Homalium (sect. Blackwellia) Benth.
Creators: name: Applequist, Wendy L.
Publisher: name: Zenodo
Year: 2018
Subjects: Biodiversity
Taxonomy
Plantae
Tracheophyta
Magnoliopsida
Malpighiales
Salicaceae
Homalium

description: Homalium sect. Blackwellia Benth. in J. Proc. Linn. Soc., Bot. 4: 33. 1859.
≡ Homalium subg. Blackwellia (Benth.) Warb. in Engler & Prantl, Nat. Pflanzenfam. III(6a): 35. 1893.
≡ Blakwellia Comm. ex Lam., Encycl. 1: 428. 1785 [nom. illeg.] [non Blakwellia Scop.].
Lectotypus (designated by SLEUMER, 1954: 52): Blakwellia paniculata Lam. (≡ Homalium paniculatum (Lam.) Benth.)
= Astranthus Lour., Fl. Cochinch. 1: 221. 1790. Typus: Astranthus cochinchinensis Lour. (≡ Homalium cochinchinensis (Lour.) Druce).
= Pythagorea Lour., Fl. Cochinch. 1: 243. 1790. ≡ Homalium sect. Pythagorea (Lour.) Kuntze in Post & Kuntze, Lex. Gen. Phan., Prop.: 285. 1903. ≡ Homalium subg. Pythagorea (Lour.) Sleumer, Fl. Males. ser. 1, 5: 52. 1954. Typus: Pythagorea cochinchinensis Lour. [= Homalium cochinchinensis (Lour.) Druce, a heterotypic synonym].
= Homalium sect. Eublackwellia Warb. in Engler & Prantl, Nat. Pflanzenfam. III(6a): 35, 36. 1893. Lectotypus (designated by APPLEQUIST, 2016b: 251): Homalium paniculatum (Lam.) Benth (≡ Blakwellia paniculata Lam.).
= Homalium sect. Paniculata S.S. Lai [as Paniculatae], Bull. Bot. Res., Harbin 14: 222. 1994 [nom. illeg.] [non Homalium [unranked] Paniculata Benth.]. Typus: Homalium phanerophlebium F.C. How & W.C. Ko.
= Homalium sect. Racemosa S.S. Lai in Bull. Bot. Res., Harbin 14: 223. 1994 [as Racemosae] [nom. illeg.] [non Homalium [unranked] Racemosa Benth.]. Typus: Homalium ceylanicum (Gardner) Benth. (≡ Blackwellia ceylanica Gardner).
Stipules axillary, free. Inflorescences racemose (to spicate) or paniculate; bracts usually small or narrow, caducous, bracteoles absent [outside Madagascar, sometimes possibly very small and rapidly caducous]. Flowers pedicellate to subsessile (sessile) with pedicels usually short, articulated (at least post-anthesis); perianth 6–10(–11)-merous [outside Madagascar, 5–12(–15)- merous]. Sepals ligulate to narrowly elliptical (linear, oblong; seldom much reduced), usually modestly accrescent; calyx tube turbinate to narrowly funnelform or tubular, at least in fruit (seldom short, funnelform), usually longitudinally ridged; sepal glands usually small and rounded, rarely elliptical. Petals similar in shape and size to sepals or narrowly elliptical to spatulate or oblanceolate (narrowly obovate, oblong-ovate, narrowly deltoid) [outside Madagascar, sometimes markedly longer than sepals], usually modestly accrescent; sepals and petals ascending to spreading (usually only in a few flowers at one time), sometimes reflexed in fruit or suberect, ciliate, usually with long or stiff trichomes [outside Madagascar, rarely not ciliate]. Stamens 1 per petal, inserted between sepal glands; anthers broader than long, usually small, with subglobose locules diverging at a broad angle, slits of dehiscence often toward the apex [outside Madagascar, sometimes dorsifixed, broadly oblong-elliptical with oblong-elliptical locules and a large connective, the slits of dehiscence nearly parallel]. Ovary conical, usually narrowly, in flower (to nearly flat), prominently conical in fruit; styles (2–)3–5(–6) [reportedly 7 in H. barandae S. Vidal ex Fern. -Vill.], free to near base. Locule of fruit usually elongated [outside Madagascar, rarely to subglobose in very short-flowered species], conical above, pubescent (to glabrate) at least on lower portion; seeds usually several per fruit, small, confined to apical portion of ovary.
