Sprengel 1825; Torrey 1826; Beck 1833) and Sprengel (1825) even called into question the validity of T.brachiatum View in CoL as a species.After Beck’s (1833) publication, most major monographs and floras recognized T. dichotomum View in CoL and I. coeruleus View in CoL with T. brachiatum View in CoL in synonymy under I. coeruleus View in CoL following the Muhlenberg-Nuttall synonymy ( Bentham 1833, 1835; Gray 1848; Gray 1878; Torrey 1843) though there were at least two exceptions that did not include T.brachiatum View in CoL in their synonymy ( Chapman 1860; Chapman 1883). The nomenclatural change of the species epithet from I. coeruleus View in CoL to I. brachiatus View in CoL (L.) Britton, Sterns, & Poggenb. ( Poggenburg et al. 1888) was done following the establishment of the rules of nomenclatural priority ( de Candolle 1867). Isanthus brachiatus View in CoL was used as the species name in most subsequent floras ( Small 1903; Small 1913; Britton & Brown 1913; Small 1933) though again Chapman (1897) still used I.coeruleus View in CoL . Isanthus brachiatum was transferred back into the genus Trichostema View in CoL by Epling (1929). Lewis (1945) also recognized I. brachiatus View in CoL as T. brachiatum and placed it in section Orthopodium Bentham (1835) based on life history, chromosome number, and morphological characteristics.

Despite the extensive treatment of these two species, typification of the names was not effected until the late 20th and early 21st centuries ( Epling 1929 and Lewis 1945 both use the term “standard” for T.brachiatum , which is not considered valid typification by the International Code of Nomenclature for algae, fungi, and plants [ICN]; Turland et al. 2018). Isanthus coeruleus had a lectotype designated by Uttal (1984) based on Michaux’s specimens in the Paris Herbarium (P). Trichostema brachiatum was determined to have no specimens present that could be considered original material, so Reveal designated the illustration from Dillenius (1732) cited by Linnaeus (1753) as the lectotype of that species ( Jarvis et al. 2001). It is clear looking at the lectotype of T. brachiatum that this species name does not apply to the plant later named I. coeruleus by Michaux. For a more in-depth discussion of the application of species names and synonymy, see McClelland (2022).