Monotes hypoleucus (Oliver) Gilg (1899: 134)

Meerts, Pierre, Rougelot, Quentin & Sosef, Marc, 2017, Revision of the genus Monotes (Dipterocarpaceae) in D. R. Congo, with implications for Angola and its distinction from Marquesia, Phytotaxa 308 (2), pp. 151-205 : 181-182

9. Monotes hypoleucus (Oliver) Gilg (1899: 134)

≡ Vatica africana Welw. ex Oliver (1868: 173) var. hypoleuca Oliv. ≡ Vatica africana Welwitsch (1869: 15) var. hypoleuca Welwitsch (1869: 17 ; Tab. 5: Fig. 12 ).

Lectotype (designated here): — ANGOLA. Huíla, Humpata , in schwach schattingen wäldern, December 1859, Welwitsch 1036 (lecto-: LISU! [barcode LISU234079]); paralectotypes: Welwitsch 1036 (LISU! [barcode LISU234080]); BM! [barcode BM001046018], COI!, P! [barcode P00389099]).

Shrub, small tree or tree up to 15(–20) m; branches grey-tomentose to tomentellous, quickly glabrescent. Leaf: petiole (3–)7–21(–40) mm long, 2(–3) mm thick; blade elliptic or ovate-elliptic to obovate-elliptic, (4–)6–16(–21) × (1.5–)2.5–9(–13.5) cm, ca. two times longer than wide, base rounded to slightly cordate, more rarely cuneate, apex rounded to emarginate, very rarely acute, markedly discolorous; secondary nerves in (8–)10–17(–20) pairs, nearly straight but incurved just before and generally without reaching the margin of the leaf, often with a tendency to form shorter subsidiary nerves; upper surface puberulent to almost glabrous, glabrescent, with dense to sparse indumentum not discernible by touch, only visible with a microscope, of more or less flexuous thin hairs 0.1–0.15(–0.25) mm long, most often stellate, in fascicles of 2–6, or, more rarely, simple, variable in shape, never straight, irregularly curled often mixed with yellow globulose glands; lower surface of leaf always discolorous, whitish, cream, beige, yellowish or grayish; secondary nerves and midrib prominent beneath, conspicuous, generally less pubescent and darker than the rest of the lamina, fulvous, brownish, reddish, glabrescent; areoles masked by hairs; reticulum masked or not by pubescence; indumentum of lower surface of leaf extremely variable in thickness and density, short to thick, comprising cottony, curly, crispate or woolly hairs (<0.1–) 0.1–1.5 mm long, often masking a layer of short stellate hairs concentrated in areoles. Inflorescence composed of pedunculated lax axillary cymes 2.5–8 cm long, peduncle densely beige-, grayishto rufous-tomentose. Flower: pedicel 1–3 mm long, sepals ovate 2–3(–5) mm long, woolly-tomentellous; petals 6–9(– 10) mm long, stamens with connective produced into a conspicuous, appendage. Fruit (6–)8–15(–20) mm in diameter, subglobose, sometimes conical and apiculate at the apex, woolly-tomentellous; wings elliptic to narrowly obovate, usually not unguiculate, (2–)2.5–5·5 × (0·4–) 1–2 cm, variable in colour, pale brown, straw-coloured, red or purple, variable in shape, generally narrowly oblanceolate to narrowly elliptic, spathulate, markedly attenuate in the lower third, sometimes suborbicular, not hiding the fruit.

Discussion: —We here propose a much more synthetic treatment than earlier revisions, treating M. angolensis (incl. M. carrissoanus , M. noldeae , M. oblongifolius , M. oxyphyllinus ), M. caloneurus (incl. M. elegans , M. dawei , M. schmitzii ), M. discolor , M. loandensis , at varietal rank within a much extended species concept for M. hypoleucus . With this broader circumscription, M. hypoleucus is easily defined by its more or less discolorous leaves, with pubescent greyish, whitish or yellowish lower surface, and upper surface almost glabrous to puberulent with indumentum of very short fasciculate or stellate hairs.This redefined species shows extensive variation in leaf size, density and persistence of upper surface indumentum, and in thickness, extent and colour of the indumentum on the lower surface. That variation is extremely difficult to translate into a coherent taxonomic system. Most characters have a continuous variation within the complex. The five varieties retained here represent the most common nodes of a reticulate variation pattern. In our judgement, however, it is useful to distinguish these varieties and thus show the structure present in the pattern, which might be linked to ecology and/or geography. A fair number of intermediate specimens exist, which can conveniently be referred to as M. hypoleucus s.l.

Bancroft (1939a) already emphasized the close affinity between M. angolensis and M. hypoleucus . Duvigneaud (1949) was aware that the abovementioned “species” might represent varieties rather than true species and mentioned the existence of intermediates (p. 47, about M. elegans , M. angolensis , M. schmitzii : “Il est d’ailleurs fort probable qu’il ne s’agit là que de variétés d’une même espèce…”; p. 48, about M. hypoleucus and relatives: “…très apparentées aux espèces du groupe caloneurus-angolensis, avec lesquelles elles semblent former souvent des hybrides.”). Verdcourt also suspected that M. caloneurus , M. elegans and M. angolensis were not distinct at species rank, but “…did not want to tamper with the classification of Duvigneaud.” ( Verdcourt 1989: 10). A critical reexamination of Duvigneaud’s collections is particularly instructive, since a fair amount of material belonging to this complex was left by him without a name; quite often, Duvigneaud’s hesitations are apparent in the form of several name changes in schedis. Obviously, Duvigneaud’s treatment, as accepted by all subsequent authors, was an overly simplified representation of a much more complex biological reality.

Taxonomic difficulties are also aggravated by the ontogenic changes in hair density on both surfaces, with the upper surface of old leaves eventually becoming glabrous in all morphs, and the lower surface changing colour with ageing.

Figures

FIGURE 12. Colour photographs of leaves: Monotes hypoleucus var. caloneurus. A. Duvigneaud 4764, B. Duvigneaud 4850, C. Duvigneaud 3203, D. Duvigneaud 3338, E. Duvigneaud 2123, F. Duvigneaud 5095, G. Duvigneaud & Timpeman 2109. Scale bar = 5 cm.