Notes. – Homalium sect. Blackwellia is the most widespread section of Homalium, ranging from Madagascar, South Africa and the Mascarenes to Asia, Malesia, Australia, and various Pacific islands. Its characteristic features include an ovary whose free portion is usually relatively narrow and strongly conical, at least in fruit; an elongated or narrow, often prominently ridged calyx tube; often narrow, often relatively numerous sepals and petals that are usually ciliate and moderately accrescent in fruit; and the absence of paired bracteoles below flowers. All Malagasy species of the section have glabrous leaves and small anthers with subglobose locules; their sepals and petals are similar or only modestly different in size and usually not very different in shape, and have similar indument.
Three species formerly recognized in Homalium sect. Blackwellia (SLEUMER, 1973), namely H. brachystylis (Tul.) Baill., H. longistaminum H. Perrier, and H. planiflorum (Boivin ex Tul.) Baill., have been transferred to Homalium sect. Odontolobus (APPLEQUIST, 2016b, 2018). That section, which like several others has only one stamen per petal, is more similar to Homalium sect. Blackwellia than any other Malagasy section and might have evolved within Homalium sect. Blackwellia as presently defined. Homalium sect. Odontolobus has usually broad and persistent bracts, and usually two small bracteoles per flower; the flowers are frequently borne in clusters. The flowers have relatively broad calyx tubes, small perianth parts that are usually more or less spreading (with the sepals often much smaller than the petals) and hardly accrescent, and an ovary that is nearly flat to shallowly conical at anthesis, becoming nearly hemispherical rather than narrowly conical in fruit. The ovary locule is sparsely pubescent to glabrous and, though mature seeds are seldom seen, there is often only one large seed that fills the locule.
Fruits of Homalium are very small capsules, with few significant characters distinguishing species, and the flowers are typically lost as a unit during maturation, so specimens with mature seeds are too rare to be useful. Hence, only floral and vegetative characters are used in the circumscription of species.
Though Blakwellia Lam. was illegitimate as a later homonym, many species of Homalium sect. Blackwellia and sect. Polyanthera Warb. sensu APPLEQUIST (2016b) were first published within that genus. The vast majority of later authors corrected the spelling of the name to Blackwellia, and authors’ original usage is followed in the provision of synonymies herein. Some nomenclaturists might dispute whether Lamarck’s spelling is an orthographic error that may be automatically corrected under Art. 60.1 of the International Code of Nomenclature (TURLAND et al., 2018). Since Lamarck’s illegitimate name cannot be treated as a basionym of sect. Blackwellia Benth., which was validly published as a new taxon, the spelling of the sectional name would be unaffected by that question.
Key to the Malagasy species of Homalium sect. Blackwellia
1. Sepal glands glabrous .................................................. 2
1a. Sepal glands pubescent to pilose ................................. 5
2. Inflorescences paniculate (rarely partly racemose) .......... ............................................................. 6. H. erianthum
2a. Inflorescences racemose ............................................... 3
3. Leaves narrowly elliptical (oblanceolate), 2.1 – 3.3 × 0.5–0.7 cm ...................................... 5. H. decaryanum
3a. Leaves variably shaped, usually mostly elliptical (broadly or narrowly) or mostly lanceolate, variable in length but consistently> (0.9–) 1.1 cm broad; widespread ............. 4
4. Leaves lanceolate (to narrowly lanceolate or ovate), leaf apex long-acuminate; petals often spreading at anthesis; Toliara ........................................ 1. H. acuminatifolium
4a. Leaves elliptical to narrowly or broadly elliptical (seldom to ovate, obovate, or rarely lanceolate), leaf apex acute to rounded or obtuse (somewhat acuminate); petals usually ascending (to irregularly spreading) at anthesis; widespread ....................................................... 2. H. axillare
5. Inflorescences paniculate, sometimes mostly racemiform with flowers in short clusters then partly cauliflorous .... ..................................................................................... 6
5a. Inflorescences racemose, not cauliflorous .................... 9
6. Panicles mostly short-branched to racemiform, partly cauliflorous on large twigs below the most distal leaves; inflorescences, calyx tube and abaxial surface of sepals and petals pilose, usually densely so; sepals and petals linear (narrowly spatulate) ............................ 4. H. cauliflorum
6a. Panicles mostly with few well-developed lower branches (partly racemose), lateral (partly pseudoterminal; occasionally borne on small twigs below leaves but not cauliflorous on large twigs); inflorescences, calyx tube and abaxial surface of sepals and petals pubescent (pilose, sparsely so on sepals and petals); sepals and petals ligulate (narrowly lanceolate) ................................................... 7
