A revision of Homalium sect. Nisa (Salicaceae) endemic to Madagascar

Abstract Wassel, A.C. & W.L. Applequist (2020). A revision of Homalium sect. Nisa (Salicaceae) endemic to Madagascar. Candollea 75: 1–23. In English, English & French abstracts. Homalium sect. Nisa (Noronha ex Thouars) Baill. is endemic to Madagascar and has included four recognized species. A new revisionary treatment of the section is presented and an identification key is provided. Eleven species are recognized, including eight that were formerly part of the Homalium nudiflorum (DC.) Baill. species complex. Four species and one subspecies are newly described: Homalium antilahimenae Wassel & Appleq., Homalium mandenense Wassel & Appleq., Homalium pachycladum Wassel & Appleq., Homalium tenue Wassel & Appleq., and Homalium stelliferum subsp. andapense Wassel & Appleq. Homalium ciliolatum (Sleumer) Wassel & Appleq. is newly recognized at species level and the new combination Homalium retusum (Blume) Wassel & Appleq. is further published. Four lectotypes are designated including a second step. Taxonomic novelties are provided with line drawings or scans of their holotypes. Risk of extinction assessments indicate that three taxa are “Endangered” and three “Vulnerable”. Occasional hybridization is observed. Some specimens are not classifiable and it is possible that other unrecognized species exist.


Introduction
Homalium Jacq. (Salicaceae) is a pantropical woody genus, historically included within the polyphyletic family Flacourtiaceae (Chase et al., 2002). The genus is distinguished by two perianth whorls, a large gland on the adaxial base of each sepal, a semi-inferior ovary and oppositipetalous stamens arranged either singularly or in fascicles. It is currently arranged into ten sections (Applequist, 2016a). Two subgenera were formerly recognized, but the sole distinguishing feature of a solitary stamen per petal (subg. Blackwellia (Benth.) Warb.) vs. fasciculate stamens (subg. Homalium) appears to be homoplasious. Before recent re-evaluation of the Malagasy species, about 150 species had been recognized within the genus (Applequist, 2013). Madagascar and Malesia are the two primary centers of diversity. The most sectional diversity is ascribed to Madagascar, with six sections, five of which are considered endemic following the recent restriction of sect. Eumyriantheia to only Malagasy species (Applequist, 2016a(Applequist, , 2016b. The Malagasy endemic Homalium sect. Nisa (Noronha ex Thouars) Baill. is distinguished by solitary stamens, sepals usually accrescent and much larger than the petals both at anthesis and especially during fruit maturation (Fig. 1), large persistent bracts and bracteoles, and stipules usually fused opposite the petiole (Applequist, 2016a). The last taxonomic revision of Homalium sect. Nisa (Sleumer, 1973) reported it to include four species. However, in the intervening decades far more herbarium material has been collected from Madagascar, so that recent revisionary studies have described multiple new species in every section so far treated (Applequist, 2016b(Applequist, , 2018a(Applequist, , 2018b. Additionally, three of the recognized species -Homalium louvelianum H. Perrier, H. stelliferum H. Perrier, and H. intercedens Sleumer -have distinctive morphological features and are consistent and well-distinguished. However, H. nudiflorum (DC.) Baill., as treated by Sleumer (1973), incorporated substantial morphological diversity, including multiple taxonomic synonyms: distinctive specimens with very densely pubescent flowers were described as H. nudiflorum var. ciliolatum Sleumer, while Nisa retusa Blume, H. scleroxylon (Tul.) Baill., and H. confertum Baker were included in var. nudiflorum. There was reason to suspect that this circumscription was overly broad and that some of these should be removed from H. nudiflorum, and that additional unnamed taxa existed within the H. nudiflorum complex. Therefore, a new modern revision based on available herbarium collections was necessary.

Materials and Methods
Materials examined included herbarium specimens at MO, duplicates of MO collections not yet distributed, and specimens from P received on loan or seen during a visit. In addition, images of specimens at P were available through Sonnerat (2020), and images of types held by other institutions available through the JSTOR Global Plants website [http://plants.jstor.org] were observed. Standard taxonomic procedures were followed consistent with the recent revision of other sections of Homalium in Madagascar (Applequist, 2016a(Applequist, , 2018a(Applequist, , 2018b. Special comment is necessary regarding certain morphological features in this section. Bark characteristics and floral color are not available for all taxa, but are reported whenever label data have provided them. In a few species, twigs have a particularly gracile or sturdy appearance that could be a useful taxonomic character. The difference among species is more prominent in twig segments below the terminal branches (which in almost all species are slender early in their growth period) but if this is to be described quantitatively, since all twigs ultimately arise from large branches, some consistent means of deciding which twigs to measure was necessary. The choice was made to measure highest and lowest leafing internodes, i.e., those that bore leaves at nodes both above and below the internode at the time of collection, except that if a single leaf were borne at the apex of a twig the internode below it was measured, and if a single leaf was borne on an older proximal twig segment the internode above it was measured.
