Homalium sect. Rhodonisa (Tul.) Sleumer

Applequist, Wendy L., 2020, A revision of Homalium sect. Rhodonisa (Salicaceae) endemic to Madagascar, Candollea 75 (2), pp. 245-268 : 247-249

Homalium sect. Rhodonisa (Tul.) Sleumer in Bull. Jard. Bot. Natl. Belg. 43: 300. 1973.

≡ Nisa [unranked] Rhodonisa Tul., as “ Rhodonisae ” in Ann. Sci. Nat., Bot. sér. 4, 8: 70. 1857.

Typus (designated by SLEUMER, 1973: 300): Nisa sanguinea Boivin ex Tul. (≡ Homalium sanguineum (Boivin ex Tul.) Baill. ).

Stipules axillary, free, usually rapidly caducous. Inflorescences axillary, spiciform to racemose or paniculate; bracts broad (sometimes rather short), bracts and bracteoles persistent. Flowers short-pedicellate, with pedicels articulated usually just below the receptacle, to subsessile or sessile; perianth 4–5(–6)-merous; sepals oblong, sometimes broadly or narrowly, to somewhat obovate, ovate, or elliptical (seldom to oblanceolate or lanceolate), slightly accrescent and usually curving over the fruit; calyx tube tubular with a narrowed base, often visibly ribbed or grooved, to funnelform, in fruit tubular to turbinate or ellipsoid; petals obovate to oblanceolate with rounded (to obtuse) apex, much longer than sepals, ascending (spreading), at least slightly accrescent (where known); sepals and petals ciliate, usually shortly, or not; sepal glands of moderate size, broadly elliptical to rounded (somewhat oblong or trapezoid), glabrous; stamens 1 per petal, inserted between glands; anthers varying from broadly oblong-elliptical with oblong-elliptical locules, the slits of dehiscence nearly parallel, to small, transversely elliptical with subglobose locules, widely separated lateral slits towards apex (sometimes apparently or nearly basifixed with connective not prominent); upper surface of ovary sometimes quite narrow, nearly flat to shallowly conical in flower, in fruit convex to conical (or little expanded); styles (3–)4–5, usually fused (or connivent) for at least half of length (sometimes only shortly). Locule of fruit narrowly cylindrical or ellipsoid (to narrowly obovoid), pubescent throughout; seeds 2–4 per fruit, small (possibly no mature seeds seen).

Notes. – Adequate molecular data to elucidate relationships among the very diverse, probably paraphyletic lineages of Homalium do not exist. The relationships of species of sect. Rhodonisa would be of particular interest. The genus as now circumscribed has an apparent division between Homalium sect. Homalium and similar sections (e.g., in Madagascar, Eumyriantheia and Nisa ) and sect. Blackwellia Benth. and related sections (primarily Polyanthera). Species that are typical of the former group have relatively few (4–7) broad sepals and petals; relatively large, broadly elliptical anthers; large sepal glands; and broadly funnelform calyx cups in flower. Sepals and petals are usually different in shape and may differ in size or accrescence. Typical species of sects. Blackwellia and Polyanthera have often more numerous, narrow sepals and petals, which usually are similar in shape; often small broad anthers with subglobose locules (though broadly elliptical anthers are also seen); and often narrowly funnelform to tubular, ridged calyx cups. The upper surface of the ovary is often prominently conical. Sect. Rhodonisa includes species that have relatively few, broad petals and sepals, like sects. Eumyriantheia and Nisa , but often elongated, narrow, ridged calyx cups, and sometimes also small “ Blackwellia -like” anthers with subglobose locules. These species are not easily classed as belonging to either group of sections, which is one reason for this author’s preference not to recognize subgenera ( APPLEQUIST, 2016a).

SLEUMER (1973) recognized two red-flowered species, H. sanguineum (Boivin ex Tul.) Baill. and H. rubriflorum Sleumer , and one white-flowered species, H. albiflorum , within sect. Rhodonisa. Red flowers are rather unusual in Homalium , though not unknown from other sections, and the two red-flowered species are also distinguished from the H. albiflorum complex in having long-branched paniculate inflorescences and flowers borne only 1 or 1–2 per node and bract (rather than in clusters of 2 or more). The taxonomy of the red-flowered species is not problematic, and no undescribed similar species were identified.