7. Leaves elliptical (to narrowly elliptical or somewhat obovate), (3–)3.5–7.6(–9) × (1.7–)2–3.3(–3.7) mm; petioles 2–12(–15) mm; sepals and petals narrowly lanceolate (ligulate), much shorter than (to slightly shorter than) calyx tube ................................................ 9. H. martialii
7a. Leaves variable in shape and size, if mostly elliptical to narrowly elliptical then mostly> 7.5 × 3.5 cm; petioles ≤ 6.5 mm; sepals and petals ligulate (to narrowly lanceolate), longer than calyx tube to slightly shorter ............ 8
8. Young twigs glabrous; leaves oblanceolate to obovate (oblong-oblanceolate, elliptical), (2.8 –)3.5 – 7.8(– 10) × (1.2–)1.6–3.8(–5.2) cm, with base cuneate to convex, apex rounded to somewhat obtuse or acute with rounded apex (shallowly emarginate, rounded-cuspidate), margins entire or subentire (revolute, slightly repand); sepals and petals longer than calyx tube .......... 14. H. thuarsianum
8a. Young twigs minutely pubescent; leaves elliptical to narrowly elliptical (broadly elliptical to obovate in abnormally small leaves), (3.4–)7.5–13 × (1.4–) 3.5–5.3 cm, with base convex (to rounded), apex cuspidate with a rounded tip (rounded), margins shallowly repand; sepals and petals slightly shorter than calyx tube to slightly longer ........... ............................................................... 7. H. fortunatii
9. Inflorescences glabrous; sepals and petals minutely pubescent with a narrow dark stripe at midrib; dry forest in Mahajanga on limestone ................. 3. H. brachyrhachis
9a. Inflorescences pubescent (to pilose or villous); sepals and petals with variable indument, without a distinct dark stripe; eastern to northeastern and central humid forests ................................................................................... 10
10. Flowers (7 –)8 – 9-merous; sepals and petals linear to narrowly ligulate (sepals narrowly spatulate), 2.5–5.5(–6.8) mm, always much longer than the narrow or short calyx tube; inflorescences moderately pilose, calyx tube and abaxial surface of sepals and petals sparsely to moderately pilose ............................... 13. H. retivenium
10a. Flowers 6– 7-merous (to 8-merous in H. micranthum); sepals and petals variable in shape, often ligulate but not linear, to 4.4 mm or less, the ratio of sepal and petal length to calyx tube length variable (often including within species); inflorescences, calyx tube and abaxial surface of sepals and petals pubescent to glabrate (to glabrous, or sparsely pilose in H. perrieri) ..................................... 11
11. Leaves narrowly lanceolate to narrowly elliptical (obovate-elliptical), 7.3– 15 × 2.6– 3.9 cm with petiole (6.5–) 9–18 mm; racemes 9–25 cm .................................. ..................................................... 8. H. longiracemosum
11a. Leaves variable in shape but not narrowly lanceolate or narrowly elliptical, at most 9.5 × 3.4 cm with petiole to 5(– 6) mm (in most species smaller); racemes at most 10 cm, in most species always <7.5 cm ...................... 12
12. Flowers sessile (short-pedicellate); sepals lanceolateoblong to oblong or obovate, petals oblong-elliptical to obovate, oblong or spatulate, apices acute to cuspidate or rounded .................................................. 15. H. viguieri
12a. Flowers pedicellate (subsessile); sepals ligulate to narrowly oblanceolate (narrowly elliptical), petals ligulate to spatulate or narrowly lanceolate (narrowly elliptical, narrowly oblanceolate-elliptical), apices always acute ............... 13
13. Leaves obovate, at most 3 × 1.5 cm, with cuneate to convex base and rounded apex, margins few-toothed near apex or subentire with small rounded glands sunk in margins, secondary veins ascending at a narrow angle to midrib, adaxial leaf surface drying pale brown, abaxial surface greenish ............................................ 11. H. myrtifolium
13a. Leaves variable in shape and size, mostly elliptical to oblong (partly somewhat obovate), margins toothed at least on apical half with elongated glands in tooth apices that often produce exudate, secondary veins at a broad angle to midrib, adaxial leaf surface drying brown to dark brown (greenish), abaxial surface brown (olive) ......... 14
14. Leaves 0.5 –2.3 × 0.3– 1.4 cm; inflorescences (0.8–)1.4 –3(–3.8) cm ............................................ 12. H. perrieri
14a. Leaves (2.4 –)3.7 – 9.5 × 1.3 – 3.4 cm; inflorescences (1.8 –) 2.5 –10 cm (often paired, sometimes very numerous) .................................................. 10. H. micranthum
description: Published as part of Applequist, Wendy L., 2018, A revision of the Malagasy species of Homalium sect. Blackwellia (Salicaceae), pp. 221-244 in Candollea 73 (2) on pages 222-225, DOI: 10.15553/c2018v732a7, http://zenodo.org/record/5724607