In most species the sepal length greatly increases during the process of floral and fruit maturation, while petal size increases to a lesser extent. There are differences among species in initial and final perianth size that provide support for recognition of taxa. However, perianth size can be used to aid in specimen identification only with caution, because the ovary and calyx enlarge only slightly as fruit develops in Homalium, so the stage of development may not always be obvious.
There are few fruit characters of taxonomic utility within sections of Homalium because the capsules are so small. Specimens with apparently mature seeds are rare, in part because the entire flower is the unit of dispersal and is consistently lost as seeds mature, but possibly also because rates of fertilization and successful seed maturation may be low. In this paper, specimens are described as fruiting if many flowers appear to be post-anthesis due to visible ovary/calyx expansion, strong inward curving of petals over the ovary, and/or the loss of most anthers. There may be differences among species in whether the basal portion of the ovary and calyx cup or the upper free portion of the ovary expands most in fruit, but this is not emphasized here because, given the tendency of flowers with the most mature ovaries to be lost, floral characters are far more useful.
To save space, locality data are not provided for all specimens seen of the most common taxa, and those that are provided are edited for brevity, especially where more complete label data are available from Tropicos (2020). "Fkt." is used throughout as an abbreviation for "fokontany". Madagascar has traditionally been divided into six provinces, but these have recently been dissolved and the country divided into 22 regions. Because provincial names will still be more familiar to many readers, localities are organized by region, with the province of which each region was formerly a part given in brackets. A complete index of specimens seen is provided as an appendix. Maps of georeferenced specimens that have been databased in Tropicos may be viewed within the Missouri Botanical Garden's Catalogue of the Plants of Madagascar project (Madagascar Catalogue, 2020), which is continually updated with new determinations and specimens.
A preliminary risk of extinction assessment using the Categories and Criteria of IUCN (2012) is provided for each taxon recognized. In instances when the extent of occurrence (EOO) and area of occupancy (AOO) could affect the assessed status, GeoCAT (Bachman & Moat, 2012) was used to estimate those values. Geographic coordinates were taken from label data or approximated from the Missouri Botanical Garden's gazetteers or coordinates estimated in prior georeferencing efforts (Tropicos, 2020). If coordinates for a specified locality could not be obtained, coordinates for the closest locatable population center mentioned on the label were used. Stipules opposite the petiole base and fused (in H. intercedens Sleumer axillary, free), usually caducous. Leaves with venation more or less brochidodromous. Inflorescences spicate; bract and bracteoles large, usually broad, thicktextured, persistent. Flowers sessile (rarely subsessile); perianth 5 -6(-8)-merous. Sepals obovate to oblanceolate (to nearly ligulate), oblanceolate-oblong or narrowly elliptic (in H. louvelianum H. Perrier oblong-ovate to ovate-deltoid), spreading, accrescent (only slightly so in H. louvelianum); calyx tube short, broadly funnelform to cup-shaped, or in fruit hemispherical. Petals ovate to oblong, usually smaller than sepals, modestly accrescent and curving over fruit (in H. louvelianum similar to sepals in size and spreading); sepals and petals not ciliate, or ciliolate in conjunction with overall surface pubescence. Sepal glands large, elliptic to elongated oblong-elliptic or roughly trapezoidal. Stamens 1 per petal, inserted between glands; anthers dorsifixed, broadly oblongelliptic with oblong-elliptic locules and a large connective, the slits of dehiscence nearly parallel. Upper surface of ovary often nearly flat in flower, in fruit becoming hemispherical or broadly conical; styles 3 -4, fused basally (rarely fused for most of length). Locule of fruit subglobose or vertically compressed, internally glabrous (short-pubescent); seeds sometimes 1 per fruit, subglobose, largely filling the locule (or several small immature seeds, possibly most never maturing).
