(3020) Proposal to conserve the name Trichostema brachiatum (Lamiaceae) with a conserved type

Abstract

(3020) Trichostema brachiatum L., Sp. Pl.: 598. 1 Mai 1753 [Angiosp.: Lab.], nom. cons. prop.

Typus: Herb. Linnaeus No. 750.2 (LINN), typ. cons. prop.

Specimen 750.2, annotated by Linnaeus as Trichostema brachiatum L. with “?” added later by J.E. Smith, was apparently added to LINN after 1753. Trichostema brachiatum, when applied consistent with 750.2, represents a species that is widely distributed in North America north of Mexico. As indicated by Linnaeus (“staminibus brevibus inclusis”), the species is distinguished on the basis of its short stamens. The only cited original material for the name T. brachiatum, a Dillenius icon (“Teucrium Virginicum, Origani folio”, Hort. Eltham. 2: 380, t. 285, fig. 369. 1732; lectotypification by Reveal in Taxon 50: 522. 2001), likely represents the species currently known as T. dichotomum L. (Sp. Pl.: 598. 1753; “staminibus longissimis exsertis”), the “short” stamens in the illustration (see Rees, Cycl. 36(I): Trichostema no. 2. 1817) the result of the coiled filaments in late-day flowers (see McClelland & Weakley in J. Bot. Res. Inst. Texas 17: 220–223. 2023). Druce & Vines (Dillenian Herb.: 181. 1907) incorrectly applied Dillenius's polynomial consistent with specimen 750.2 (LINN).

Michaux (Fl. Bor.-Amer. 2: 3–4, t. 30) published the names Isanthus and I. coeruleus and Redouté's illustration in Michaux of I. coeruleus clearly represents the same species as specimen 750.2 (LINN). Like Michaux, many subsequent authors also recognized I. coeruleus, some without citing Trichostema brachiatum (e.g., Eaton, Man. Bot., ed. 2: 285, 467. 1818; Torrey, Comp. Fl. N. Middle Stat.: 233, 238. 1826; Wood, Class-book Bot.: 541. 1881; Gray, Manual, ed. 6: 405–406. 1889; Chapman, Fl. S. U.S., ed. 3: 389. 1897) and others (e.g., Muhlenberg, Cat. Pl. Amer. Sept.: 56. 1813; Nuttall, Gen. N. Amer. Pl. 2: 27, 39. 1818; Bentham, Labiat. Gen Spec.: 166–167. 1833, 658–660. 1835; Steudel, Nomencl. Bot., ed. 2, 1: 823–824. 1840, 2: 703. 1841; Torrey, Fl. New York 2: 55, 81–82. 1843; Candolle, Prodr. 12: 572–574. 1848) citing T. brachiatum as a synonym of I. coeruleus, their adoption of the later I. coeruleus instead of a new combination in Isanthus based on the earlier T. brachiatum the result of their following the “Kew Rule”, in which priority was applied only within the genus.

Only a few authors attempted to apply Trichostema brachiatum consistent with Dillenius's illustration. Lamarck & Poiret (Tabl. Encycl. 2: t. 515. 1794, 3: 71. 1823; Encycl. 6: 572. 1805, 8: 84–85. 1808) applied the name consistent with the species now known as T. setaceum Houtt. (= T. lineare Walter; T. lineare Nutt.), a species taxonomically aligned with T. dichotomum. Others (Rees, Cycl. 19(I): Isanthus no. 1. 1811, l.c. 1817; Pursh, Fl. Amer. Sept. 2: 405, 414. 1814; Steudel, Nomencl. Bot. 1: 440, 847. 1821; Guillemin in Audouin & al., Dict. Class. Hist. Nat. 9: 22. 1826, 16: 362. 1830) recognized T. brachiatum as a distinct species, although Rees (l.c. 1817) doubted that T. brachiatum was distinct from T. dichotomum and Steudel (l.c. 1840 & 1841) later changed his mind and applied T. brachiatum consistent with 750.2 (LINN). Sprengel (Syst. Veg. 2: 694, 719. 1825) treated T. brachiatum as a nomen dubium, noting “Trichostemma [sic] brachiatum L., soli Dillenio notum, quod Nuttallius huc trahit [i.e., a synonym of Isanthus coeruleus], alienum est et valde dubium.”