However, the populations with clustered whitish (occasionally to yellowish or greenish) flowers, which SLEUMER (1973) subsumed within H. albiflorum , appear actually to represent no fewer than ten distinct taxa. These seem to fall roughly into two clusters. Homalium albiflorum s.str. ( Fig. 1 ) and three new species, H. analavelonae Appleq. , H. ihosyense Appleq. , and H. phillipsonii Appleq. , have usually five petals (varying to four or six); the outer surfaces of the sepals are sparsely pubescent or pilose, and the petals are at least sparsely pubescent (to pilose or glabrescent) with ciliate margins. Two previously described segregate species, H. leucophloeum (Tul.) Baill. and H. baillonii Scott-Elliot , and four new species, H. megaphyllum Appleq. , H. pseudoracemosum Appleq. , H. rakotovaoi Appleq. , and H. vohitsiandrianense Appleq. , have usually four petals (seldom to five in two species), glabrous or glabrate sepals, and glabrous petals without ciliate margins (except that in H. pseudoracemosum sepals are sometimes sparsely pubescent and petal margins are sometimes ciliate). These groups differ in ecological preferences, with the H. albiflorum group preferring arid to subhumid western and northern habitats and the H. leucophloeum group mostly in central and eastern (or extreme northern) forests that are humid or at least less arid ( Fig. 2 ). An outlier is H. pseudoracemosum , a western species which is morphologically more similar to H. albiflorum than others in this group, and which may represent a transitional form between the two groups.

Key to the species of Homalium sect. Rhodonisa

1. Inflorescences usually panicles with well-developed branches (reduced to racemes); flowers borne singly or 1–2 per node; petals red ............................................... 2

1a. Inflorescences racemes, spikes, or short-branched racemiform panicles; flowers mostly borne in clusters of at least 2–3 per node (very rarely, most borne singly); petals whitish (to yellow or pale green) ................................. 3

2. Leaves elliptical to broadly (narrowly) elliptical, (4.5–)5–16 × 2.1–5.5(–8) cm with petiole 4–8(–10) mm; flowers 1–2 per node, pedicellate; petals 2.4–4 mm, sparsely appressed-pubescent on abaxial surface mostly towards base; Alaotra-Mangoro, mid-elevation forests ................. .......................................................... 10. H. rubriflorum

2a. Leaves broadly elliptical (elliptical, somewhat obovate), (5.6 –)7.7 – 11.7 × (3.8 –) 4.2 – 8.2 cm with petiole (9 –) 12 – 22 mm; flowers 1 per node, sessile; petals 4.4 – 6.3 mm, glabrous except for sometimes ciliate margins; Analanjirofo, low-elevation forests ................... .......................................................... 11. H. sanguineum

3. Young twigs pilose; leaves pilose, especially on veins; secondary veins strongly prominent, sometimes with small domatia in vein axils ............................ 7. H. phillipsonii

3a. Young twigs glabrous (rarely pubescent); leaves glabrous (rarely sparsely pubescent); secondary veins moderately to little prominent, almost always without (very rarely with) domatia ....................................................................... 4

4. Inflorescences at least partly short-branched racemiform panicles ........................................................................ 5

4a. Inflorescences racemose to spicate ............................... 6

5. Leaves usually mostly lanceolate to ovate, to narrowly elliptical or elliptical; inflorescences consistently racemiform panicles; rachis glabrous or glabrate (branches may be sparsely pubescent); petals 4(5), 3–5.5 mm, glabrous except margins sometimes sparsely short-ciliate ............. .................................................. 8. H. pseudoracemosum

5a. Leaves elliptical to broadly elliptical (ovate, perhaps very rarely lanceolate or narrowly elliptical); inflorescences usually mostly racemose; portions of rachis moderately to densely pubescent; petals 5(6), 2.6–8 mm, sparsely to moderately short-pubescent on both surfaces (rarely glabrescent) ............................................... 1. H. albiflorum

6. Free portions of sepals sparsely pubescent or pilose; petals pubescent (or pilose), usually sparsely (rarely glabrescent), the margins ciliate ....................................................... 7

6a. Free portions of sepals glabrous or glabrate except for sometimes ciliate margins; petals glabrous, the margins not ciliate (in H. leucophloeum rarely sparsely ciliate) ... 9