Etymology. -Homalium antilahimenae is named for botanist Patrice Antilahimena, one of the most experienced living field botanists in Madagascar, who has made over 9500 collections, including the type, and additionally made valuable contributions to ecological and floristic studies (e.g., Andrews et al., 1998;Phillipson et al., 2010). Wood is used to construct houses (Rakotonirina et al. 180).
Distribution, ecology and conservation status. -Homalium antilahimenae is native to the northeastern regions of SAVA and Analanjirofo, where it usually occurs in low-to mid-elevation humid forests (with one specimen labeled at a range of 740 -1200 m). It appears to be uncommon. As few as six distinct locations may be known; the Extent of Occurrence (EOO) is calculated as c. 11,190 km2 (which includes a significant area of open water) and the Area of Occupancy (AOO) as 32 km2. Its habitat includes the protected area of Masoala, but other portions of the known habitat are more subject to ongoing anthropogenic degradation. A preliminary assessment of its conservation status is therefore "Vulnerable" [VU B1ab(iii)+2ab(iii)].

Homalium ciliolatum
Distribution, ecology and conservation status. -Homalium ciliolatum is confined to the southeastern regions of Atsimo-Atsinanana, Vatovavy-Fitovinany, and Anosy. It is usually found near the coast at low elevations, but the type was probably from a mid-elevation forest. Reported substrates include basalt and rocky soil; habitats include forest and savoka (disturbed grassland). As many as ten subpopulations have been collected in the past (older locality data are imprecise). However, it has only been collected twice in the past 54 years, though much of its habitat is relatively accessible, so it is evidently rare. From all historical localities, the Extent of Occurrence is calculated as c. 15,980 km2 and the Area of Occupancy as 40 km2. Most historical populations were from land that has been subject to serious anthropogenic damage in the intervening decades and is still not protected. Therefore a preliminary assessment of its conservation status is "Vulnerable" [VU B1ab(iii)+ 2ab(iii)], but this is likely to underestimate the degree of threat it faces.
Notes. -Homalium ciliolatum was described by Sleumer (1973) as a variety of H. nudiflorum, from which it is most obviously separated by its densely pubescent flowers. The petals are velutinous on both surfaces; the basal part of the calyx externally is densely sericeous, and the remainder of the sepals are short-pubescent and ciliate. Also, the upper portions of the inflorescence rachis and the bracts and bracteoles are pubescent, and the bracts and bracteoles short-ciliate. The petals appear often to remain curved over the ovary before fruit develops. It is likely that no fully mature perianths have been seen and the final size is larger than the range reported here. The distribution of H. ciliolatum is more southerly than that of H. nudiflorum and it is confined to lower altitudes. Two other species herein excluded from the H. nudiflorum complex, the newly described H. antilahimenae Wassel & Appleq. and H. tenue Wassel & Appleq., also have calyces much more pubescent than those of H. nudiflorum. Homalium ciliolatum is distinguished from these by the distinctive long-sericeous indument of the calyx cup, which is not seen in any other species, and by having most flowers borne in pairs.
Wood is used to make unspecified products (Service Forestier 25277). Distribution, ecology and conservation status. -Homalium confertum is widespread in eastern and central regions of Madagascar. It usually occurs in humid forests at mid-to high elevations (range 445 -1860 m), but also extends into relatively dry western forests (Ankarafantsika). Reported substrates include gneiss. It is sometimes common and locally dominant (Randrianaivo et al. 526). It has been collected in several protected areas, including Ambohitantely, Anjanaharibe-Sud, Ankarafantsika, Montagne d'Ambre, and Zahamena. Its conservation status is estimated as "Least Concern" [LC].
Notes. -Homalium confertum was lumped into H. nudiflorum by Sleumer (1973). It differs from H. nudiflorum s.s. in its larger, often strongly toothed leaves with many marginal glands and its more densely pubescent petals; the final perianth size may also be larger. The two overlap in range, but H. confertum is more tolerant of dry climate and high altitude, with much of its habitat on the central plateau. It is similar to H. pachycladum and H. retusum, which also overlap in range, in maximum leaf size, but H. pachycladum and retusum are distinguished by entire leaf margins, fewer leaf glands, and petals glabrous on the abaxial surface.
