Monographs Details: Borreria capitata var. tenella (Humb., Bonpl. & Kunth) Steyerm.
Authority: Maguire, Bassett. 1972. The botany of the Guayana Highland--part IX. Mem. New York Bot. Gard. 23: 1-832.
Family:Rubiaceae
Discussion:

Spermacoce tenella H.B.K., Nov. Gen. & Sp. 3: 345. 1819.

Spermacoce aturensis H.B.K., Nov. Gen. & Sp. 3: 345. 1819.

Borreria suaveolens of most authors, not G.F.W. Mey., Prim. Fl. Esseq. 81. pl. 1. 1818.

Borreria tenella (H.B.K.) Ch. & Schl., Linnaea 3: 317. 1828, as to synonymy and to small part of description with reference to Humboldt’s specimens.

Spermacoce orinocensis Willd. ex R. & S., Syst. Veg. 3: 531. 1818.

Spermacoce tenella has been interpreted quite differently by various authors. Present day material collected from the area of the type locality of San Fernando de Atabapo agrees with the description and photo of the type collection of Spermacoce tenella, as exemplified by Williams 16036 from sabanas de Sanariapo, while other collections from the type locality (Cowan & Wurdack 32087, Curran 93, and Medina 496) represent a different taxon, described elsewhere in this treatment. The original description, substantiated by a photograph of the type, states that the internodes are 3.5-5 cm long. The leaves are stated to be 2.4-3 cm long (the photo shows them 1.6-4 cm long), 1 mm wide, and glabrous with a prominent midrib beneath. The Williams 16036 specimen agrees in having internodes elongated, 2-8 cm long, but with leaves longer and 3.5-6.5 cm long. The description of Spermacoce tenella states the calyx to be “quadri- aut quinquepartitus.”

Williams 15885, collected at Puerto Ayacucho, near the type locality (“prope Atures”) of Spermacoce aturensis H.B.K., likewise matches the description and photo of Spermacoce tenella H.B.K. with elongated internodes 3-5.5 cm long and opposite linear elongated leaves 2.7-5.5 cm long and 1.5-3 mm broad (the internodes in S. aturensis are described as 3.5 cm long, the leaves 3.5-5 cm long and 2-3 mm broad). It is similar to the Williams 16036 collection, but shows more abundant retrorse hairs on the angles of the stems. In other respects as to the details of calyx and corolla, both of the Williams collections are identical, with corollas 4-5 mm long, and 4 calyx lobes 1-2 mm long, while the flowering heads are 10 mm high by 15 mm broad and subtended by elongated bracts 22-35 mm long which exceed the head.

Chamisso & Schlechtendal (Linnaea 3: 317. 1828) attempted to enlarge the original concept of HBK’s Spermacoce tenella by inclusion of Brazilian specimens with pubescent stems, and pubescent leaves varying from lanceolate-linear to oblong-lanceolate and up to 0.62-0.63 cm broad. Whereas Spermacoce tenella H.B.K. was originally described with leaves glabrous above and mainly so below, Chamisso and Schlechtendal included in their description of Borreria tenella leaves varying from the upper surface scabrid with short, erect hairs and the lower surface pubescent, to those with both sides softly tomentose, especially on the lower surface. The Brazilian specimens included by Chamisso and Schlechtendal refer to plants having much larger heads of flowers than those from the type locality of Venezuela, and the Brazilian plants are described as more robust, with more rigid and erect stems than the material collected originally by Humboldt & Bonpland. Schumann (Mart. Fl. Bras. 6(6): 54-57. 1888), likewise, treated Borreria tenella as a highly variable taxon at great variance from the original description and photo of the type of Spermacoce tenella H.B.K. Schumann attempted to keep B. tenella and B. capitata apart on the basis of the transversely sulcate seed character of B. capitata, but this difference is not obvious in the flowering heads. Although the heads in B. tenella average slightly smaller than those of B. capitata, too many intergradations exist between these taxa, and no differences appear in the fruiting stages.

In summary, the narrow-leaved, glabrous-leaved plants described as Spermacoce aturensis and S. tenella by Humboldt and Bonpland appear to be conspecific with those of B. capitata so far as concerns their morphological details of flowers and inflorescences. Like B. capitata, the plants originating from Territorio Federal Amazonas, Venezuela, have relatively large infundibuliform, elongated corollas 3-6 mm long with lobes mostly 1.2-2.2 mm long, constantly 4-5 densely hispidulous-ciliolate calyx lobes which are relatively elongated (0.8-2.2 mm) and exceeded by the relatively elongated corolla tube, fruiting cocci 2-3 mm long densely pilose in their upper portion, and elongated leaf-blades mainly 3-8 cm long. The narrow, glabrous-leaved Spermacoce aturensis cannot be separated from the narrow, glabrous-leaved forms of B. capitata f glabra and are here interpreted as merely another variation of the B. capitata complex. It has, therefore, not been possible to maintain B. tenella as separate from B. capitata, as has been the view taken by various other authors, and in the present treatment B. tenella has been retained in varietal category under B. capitata.

It is not evident why Spermacoce aturensis was not taken up by later authors. Schumann (Mart. Fl. Bras. 6(6): 55. 1888) did not take up the name in his treatment of Borreria, and Standley was in doubt about the name, but considered it as a probable synonym of B. suaveolens (Field Mus. Publ. Bot. 7: 483. 1931).