7. Peduncles (0.6–) 2.5–6 cm; rachis at least partly pubescent; anthers transversely (to broadly) elliptical; widespread ... .............................................................. 1. H. albiflorum

7a. Peduncles <0.6 cm, bracts and usually flowers borne to near base of inflorescence; rachis villous or pilose to glabrate; anthers broadly elliptical; endemic to southern regions ......................................................................... 8

8. Leaves broadly elliptical to elliptical or broadly obovate, 5.4–12.2 × 3–7.5 cm, relatively thin-textured; immature racemes 2–4 cm with rachis villous, possibly zigzag; pedicels 1–2 mm; sepals at anthesis oblong-ovate to oblonglanceolate, 1.5–1.7 mm, sparsely pubescent with few long appressed hairs outside, long-ciliate; petals at anthesis 3– 3.7 mm, accrescence unknown, sparsely appressedpubescent with sometimes long trichomes; Analavelona ......................................................... 2. H. analavelonae

8a. Leaves elliptical (to somewhat obovate or broadly elliptical), (4.7–)6.7–14 × (3–) 4–6.3 cm, thick-textured; racemes (3–) 4.5–11 cm with rachis moderately pilose to glabrate; pedicels 0.5–4.5 mm; sepals narrowly oblong, 1.8–3 mm, sparsely pilose outside, ciliate; petals 3.5–12 mm, strongly accrescent, sparsely pubescent to sparsely pilose; near Ihosy ....................................................... 4. H. ihosyense

9. Leaves ovate to broadly ovate (to lanceolate, elliptical), with margins irregularly and shallowly crenulate to crenate or crenate-denticulate (somewhat revolute); pedicels (1–)2.5–6(–8.5) mm ............................................ 10

9a. Leaves elliptical, broadly elliptical or oblong-elliptical (to narrowly elliptical, ovate) with margins subentire to shallowly repand or undulate; pedicels short, 0.5–1 mm, or flowers sessile ............................................................ 11

10. Leaves ovate (lanceolate to elliptical), (5.5–)6.7 – 15.7 × (2.8 –) 3.5 – 6.5 cm with acuminate (acute, obtuse, or emarginate) apex; racemes (3 –) 5 –16 cm; sepal glands 0.4–0.6 × 0.3–0.4 mm; northern Madagascar................ ............................................................. 9. H. rakotovaoi

10a. Leaves of type ovate to broadly ovate (aberrantly broadly elliptical), (4 –)5 – 7.8 × (2 –) 2.6 – 4.8 cm with acute (rounded, emarginate) apex; racemes (1.7–) 3–4 cm; sepal glands 1.2–1.3 × 0.8–0.9 mm; southeastern Madagascar (Vohitsiandriana) .................. 12. H. vohitsiandrianense

11. Leaves elliptical to oblong-elliptical (narrowly elliptical, ovate), 13–35 × (5–) 5.7–11.5 cm, usually thin-textured in proportion to their size (to moderately thick); inflorescences often partly cauliflorous on twigs below leaves; northern Madagascar...................... 6. H. megaphyllum

11a. Leaves elliptical or broadly elliptical to broadly obovate (obovate), (4.3–)5.2– 12(–15) × 2.1– 8.5 cm, moderately thick-textured; inflorescences usually all borne in leaf axils, on distal portions of twigs; southern or central Madagascar............................................................... 12

12. Tree to 12(–20) m or shrub; leaves broadly elliptical to elliptical, (5.3–)6.2– 12(– 15) × (2.8–) 3.5– 8.5 cm, with petiole (10–)14–28(–33) mm; flowers short-pedicellate or sessile; southeastern Madagascar, littoral or low-elevation forests ...................................................... 3. H. baillonii

12a. Tree to 30 m; leaves elliptical (ovate, narrowly elliptical), (4.3–)5.2–10(–12.4) × 2.1–4.5(–5.8) cm, with petiole (6–)9–14(–27) mm; flowers normally sessile (in rare individuals short-pedicellate); central to south-central Madagascar, mid-elevation forests ............. 5. H. leucophloeum

Figures

Fig. 1. – Homalium albiflorum (Boivin ex Tul.) O. Hoffm. s.str. [Schatz 4235] [Photo: G.E. Schatz]

Fig. 2. – Map of representative distributions of species in the Homalium albiflorum complex, approximated as necessary.