Several possible hybrids between H. confertum and H. nudiflorum are seen in two regions where both are known, and gene flow is suspected to be responsible for some of the range of variation in H. nudiflorum; these are discussed under the latter species. Hybridization is seen in most sections of Homalium, even rarely among species of different sections (Applequist, 2018b), so the ability of these species to hybridize does not suggest that they should not be recognized as distinct, given their significant morphological and ecological differences.  Tree to 25 m tall, 30 cm dbh; bark gray-white, exfoliating, platanoid; twigs dark brown, glabrous, leafing internodes ca. 0.7 -1.3 mm diam.; stipules free, caducous. Leaves lanceolate, 3.4 -6.7 × 1.2 -2.4 cm; base convex; apex acuminate; margin serrulate with round to elongated glands in tooth apices; secondary veins 10 -12(-14) per side; both surfaces glabrous, drying dark brown with paler veins on adaxial surface, lighter to greenish on abaxial surface; petiole glabrous, 7 -16 mm. Inflorescences racemose, axillary, 2.1 -2.8 cm; rachis glabrous; bracts and bracteoles glabrous, ca. 1.5 -2.5 mm; flowers borne singly. Flowers (5 -)6-merous, yellowish-green; sepals narrowly oblanceolate to nearly ligulate, 6 -8 mm in fruit, adaxial surface with appressed pubescence on calyx cup and sparsely pubescent on sepal bases, adaxial surface glabrous; sepal glands glabrous; petals deltoid-ovate, 2.5 -3 mm in fruit, abaxial surface short-pubescent, adaxial surface glabrous; filaments glabrous, ca. 2 mm; anthers not seen; styles ca. 1.5 -1.8 mm.

Homalium intercedens
Distribution, ecology and conservation status. -Homalium intercedens is known only from Ankarana in the DIANA region in the extreme north of Madagascar. It is reported on limestone and at a relatively low altitude. It is known from only one subpopulation (or perhaps two subpopulations close together) and rarely collected. Ankarana has been visited by many collectors over the years. Tropicos (2020) has databased over 2100 historical and recent specimens from the area so the small number of collections is likely to indicate that the taxon is either narrowly endemic or uncommon within its range. However, the population is in a protected area that is known for inaccessible terrain, so it is at little risk of loss due to human activity. Therefore, its conservation status is tentatively assessed as "Least Concern" [LC].
Distribution, ecology and conservation status. -Homalium louvelianum is widely distributed along most of the eastern coast of Madagascar. It is found in littoral forests, or less often low-altitude humid forests near the coast, on sand and ferralitic soil. Although littoral forests are highly fragmented and threatened, H. louvelianum is still frequently collected over a wide range. Its conservation status is assessed as "Least Concern" [LC].
Notes. -Homalium louvelianum displays less perianth accrescence than any other species of Homalium sect. Nisa. It has the smallest flowers within the section, and is unique in having sepals similar in size to the petals and broadest towards the base. The flowers often appear crowded on short racemes, though occasionally the rachis is relatively elongated and flowers more widely spaced. In these characters and the unusually short styles and filaments, the species bears a strong resemblance to those species of Homalium sect. Odontolobus Warb. s.l. (Applequist, 2018a) that display less extreme floral reduction. While no molecular data adequate to resolve relationships within Malagasy Homalium are available, this species suggests an intermediate or transitional form between those two sections which may provide a clue as to their relationships.
Razakamalala et al. 3796 is identified as a hybrid between H. louvelianum and H. tenue. The leaves are consistent with H. louvelianum, narrowly elliptical with acute to acuminate apices. The sepals are only slightly longer than the petals, but resemble those of most species of Homalium sect. Nisa in shape and texture and are slightly reflexed, while the petals are densely pubescent and slightly inward-curving. Homalium tenue is found at this locality and its leaf shape and dense petal indument make it the presumptive second parent.
Distribution, ecology and conservation status. -Homalium mandenense is or has been endemic to four littoral forests in a very small portion of the southeastern region of Anosy, near Toliara. The potential habitat has been declining in area and quality, but Mandena is now a protected area. The Extent of Occurrence is calculated as 72 km2 and the Area of Occupancy as 20 km2. While some subpopulations are in protected areas, others are not, and their habitat is threatened by mining and wood harvest for firewood and charcoal. Therefore, the preliminary assessment of this species' conservation status is "Endangered" [ENB1ab(iii)+2ab(iii)].
