Homalium axillare (Lam.) Benth.

Applequist, Wendy L., 2018, A revision of the Malagasy species of Homalium sect. Blackwellia (Salicaceae), Candollea 73 (2), pp. 221-244 : 225-226

2. Homalium axillare (Lam.) Benth. in J. Proc. Linn. Soc., Bot. 4: 34. 1859.

≡ Blakwellia axillaris Lam., Encycl. 1: 428. 1785.

Lectotypus (designated by SLEUMER, 1973: 255): MADAGASCAR: sine loco, s.d., fl., Commerson s.n. ( P-JU 14407 image seen; isolectotype: G [ G00018417 , G00018418 , G00018419 ] images seen, P [ P00624058 ]!) .

Tree to 16 m, 26 cm dbh, or large shrub; young twigs glabrous or minutely papillate. Leaves elliptical to narrowly or broadly elliptical (seldom to ovate, obovate, or rarely lanceolate), (2–)2.5–7.5(–9.3) × (0.9–)1.1–3.8(–4.3) cm; petiole (1.5–) 2–8 mm; base convex (rounded, cuneate); apex acute to rounded or obtuse (somewhat acuminate); margins crenateserrate, with small or elongated glands at apices of teeth, to subentire; adaxial surface drying darker, brown to greenish or grayish, abaxial surface pale to medium brown (greenish). Inflorescences racemose, 1.5–14.5 cm, pubescent; flowers pedicellate with pedicels 0.2–1.8(–2.5) mm (subsessile); bracts oblong-ovate to ovate or lanceolate (suborbicular, oblongobovate, deltoid), 0.5– 1.1(–1.6) mm. Flowers 7– 10-merous (most commonly 8-merous); calyx tube narrowly funnelform to basally rounded, pubescent; sepals narrowly oblanceolateoblong to ligulate or narrowly oblong with acute apex, 0.8–3.7 mm; petals narrowly oblanceolate-oblong to ligulate or narrowly oblong with acute apex, 0.8–3.5 mm; sepals and petals whitish to cream or yellow-green (possibly becoming pink or reddish in fruit), accrescent, longer than calyx tube (very rarely similar in length), ascending to irregularly spreading or partly reflexed (most often in fruit), ciliate with long trichomes, both surfaces sparsely short-pubescent to glabrous; sepal glands glabrous; filaments 0.8–2.4(–3.5) mm, sparsely villous (to glabrate); ovary hispid or pilose to long-pubescent; styles 3–4(–5), 1.3–3 mm (apparently accrescent), basally hispid to pilose.

Notes. – There is substantial geographic variation in morphology within H. axillare , which is the commonest species in its section. Specimens from the province of Toamasina have usually relatively large, crenate leaves with usually large, elliptical marginal glands that may occupy much of the apical margins of teeth; the leaves usually dry brown. The type of the species, the locality of which is unknown, falls into this group, as do a few specimens from Antananarivo, Fianarantsoa, and eastern Mahajanga. Their inflorescences are usually 4–14.5 cm long and mostly borne near twig apices, so that they often extend much beyond the leaves. (However, rare specimens from this range have short or poorly developed inflorescences.) Most specimens, including all from Toamasina, have filaments 2–3.5 mm long, though outlying specimens sometimes have short filaments <1.5 mm long.

Specimens from the remainder of the species’ range have relatively short inflorescences (1.3 – 6 cm, rarely to 8 cm). Their leaves may be toothed or virtually entire, and when they are toothed, the marginal glands are usually small and nearly round; the leaves are much more likely to dry greenish to grayish brown on at least one surface. Their filaments are usually short, 0.6–1.4 mm, though a few specimens with filaments up to 2.4 mm are placed in this group because of their overall morphology. These populations are herein recognized as H. axillare subsp. delphinense (H. Perrier) Appleq. , while the variant native primarily to Toamasina is treated as subsp. axillare . Because there is variation within as well as among subspecies in all of the potentially distinguishing characters, they are presumed not to be reproductively isolated. A group of specimens collected contemporaneously at Manongarivo (McPherson & van der Werff 16356, 16379, 16381, van der Werff & McPherson 13533) show particularly large variation within a single population, including some characters typical of H. axillare subsp. axillare .

In northern Madagascar and the province of Fianarantsoa, H. axillare subsp. delphinense generally occurs at moderate to high altitudes in humid forest, whereas in Toliara, most populations are from littoral or low-altitude coastal forests. Most specimens from Toliara have quite small, entire, pale-colored leaves, and the difference in ecological preferences might well suggest that they should be treated as a distinct subspecies. However, coastal specimens are known that have moderately large, prominently crenate, and/or brown-drying leaves, while a very small-leaved specimen from a high altitude in Toliara is known. Since it is not possible to separate ecologically distinct groups using morphology, they have not been formally recognized. However, the likelihood of geographically correlated genetic variation should be noted.

SLEUMER (1973) quoted among specimens seen “ Commerson (P, P-LA), ‘ Blackwellia de Madagascar’, P-JU 14407 holotype Blackwellia axillaris ”. Since multiple unnumbered Commerson duplicates exist that have been presumed to be part of the same gathering, this actually constitutes a lectotypification with an erroneous term used. No material corresponding to this collection is locatable in the Lamarck herbarium. There is a second sheet in the main collection in Paris [P00624058], which Sleumer labeled as the “type” of B. axillaris ; this is an isolectotype. A second specimen at P [P00624059], which bears a Herb. Mus. Paris, Herbier de A.N. Desvaux label, was determined by Sleumer as a “prob[able] isotype” of B. axillaris . Only the piece on the upper left of that sheet is actually Homalium axillare . The remainder appears to be H. planiflorum (Boivin ex Tul.) Baill. The former portion does resemble the Commerson material but there is no clear evidence that it derives from the same collection; the handwriting on the main labels is similar but does not appear to be identical. Hence that sheet is not considered here to be an isotype. There are also three specimens at G that do bear the name of Commerson in their labeling (which is variable) and that have been considered to be type material.

Key to the subspecies of Homalium axillare

1. Leaves mostly elliptical to broadly elliptical, (2.5–)3.6– 7.5(–9.3) × 1.5– 3.8(–4.3) cm; petiole 4–8 mm; margins crenate-serrate (crenulate, subentire), with glands usually elongated in tooth apices; adaxial surface drying darker, brown (greenish), abaxial surface brown (greenish); racemes (2.6–) 4–14.5 cm; filaments (1.3–) 2.1–3.5 mm ... ......................................... 2a. H. axillare subsp. axillare

1a. Leaves mostly elliptical to narrowly (broadly) elliptical, (2 –)2.5–6.5 × (0.9–) 1.1–3.6 cm; petiole (1.5–)2–4(–5) mm; margins crenate-serrulate to subentire, with glands round or slightly elongated; adaxial surface drying darker, usually greenish or grayish, abaxial surface pale to medium brown (greenish); racemes 1.5–8 cm; filaments 0.8–2.4 mm ..... ................................... 2b. H. axillare subsp. delphinense