Monotes hypoleucus var. angolensis (De Wild.) Meerts, 2017

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 : 184-185

9b. Monotes hypoleucus var. angolensis (De Wild.) Meerts , comb. et stat. nov.

Basionym: Monotes angolensis De Wildeman (1927a: 168) ; Bancroft (1937: 140; 1939a: 346, 379); Catarino et al. (2013: 267); Duvigneaud (1949: 46; 1961: 417); Exell & Mendonça (1951: 371); Figueiredo & Smith (2008: 67); Lebrun & Stork (1991: 144); White (1962: 262, fig. 46B).

Lectotype (designated here): — ANGOLA. Huila, Dongo, Forte Maria Pia , 8 February 1907, Gossweiler 2910 (lecto-: BR! [barcode BR0000008891259], isolecto-: BM! [barcode BM001046015], COI!, K! [barcode K000240362], LISC).

= Monotes oblongifolius Hutchinson (1931: 248) ; Bancroft (1939a: 349). ≡ Monotes angolensis var. oblongifolius (Hutch.) Duvigneaud (1949: 47) . Type:— ZAMBIA. Kaloswe, Hutchinson & Gillet 3765 (holo-: K! [barcode K000240340]).

= Monotes noldeae Bancroft (1936b: 226) ; Bancroft (1937: 137); Bancroft (1939a: 355); Catarino et al. (2013: 270); Figueiredo & Smith (2008: 67). Type:— ANGOLA. Malange, Huila, April 1933, Nolde 202 (holo-: BM! [barcode BM000603393]; iso-: BRLU! (fragm.), LISC!), syn. nov .

= Monotes carrissoanus Bancroft (1939a: 356 , 1939b: 110); Catarino et al. (2013: 267); Exell & Mendonça (1951: 371); Figueiredo & Smith (2008: 67). Type:— ANGOLA. Moxico, between Caxipoque and Munhango, 7 May 1937, Exell & Mendonça 1788 (holo-: BM! [barcode BM000603395]; iso-: BR!, BRLU! (fragm.), COI), syn. nov.

Diagnosis:— Differs from the type variety by the following traits: leaf blade smaller, 4–9(–10) × 1.5–4(–5) cm; lower surface with indumentum of extremely short (<0.1(–0.2) mm), generally yellowish, crispate hairs, generally denser in areoles than on reticulum; upper surface reticulate, generally almost glabrous.

Tree up to 17 m high; branchlets tomentellous, soon becoming glabrous, often striated. Leaf: petiole (3–) 7–20 mm long, relatively slender (1.5–2 mm in diameter); blade elliptic to ovate elliptic or obovate-elliptic, 4–9(–10) × 1.5–4.5(–5) cm, obtuse to rounded to slightly cordate at the base, more rarely cuneate, obtuse to truncate or slightly emarginate at the apex, sometimes acute ( f. oxyphyllinus ), discolorous; upper surface generally reticulate, at first with minute flexuous simple or fasciculate scattered hairs (<0.2 mm), most often becoming almost glabrous except on nerves (more rarely persistently puberulent), sometimes viscid, rarely glandular-granulose; lower surface smoothly tomentellous, often more or less yellowish, much more rarely whitish, with extremely short coiled or crispate hairs covering the interreticular areoles most often <0.2 mm long, with the lumen occupying a third of the diameter (ca. 5 μm) or less, yellowish to whitish; midrib prominent beneath; lateral nerves in (8–)10–13 pairs, with a very strong tendency to the formation of subsidiaries, nearly straight but incurved just before and generally without reaching the margin of the leaf, pubescent to almost glabrous; reticulum on undersurface often well visible, not hidden by hairs, glabrescent, more rarely tomentellous as the areoles and less visible. Inflorescences up to 2.5 cm long, axillary, relatively lax, few-flowered, greyish-tomentose, on a long slender peduncle. Flower: pedicel 3 mm long, sepals ovate 2–3 mm long, woolly-tomentellous; petals 6–8 mm long, densely sericeous-tomentellous; stamens with connective produced into a short triangular appendage. Fruit (6–) 8–12 mm in diameter, subglobose, woolly-tomentellous, generally conical and apiculate at the apex; wings shiny, red or purple, sometimes pale brown or straw coloured, (2–)3–5 × 0.4–1(–2) cm, generally narrowly oblanceolate to narrowly elliptic, markedly attenuate in the lower third, not hiding the fruit.

Variation:— Our circumscription of var. angolensis is somewhat broader than M. angolensis De Wild. sensu stricto. In its typical form (leaf blade with yellow lower surface, conspicuous dark reticulum, base and apex rounded), M. angolensis is a striking and easily recognized taxon; however, typical forms are less frequent than intermediates with other forms in the complex. First, the taxonomic value of the yellow pigment in the indumentum of the lower surface of the leaf has been overestimated. Some collections with greyish indumentum do not differ from typical collections in other traits (e.g. Liben 3816; Desenfans 2619; Duvigneaud 2545M1, 4636M1 (BRLU!); Lisowski, Malaisse & Symoens 11401 (POZG!). Secondly, in its typical form, var. angolensis has a very conspicuous reticulum not covered by hairs and darker than the areoles. However, in some forms the indumentum does cover the reticulum. Monotes noldeae , known from one or two collections from Angola, is one of such forms; specimens matching the type have been found in D.R. Congo (e.g. Duvigneaud & Timperman 2402M (BRLU!)). Such forms are closely related to Monotes oblongifolius , already synonymized with M. angolensis by Duvigneaud (1961) and because of a lack of any reliable discriminant character to maintain those taxa even at a low rank, we decided not to recognize them.

The four syntypes of M. angolensis differ in pubescence of the upper surface of the leaf; Gossweiler 2893 and Gossweiler 2910, from Angola, are persistently puberulent, while Delevoy 502 and Delevoy 523, from D.R. Congo, are almost glabrous (except on nerves); those four specimens are otherwise very similar. There is apparently geographic variation in pubescence, since most of the copious material from D.R. Congo is glabrous, while material from Angola tends to be more pubescent. This once again illustrates extensive species polymorphism in Monotes , and the need of a broad species concept. It seems appropriate to choose a specimen from Angola as the lectotype ; Gossweiler 2910 [barcode BR0000008891259] is here chosen because it has flowers while the specimen Gossweiler 2893 [barcode BR0000009860414] does not.

Monotes carrissoanus , another putative endemic of Angola known from only two collections, is very similar to typical var. angolensis , having a yellowish lower surface of the leaf and strikingly contrasting darker reticulum; phenotypes matching the type were collected in Upper Katanga (e.g. Plateau de la Manika, Uapacetum robynsii en bordure de steppe, 1957, Duvigneaud 2545M2 (BRLU!); Nzilo, dépression avec Monotes angolensis , 14 June 1957, Duvigneaud 3517 (BRLU!)); the new material from D.R. Congo shows that the supposed discriminant characters (particular pattern of tertiary venation, long petioles) represent individual variation occurring within the polymorphic var. angolensis and intermediates with other morphs occur in Katanga (e.g. Duvigneaud & Timperman 2704; Duvigneaud 4541M1 (BRLU!); Duvigneaud 4653 (BRLU!)). Thus, M. carrissoanus cannot be maintained even at low rank.

Dwarf forms are sometimes found (Plancke 121/1590, 121/1600 (BRLU!)), but it is unclear if such specimens represent suckers or a genuinely suffruticose variety.

The shape of leaf apex and the colour of lower surface of leaf are variable; two forms can be recognized, with a range of intermediates.