Distribution, ecology and conservation status. -Homalium nudiflorum as circumscribed herein is a very widespread, though scattered species in eastern forests. It usually occurs in mid-elevation humid forests up to 1220 m, but is occasionally found near sea level. Reported substrates are lateritic soils, including ferruginous crust and cuirasse, volcanic soil, and gneiss or granite. It occurs in several protected areas, including Galoko, Manombo, Mantadia, Midongy du Sud, Montagne d'Ambre, and Zahamena. Its conservation status is estimated as "Least Concern" [LC]. However, it should be noted that regional variants of the species may be genetically distinct and at greater risk. Wood is used for construction of houses (Randriamanarivo et al. 10, Service Forestier 16192) and for heating and charcoal production (Service Forestier 16192).
Notes. -Homalium nudiflorum as defined by Sleumer (1973) was much too broadly circumscribed, including in a single species with one variety all of sect. Nisa that was not obviously excluded by having greatly different perianth morphology (H. louvelianum), stipule morphology (H. intermedium), or sepal gland and foliar indument (H. stelliferum). In this treatment, eight species are recognized within what was formerly the H. nudiflorum complex. Homalium nudiflorum as herein defined is much less heterogeneous, but remains a widespread and morphologically variable taxon. Variation in morphology is probably geographically correlated, but since most parts of the range are represented by few individuals, it is impossible to determine how consistently so.
Most conspicuously, most collections from the far north (DIANA and SAVA) have broadly elliptical leaves (providing the upper extreme of the size range) with usually rounded bases, a glaucous upper surface, and often shallowly repand rather than entire margins. Additionally, these collections have moderate (to dense) indument on the apical half of the petals, while material from other parts of the country usually has trichomes only at the extreme petal apices. These populations might merit recognition as a species. However, most of their distinctive characters may be found individually in specimens from other regions (e.g., there are southern collections with large, broad-based, broadly elliptical leaves or with heavy petal indument) and there are both typical northern collections and potential intermediates. Because hybridization between H. nudiflorum and H. confertum is suspected to occur primarily in the north (see below), it is also possible that some characters of these specimens are attributable to gene flow or incomplete separation from H. confertum. Sleumer (1973: 299) stated that Petit Thouars s.n. "P & P-Juss 14 411" was the "holotype" of Homalium nudiflorum, and that no duplicate was found at G-DC, where material possessed by De Candolle might have been expected to be located. Two sheets of Petit Thouars s.n. were located at P, both of which were labeled "holotype" by Sleumer; these should be considered syntypes. Both are of poor quality, but the sheet barcoded [P00418086] includes a fragment packet containing flowers of H. nudiflorum along with mounted material that appears to be H. louvelianum. Therefore, [P00418087] is chosen as lectotype. The type collection has narrowly elliptic to obovate or narrowly obovate leaves (some fairly large, though not broad for their size), a glabrous rachis, bracts (except for cilia) and calyx, and sparsely pubescent petals. Without locality data, it is not possible to identify this as pertaining to any specific geographic variant of H. nudiflorum, which favors the maintenance of a relatively broad species concept for pragmatic reasons, while excluding several segregate taxa that are clearly not conspecific with the type.
Several specimens (see below) are observed that have the fairly small leaves and most other features of Homalium nudiflorum s.str. combined with features of H. confertum, in particular dense indument on most, or sometimes all, of both petal surfaces. A few have variable leaf morphology with some leaves being crenulate or dentate-crenulate, with some elongated glands. A few other specimens with atypical morphology are herein treated as H. nudiflorum. All of the putative intermediates come from regions where both H. nudiflorum and H. confertum are found. Hybridization is observed in other sections of Homalium (Applequist 2016a(Applequist , 2018b. Hybridization or gene flow between these species, which are probably closely related, is suggested to be responsible for the features of these specimens. One or two specimens, e.g., Service Forestier 23750 with very small anthers and short filaments and styles, might be suspected of other hybrid ancestry, but their morphology is otherwise consistent with H. nudiflorum. Service Forestier 18334 has leaves similar to those of one form of H. nudiflorum (except that the adaxial surface is glaucous) but relatively gigantic flowers, with sepals reaching 20 mm in fruit. No other material of this section has flowers of similar size. As three characters to distinguish it from H. nudiflorum, as herein circumscribed, are not seen, it is tentatively placed in that species. However, further collections of this population would be highly desirable.
