Megalastrum lanuginosum (Willd. ex Kaulf.) Holttum

Description

by: G. Rouhan & R.C. Moran

Type: Mauritius. S. loc., s.d., Aubert du Petit-Thouars s.n. (holotype, B-W 19808!, 3 sheets).

Description: Rhizomes massive, erect or procumbent, can be arborescent (like a small tree fern) forming a trunk to 50(-100) cm high and 15 cm thick, scaly, the scales up to 30 × 3 mm, linear, thin, translucid, light golden brown, papyraceous, subentire and irregularly dentate-serrulate toward the apex; fronds up to 2.5 m long, petiole sulcate; petiole bases subglabrous, rarely bearing few scales similar to those of the rhizome, but smaller, up to 4 × 0.5 mm; laminae up to 1.5 m long, deltate, triangular to pentagonal; tripinnate-pinnatifid to tripinnate-pinnatisect (rarely quadripinnate-pinnatifid) at base, bipinnate-pinnatifid or bipinnate-pinnatisect medially (rarely tripinnate-pinnatifid); basal pinnae up to 100 cm long, the stalk up to 3.5 cm long, strongly inequilateral with the basal basiscopic pinnules much more elongate, the most basal basiscopic pinnules (to 35 cm long) up to 5 times the length of the most basal acroscopic ones, the pinnules on the acroscopic side slightly reduced toward the base of the pinna; pinna rachises abaxially puberulent to pubescent, eglandular, the hairs erect to spreading, 0.2-1.0 mm long, to 8-celled, very sparsely scaly, the scales 0.5-1.0 × 0.2-0.3 mm, linear, entire, flat (non-bullate), dark brown to light brown, adaxially densely pubescent, the hairs erect, 0.3-1.0 mm long, 5-8-celled, septate, eglandular; costules abaxially pubescent, glandular or eglandular, the glands spherical, yellowish, shiny, sessile or stalked, ca. 0.1 mm diam., 1-2-celled, the hairs spreading, (0.2-)0.5-1.7 mm long, 6-8-celled, very sparsely to abundantly scaly, the scales 0.3-1.5 × ca. 0.3(-0.5) mm, flat to conspicuously bullate, entire, light brown, dull, rarely clathrate, adaxially pubescent, non glandular, the hairs erect to spreading, 0.5-0.7(-1.2) mm long, 3-5-celled, septate; laminar tissue between veins from pubescent on both surfaces to subglabrous on both surfaces, abaxially eglandular or sparsely glandular, the glands like those of the costules, hairs erect to spreading, 0.2-1.2 mm long, 3-8-celled, thicker on the adaxial surfaces; veins 3-5 pairs per pinnules lobe, visible on both surfaces, ended at or close to (rarely behind) lamina margins (excepted the veins with sori which end well behind lamina margins) in enlarged clavate tips (conspicuous hydathodes), abaxially pubescent and sparsely glandular to non glandular, glands spherical, yellowish, shiny, sessile or stalked, ca. 0.1 mm diam., hairs erect, 0.3-1.0 mm long, usually 4-6-celled, adaxially pubescent, hairs erect to spreading, 0.4-0.8 mm long, 4-6-celled; laminar margins ciliate, hairs similar to those on laminar tissue; sori on the acroscopic branches of forked veins, indusia present and persistent, covering sporangia, orbicular-reniform, up to 1.5 mm diam., dark brown, shiny, densely pubescent to glabrous, densely glandular or eglandular, glands 1- or 2-celled, yellowish, shiny, hairs up to 0.7 mm long, margins dull and erose.

Etymology: From the Latin lanugo, wool, down, referring to the pubescence of the leaves. Although this is the condition of most specimens of Megalastrum lanuginosum, some specimens are only sparsely pubescent.

Distribution: Disjointed distribution between tropical West Africa, where restricted to Bioko and São Tomé, and eastern Africa and Madagascan region, the latter including East tropical Africa, eastern South Africa, Madagascar, Mascarene Islands (La Réunion and Mauritius), and the Comoros (Figure 1). It is rare or perhaps extinct in the Mascarene Islands (no collection since 1925 although it was recently searched in La Réunion; E. Grangaud, pers. com.), rare and local in Gulf of Guinea Islands (Benl, 1991), but can be locally common in East Africa. Terrestrial in deep shade of evergreen forests, in swampy areas, most frequently observed near or on banks of permanent stream. Also present in cloud forests, and mentioned once in rock fissure. It can be like a small tree fern, likely up to 1 m high, according to the annotation on Schelpe 1689.

Comments: Megalastrum lanuginosum differs from the two endemic Mascarene species by laminae at least tripinnate-pinnatifid at base (rarely quadripinnate), appearing more finely dissected compared to the bipinnate-pinnatifid laminae in M. lanatum and M. canace. Other useful differentiating characters include the scales of the rhizomes that are golden brown, papyraceous, with small irregular marginal teeth toward the apex, and the large, conspicuous and persistent indusia. Veins usually end at or close to (rarely behind) the laminar margins, which is remarkable in the genus. Only the Neotropical M. inaequale (Kaulf. ex Link) A. R. Sm. & R. C. Moran, shows this character. Other morphological characters are extremely variable in M. lanuginosum. Although bullate scales are found only in M. lanuginosum and observable on almost all specimens, their density and size show great variation among specimens. Contrary to Holttum (1983), we found few small bullate scales on Mascarene specimens (to see these scales, careful inspection is required on the abaxial surfaces of the costules with at least 10 x). Because numerous morphological intermediates exist from densely to sparsely scaly costules, and from large bullate to small bullate scales, separate species cannot be distinguished on the basis of this character. Also, the indusia vary in the presence and abundance of hairs and glands, and in the length of hairs. No other character apparently correlates with this, and therefore we interpret the variations as phenotypic plasticity within a single species. Megalastrum lanuginosum varies greatly in indument, especially as to the location, abundance, and length of hairs. These characters, overlap with those of M. lanatum. The variation does not correlate with geography. For example, the length of hairs on pinna-rachises, costules, and veins varies even within the Mascarene Islands: most of the specimens have long hairs (> 0.5 mm long) whereas two specimens (collector unknown s.n. in Herb. Desvaux in La Réunion, P00610737; Bory de St Vincent s.n. in Mauritius, P00483107 and P00477882) have shorter hairs (< 0.4 mm long). Similarly, in Bioko, laminar tissue between veins is glabrous in some specimens, pubescent in others. The two following names should refer to M. lanuginosum: Aspidium odoratum Mett., nom. superfl., Abh. Senckenberg. Naturf. Ges. 2: 399. 1858, non Bory ex Willd., Sp. Pl., ed. 4, 5: 286. 1810; Lastrea odorata C. Presl, nom. nud., Tent. Pterid: 77. 1836.

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