Costularia hornei (C.B. Clarke) Kükenthal (1939a: 27)

Henriette, Elvina, Larridon, Isabel, Morel, Charles, Goetghebeur, Paul & Senterre, Bruno, 2015, Revision of the genus Costularia (Cyperaceae: Schoeneae) for the flora of the Seychelles, including the rediscovery and resurrection of a rare endemic species, Phytotaxa 231 (1), pp. 31-41 : 33

Costularia hornei (C.B. Clarke) Kükenthal (1939a: 27)

Schoenus hornei Clarke (1894: 657) . Type:— SEYCHELLES. Horne 626 (lectotype designated here K!, 2 sheets K-000244890, K-

000244891). Lophoschoenus hornei (C.B. Clarke) Stapf in Gibbs (1914: 181). Costularia hornei var. rectirhachilloidea Kükenthal (1939a: 28) . Type:— SEYCHELLES. Mahé: Cascade Estate, Thomasset 171 (lectotype designated here K!). Tetraria hornei (C.B. Clarke) Koyama (1961: 75) . [ Asterochaete elongata auct. non Kunth (1837: 312) sensu Baker (1877: 417). Specimen cited:— Horne 626.] [ Schoenus xipholepis (Baker) Summerhayes (1928: 394) . p.p. quoad Horne 626 sed excl. holotypus.]

Representative specimens examined: — SEYCHELLES. Curieuse, G. Cusset & B. Jeune 290 ( K), 27 June 1974 ; D. Vesey-Fitzgerald 5436 ( K), 17 January 1987 ; Mahé, J. Procter 4468 ( K); Mahé, Cascade, H. P. Thomasset 171 ( K), February 1900 ; Copolia, B. Senterre & E. Henriette 7107 ( SEY, P, GENT), 14 August 2014 ; Montagne Palmiste (= Bernica), S. A. Robertson 2521 ( SEY, K), 20 January 1978 ; Pérard, B. Senterre & E. Henriette 7116 ( SEY), 11 December 2014 ; Praslin, R. Alluaud s.n. ( K), 09 June 1892 ; Midlands, D. Vesey-Fitzgerald 6117 ( K), 9 January 1938 ; Silhouette, Mont Pot à Eau, B. Senterre & J. Gerlach 5294 ( SEY, P, GENT), 18 March 2008 ; B. Senterre & C. Morel 7108 ( SEY, P, GENT), 24 September 2014 .

Distribution: —Endemic to the Seychelles. Widespread on the main granitic islands (Curieuse, Félicité, Mahé, Praslin, Silhouette).

Ecology: —From sea level to the highest summits (ca. 900 m elevation; Fig. 2 ), typically dominant in the herbaceous fringes of inselbergs, but also colonizing actively the degraded soils in the lowland belt, for example on Praslin and Curieuse.

Conservation: —Least Concern according to IUCN Red List (2014).

Vernacular name: —This plant is known locally as “Lerb razwar”, which means ‘razor herb’, due to the very sharp leaf margins.

Taxonomic note: — Costularia hornei is endemic to the Seychelles where it represents the western limit of distribution of the subgenus Lophoschoenus (with tristichous leaves). It was first recorded there under a name misapplied by Baker (1877: 417, as Asterochaete elongata ), who had seen only the specimen from the Seychelles (Horne 626), and provided a description based on the latter (for a complete description see also Kükenthal 1939a: 27). Nevertheless, the true Asterochaete elongata , currently treated as Costularia melicoides (Poiret in Lamarck & Poiret 1806: 273) Clarke (1908: 48) (Type: REUNION. du Petit-Thouars s.n. (P-02284597!, P-00552880!)), is clearly a species of the subgenus Costularia , with distichous leaves, restricted to the Mascarenes.

Costularia hornei differs from most other species in the subgenus Lophoschoenus by having the rachilla elongated and curved ( Kükenthal 1939a: 27). A second variety of that species, named Costularia hornei var. rectirhachilloidea , was proposed by Kükenthal (1939a: 28) based on the rachilla internodes being short and straight (not elongated and curved as in the type variety) and on the glumes being of similar length (the basal ones reduced in the type variety). Nevertheless, current field explorations and study of a type specimen in Kew (Thomasset 171!) showed that the variety rectirhachilloidea does not represent a distinct variety but rather specimens at an early stage of spikelet development. Note that the basionym of Costularia hornei , i.e. Schoenus hornei , was published without description but it is nevertheless a valid name because it included an explicit reference to a previously published description ( McNeill et al. 2012: Art. 38.1): Baker’s Asterochaete elongata was cited as a synonym, the specimen Horne 626 was cited and the epithet “ hornei ” was chosen to honour the collector. Although Schoenus hornei also included among the cited synonyms a previously validly published name, i.e. Cladium xipholepis , and could therefore be considered as an illegitimate name ( McNeill et al. 2012: Art. 52), Clarke (1894) implicitly considered such synonymy as uncertain since he excluded the type of Cladium xipholepis ( McNeill et al. 2012: Art. 52 and 7.5) which he considered as a “young state of Baker’s Asterochaete elongata ”. Clarke also wrote on the specimen Horne 626 “ Schoenus hornei sp. nov. ”, in March 1891, and therefore it is clear that the name Schoenus hornei is typified by Horne 626 and that Clarke was making reference to the description given by Baker under Asterochaete elongata . Note that another syntype was cited by Clarke (1894), i.e. Wright 81, which we could not examine but which is clearly not the same sheet than the specimen Wright s.n. (type of Cladium xipholepis ; ‘s.n.’ being sometimes an omission of a number).

Figures

FIGURE 2. Distribution maps on the island of Mahé (Seychelles) for Costularia hornei (A) and Costularia xipholepis (B), showing also the distribution of their respective habitats (based on Senterre & Wagner 2014).