Baron 6251 is a specimen in poor condition with a few small flowers and a few elliptical to ovate leaves, one of them very large. Though leaf shape is quite variable in this species complex, ovate leaves are normally not seen. This may represent a hybrid with a larger-leaved species, possibly of another section, or a distinct species. Baron 6270 bears a fragment with short inflorescences and very small (2.2 -3.3 × 1.5 -2.2 cm), closely spaced, broadly elliptical leaves with entire margins, which are not glaucous. As the inflorescence is well past anthesis, the small size of the leaves and internodes is not due to immaturity. The specimen might be an aberrant individual of H. nudiflorum or an otherwise unknown related taxon. No locality data are available for Baron's collections, so it is not possible to attempt to relocate the collected populations, and the available material is not adequate to recognize new species.  Shrub or tree to 10 m tall, 30 cm dbh; twigs medium brown or pale gray, glabrous, leafing internodes (1.7 -)2 -4(-5.5) mm diam.; stipules fused, caducous. Leaves broadly elliptic to elliptic or obovate (somewhat ovate), (3 -)5.8 -14.5 × (1.8 -)3.5 -7 cm; base rounded to cuneate; apex rounded to obtuse, apiculate, or emarginate; margin entire to subentire, with small, inconspicuous glands; secondary veins 8 -11 per side; both surfaces glabrous, drying yellowish brown to greenish on adaxial surface, darker to pale brown on abaxial surface; petiole glabrous, 6 -15 mm. Inflorescences paniculate, axillary, 4.5 -11 cm; rachis glabrous to sparsely short-pubescent; bracts and bracteoles glabrous, (1 -)1.8 -3.5 mm; flowers borne singly.
Distribution, ecology and conservation status. -Homalium pachycladum is endemic to mid-elevation humid forest west of Maroantsetra. The Area of Occupancy is 12 km2; the Extent of Occurrence (EOO) would be calculated as c. 6 km2, but following the IUCN (2017) guidelines for application of Red List Criteria, the EOO must be at least equal to the AOO and is therefore considered to be 12 km2. All three collections were made inside the eastern edge of the recently delineated protected area of Makira. A preliminary assessment of this species' conservation status is therefore "Least Concern" [LC].
Distribution, ecology and conservation status. -Homalium retusum is found near the eastern coast in the northern part of Madagascar at low elevations. Few specimens are known, suggesting that this is an uncommon to rare species. Only five clearly distinct subpopulations have been recorded (Ile Ste.-Marie in Analanjirofo, Foulpointe and Ambila-Lemaitso in Atsinanana, and Anjana and Andempona in SAVA). The Extent of Occurrence (EOO) is calculated as c. 9847 km2 and the Area of Occupancy (AOO) as 28 km2. Much of the habitat is subject to ongoing anthropogenic damage, especially in view of its relative accessibility. Therefore, a preliminary assessment of its conservation status is Endangered [EN B1ab(iii)+2ab(iii)]. Wood is good for construction (Bernier 173). Notes.
-Homalium retusum has previously been lumped into H. nudiflorum s.l. (Sleumer, 1973), which as herein circumscribed, differs in having smaller, narrower, shorterpetioled leaves; its inflorescences are usually racemose, not over 9 cm, glabrous or sparsely pubescent, with flowers borne singly. Homalium retusum most closely resembles H. pachycladum: both have relatively large leaves, paniculate inflorescences, and flowers with glabrous sepals and near-glabrous petals. Homalium retusum is known only from low-altitude and littoral forests, while H. pachycladum occurs farther inland and at much higher elevations. Homalium pachycladum has elliptic to obovate (or rarely ovate) leaves with shorter petioles (6 -15 mm) and flowers all borne singly; its twigs seem usually to be thick, giving a robust appearance. Another inland species with large leaves, H. confertum, is distinguished by having usually elliptic to oblong-elliptic leaves with a serrate to crenate margin, racemose inflorescences with flowers borne singly, and petals densely pubescent on most of both surfaces.
The history of the names applying to this species resembles that of Homalium planiflorum (Boivin ex Tul.) Baill. (Blackwellia planiflora Boivin ex Tul.) versus Blackwellia gracilis Blume (see Applequist, 2017Applequist, , 2018b, but without disruptive nomenclatural consequences. Blume published Nisa retusa based on material of Bernier 173 a year before Tulasne published N. scleroxylon based mostly on the same collection. Blume specified that his name was based only on material sent from P to L. That material was described in the protologue (Blume, 1856(Blume, -1857 as "Nomine Asteropeia et Mourangia Touditsch ex Herb. Mus. Paris. mecum benevole communicata." Three sheets at L bore labels initially marked "Asteropeia [or Asteropeja] multiflora" then "Mourangia [or Morangia] touditch" (a vernacular name, though not identified as such on those sheets); Blume later added "Nisa retusa" in the top margins. Much more recent pink typed labels identify these sheets as being type material of N. scleroxylon, "leg. Bernier 173".