Like most previous workers, Britton & al. (in Poggenburg & al., Prelim. Cat.: 44. 1888) applied the name Trichostema brachiatum consistent with specimen 750.2 (LINN) and, being adherents of strict priority, effected the combination Isanthus brachiatus, citing I. coeruleus in synonymy. Subsequent authors followed suit (e.g., Briquet in Engler & Prantl, Nat. Pflanzenfam. 4(3a): 215. 1895; Fernald & Robinson, Manual: 693. 1908; Britton & Brown, Ill. Fl. N. U.S., ed. 3, 3: 104. 1913; Small, Fl. S.E. U.S., ed. 2: 1019. 1913; Taylor, Fl. Vicin. New York: 529. 1915; Adams, Short Guide Canad. Gen. Seed Pl.: 66. 1938; Kearney & Peebles, Fl. Pl. Ferns Arizona: 770. 1942; Fernald, Manual: 1216. 1950).

Lewis (in Brittonia 5: 589. 1945) in his revision of Trichostema did not recognize Isanthus and he accepted the name T. brachiatum (I. coeruleus cited in synonymy) with specimen 750.2 (LINN) cited as the “standard” (see also Epling in J. Bot. 67: 11. 1929). Lewis's use of “standard” may have been an acknowledgement that 750.2, while not the type, was nonetheless serving as the application standard for the name T. brachiatum.

Today most follow Lewis's generic concept and the name Trichostema brachiatum is in widespread use throughout the range of the species, which includes over 30 states in the U.S. as well as the District of Columbia, and Ontario and Quebec, Canada. The name has been used in well over 100 floras, checklists, and other works on the flora of North America north of Mexico (e.g., Haines, Fl. Nov. Angl.: 653. 2001; Weakley, Fl. S.E. U.S.: 1397. 2020; Canadensys. 2023 [https://data.canadensys.net/vascan]; NatureServe. 2023 [https://explorer.natureserve.org/]; Plants of the World Online. 2023 [http://www.plantsoftheworldonline.org/]; USDA PLANTS. 2023 [https://plants.sc.egov.usda.gov/home]). The species is also tracked or listed, under the name T. brachiatum, as a species of conservation concern in the U.S. in eleven states, as well as Quebec, Canada (NatureServe. 2023 [https://explorer.natureserve.org/]). Some current works still recognize the genus Isanthus and use the name I. brachiatus (e.g., Gleason & Cronquist, Man. Vasc. Pl. N.E. U.S. Can.: 435. 2004; Jones, Pl. Life Kentucky: 412. 2005; Jarvis, Order Out Chaos: 896. 2007; Wilhelm & Rericha, Fl. Chicago: 619–620. 2017).

Recently, McClelland & Weakley (l.c.) conducted an analysis of the Dillenius icon designated as lectotype by Reveal (l.c.) and concluded that it represents the species currently known as Trichostema dichotomum. Neither Reveal nor Jarvis (l.c.) indicated that the lectotypification altered the name's current application. Contrary to Art. 57.1 of the ICN (Turland & al. in Regnum Veg. 159. 2018), McClelland & Weakley (l.c.: 221) rejected the idea of conserving T. brachiatum with a conserved type, shifted the long-standing application of the widely used T. brachiatum by treating it as a synonym of T. dichotomum, and effected the new combination T. coeruleum (Michx.) K.S. McClell. & Weakley (l.c.: 222) for the species currently known as T. brachiatum.

Using Art. 14.9 of the ICN (Turland & al., l.c.), this proposal seeks to avoid this disadvantageous nomenclatural change by making what has effectively been the de facto type of Trichostema brachiatum (i.e., 750.2 in LINN) the de jure type through conservation. Acceptance of this proposal will permit the continued use of the name T. brachiatum in its current sense, as well as I. brachiatus when Isanthus is recognized. Rejection of this proposal will result in the widely used T. brachiatum becoming a synonym of T. dichotomum, with the correct name for the species now known as T. brachiatum being the recently published T. coeruleus or, when Isanthus is recognized, I. coeruleus, a name that has not had any usage since the late 1800s.