Homalium planiflorum (Tul.) Baill. in Bull Mens. Soc. Linn. Paris 1: 574. 1886

Applequist, Wendy L., 2018, A revision of Homalium sect. Odontolobus (Salicaceae) endemic to Madagascar, Candollea 73 (1), pp. 27-48 : 42-43

9. Homalium planiflorum (Tul.) Baill. in Bull Mens. Soc. Linn. Paris 1: 574. 1886 .

Ξ Blackwellia planiflora Boivin ex Tul. in Ann. Sci. Nat., Bot. ser. 4, 8: 64. 1857 [nom. conserv. prop.; APPLEQUIST, 2017].

Ξ Blackwellia gracilis Blume in Mus. Bot. Lugd.-Bat. 2: 26. 1856 .

Holotypus: MADAGASCAR: “ Ile Ste. Marie ”, s.d., Richard 297 ( L [ L0010991 ] image seen ; iso-: G [ G00018396 ] image seen, P [ P00375177 , P00375178 ]!) .

Tree to 30 m tall, 60 cm dbh; bark young twigs glabrous (minutely pubescent when very young). Leaves: petiole (2-)3- 8(-14) mm long, glabrous (to sparsely and minutely pubescent); blade narrowly elliptical to elliptical or oblanceolate (aberrantly obovate), (2.8-)3.5-10.3 × 1-4 cm; base convex (cuneate to attenuate); margins crenate-serrate to serrulate, at least apically (partly subentire); apex acute to acuminate (to rounded, emarginate, cuspidate). Inflorescences racemose (paniculate with a few long branches, racemiform panicles), sometimes clustered terminally, (2.2-)5-10(-22) cm long, moderately to sparsely short-pubescent; flowers mostly in small clusters, pedicellate with pedicels (0.4-)1-3(-3.5) mm long; bracts broadly ovate-deltoid to transversely ovate; bracteoles ovate to deltoid, small, not thickened, caducous. Flowers 7-8 (-9)-merous, whitish to pale green, pale yellow or creamcolored or pink to reddish; sepals lanceolate to narrowly oblong-lanceolate (ovate), 0.5-0.9(-1.1) mm long; petals oblanceolate (to broadly spatulate, narrowly obovate, to somewhat oblong or narrowly elliptical), 1-1.7 mm long, margins ciliate and both surfaces sparsely pubescent to glabrate, apex acute to rounded; filaments (0.5-)0.6-1.2(-1.4) mm long; upper surface of ovary moderately pubescent; styles 3-4, (0.4-) 0.8-1.4 mm long.

Notes. – Most specimens assigned to H. planiflorum have white to pale green or yellowish flowers. A group of specimens from southeastern Madagascar that are characterized by pink or reddish flowers are herein segregated as subsp. roseiflorum Appleq. While specimens of subsp. planiflorum have sometimes large leaves and racemes usually borne along the length of the twigs, those of subsp. roseiflorum often have relatively small leaves and relatively large (though not long) distal clusters of racemes, or occasionally paniculate inflorescences borne well below twig apices; however, these characters are not fully consistent. Several specimens from the extreme southeast, where both subspecies occur, are not assigned to either subspecies, though these collections generally have narrow leaves of modest size, typical of subsp. roseiflorum . One is sterile and two are only weakly flowering with one or two racemes, so subspecies identity cannot be confidently assigned without information on flower color. Two other collections are described as having possibly reddish flower color (“terre brûlée” according to Cloisel and “fulvo-rosei (sordid)” according to Bernardi) but the distribution of their well-developed inflorescences appears to be more consistent with subsp. planiflorum , and flowers may brown as they turn to fruit. The inability to assign identities to some specimens in the zone where these taxa are sympatric supports a view that they are not fully genetically isolated, and that the distinctions between them are not adequate to justify recognition at the species level. Alternatively, the intermediate specimens could be interpreted as hybrids between two distinct species; however, because three fixed differences between the two morphological forms have not been observed, the conservative approach of treating them as conspecific has been preferred.

Service Forestier 9952 (Andovolava, Nosy-Varika, 14.XI.1953) is a specimen in poor condition with few flowers; the leaves are very large and broad (up to at least 10 × 5.5 cm). It may represent a regional variant, an aberrant individual, or an undescribed species; the available material is inadequate to clarify its status. Additional collections from this population would be highly desirable.

Blackwellia planiflora was published in 1857, the year after B. gracilis Blume , though its author, Tulasne, was apparently unaware of the latter publication. The citation of Richard 297 in the protologue of B. gracilis explicitly referred only to the duplicate at L, which is therefore its holotype. The protologue of B. planiflora cited two syntype gatherings, Boivin 1847 and Richard 297; three duplicates each of the former are present at P and G, and two of the latter at P. PERRIER DE LA BÂTHIE (1946: 92) chose Boivin 1847 as “ type ” (i.e., the lectotype) without selecting among the available duplicates. Under Art. 52.2 of the ICN ( MCNEILL et al., 2012), the citation of Richard 297, which included the holotype of B. gracilis , in the protologue of B. planiflora would appear to make the latter a superfluous and illegitimate name. Past literature has implicitly treated it as being the citation only of duplicates that were available to Boivin and Tulasne, not the duplicate at L that they presumably did not have access to (e.g., SLEUMER, 1973: 248). By that view, B. planiflora need not be treated as illegitimate. However, Art. 9.5 of the ICN states that the citation of a gathering, if not qualified or limited, is equivalent to the citation of all duplicates of that gathering, even those not seen or known of. Tulasne did not indicate that only material at P was used. The informal practice of treating a duplicate in an author’s home institution as a holotype despite the absence of a qualifying statement is seen in the literature (see Homalium lucidum above), but normally in cases that do not involve questions of legitimacy. Hence Blackwellia planiflora is illegitimate due to the citation of the holotype of B. gracile , which is therefore also the obligate type of B. planiflora , regardless of Perrier de la Bâthie’s attempt to select another gathering as the (lecto) type.

This creates serious problems, because under the ICN, the species under question here would have no legitimate name. Blackwellia gracilis Blume cannot now be transferred to Homalium and retain that epithet because of the existence of H. gracile Briq. , a replacement name for Blackwellia gracilis Vieill. [nom. illeg., non Blume]. However, because Homalium gracile had not yet been published when H. planiflorum was described, H. planiflorum as a replacement name credited to Baillon would also be illegitimate (see Art. 58, Note 1, of the ICN; MCNEILL et al., 2012). The publication of a new name would therefore be required. However, since this is the most common species of its section, introducing an entirely unfamiliar name would cause some confusion among botanists. A proposal to conserve Blackwellia planiflora to permit the continued use of Homalium planiflorum (the preferable approach, since Prop. 235 to amend the ICN to permit this ( WIERSEMA et al., 2016) was accepted at the 2017 Botanical Congress) has therefore been offered ( APPLEQUIST, 2017) but will not be considered by the relevant Committees for some time yet.

Key to the subspecies of Homalium planiflorum

1. Inflorescences usually racemose (rarely a racemiform or few-branched panicle), usually borne along much of the length of small twigs; flowers whitish to pale green, pale yellow, or cream-colored; widely distributed ..................... ....................................................... 9a. subsp. planiflorum

1a. Inflorescences racemose, usually clustered near twig ends, or paniculate with few long racemoid branches; flowers pink to reddish; confined to southeastern Madagascar ..... ........................................................9b. subsp. roseiflorum