One year later, Tulasne published N. scleroxylon, citing "Bernerii herb., n. 173; Galdich. herb., n. 296, in phytotheca Jalbertiana" (Tulasne, 1857: 70). These are Latinizations for Bernier and Gaudichaud-Beaupré respectively. Although Tulasne presumably used only those duplicates of Bernier 173 to be found in France, he did not say so. The protologue (Tulasne, 1857: 70) further states: "Marangue-Touditch (1), auctore Bernerio, vernaculare audit…", explaining the labeling seen, and apparently misinterpreted, by Blume. Three duplicates of Bernier 173 exist at P; two were labeled as having the vernacular name "Maranguá touditch" and one had initially been labeled "Asteropeia". Though the sheets sent to L were not initially labeled as Bernier 173, they can be confidently assigned to that collection. Sleumer (1973: 299) referred to Bernier 173 (L) as the "holotype" of Nisa retusa and to Bernier 173 (P) as the "holotype" of Nisa scleroxylon. For N. retusa, the original material was already restricted to the three sheets at L, which are syntypes, and Sleumer made no choice between them. The sheet that appears most complete is here chosen as a lectotype. For N. scleroxylon, Sleumer's statement constitutes a first-step lectotypification (Art. 9.17 of ICN; Turland et al., 2018). This action means that N. retusa and N. scleroxylon are not nomenclatural synonyms, since their types are different specimens, even though they are from the same gathering. The best of the sheets at P is here chosen as a second-step lectotypification.  upper (inward-facing) surface; petals oblong-ovate to elliptic, 3.5 -4 mm after anthesis to in fruit, both surfaces densely (to moderately) pubescent at least on apical half; filaments sparsely pubescent to sparsely pilose, 1.6 -2.5 mm; anthers 0.6 -0.8 mm; styles ca. 2.4 -3.4(-4?) mm. Distribution, ecology and conservation status. -Homalium tenue is found in low-elevation forests in the southeastern regions of Anosy and Atsimo-Atsinanana; it is reported on sand and laterite. Although the localities in Anosy are variously described, all come from a single small region, so that not more than five distinct subpopulations are known. The Extent of Occurrence and Area of Occupancy are calculated including an unusual collection that may be of hybrid origin (Service Forestier 23622, discussed below) on the grounds that if it is a hybrid, the parental species is likely also to be present in the forest in question. The Extent of Occurrence (EOO) is calculated as c. 908 km2 and the Area of Occupancy as 28 km2. Habitat is largely unprotected and because low-elevation forest is easily accessed, it is very vulnerable to anthropogenic damage from wood harvest and forest clearing. A preliminary assessment of its conservation status is therefore "Endangered" [EN B1ab(iii)+2ab(iii)].
Notes. -Homalium tenue overlaps in distribution with H. nudiflorum, which has sometimes similar leaves. Homalium tenue differs in the denser indument on the petals and calyx cup, usually extending to the basal medial portion of the sepals, and its flowers are smaller; its slender twigs and inflorescences give it a more delicate appearance than H. nudiflorum, though it is reportedly a sometimes large tree. The leaves are usually quite small (the inclusion of Service Forestier 23622, which in most other features is consistent with the species, significantly increases the leaf size range). Collections from Atsimo-Atsinanana have more prominently acuminate and often larger leaves than those from Toliara. Two other species, H. antilahimenae and H. ciliolatum, have dense pubescence on both petals and portions of the calyx; these two and H. tenue are suspected to be a natural group. Those two species have at least some indument on both sepal surfaces, with longer indument on the calyx cup, and their flowers are larger at maturity; they lack the gracile appearance of H. tenue. Homalium ciliolatum, which overlaps in geographic range, has usually larger and broader leaves, flowers usually borne in pairs, and large pubescent bracts and bracteoles; the calyx cup is long-sericeous and the sepals pubescent throughout on both surfaces. Homalium antilahimenae is native to northern Madagascar and has often larger leaves with some marginal glands, often densely pubescent rachises, and bracts and bracteoles often pubescent throughout.
Hybridization between Homalium tenue and H. louvelianum has been observed (see under the latter species above).