Megalastrum taafense Rouhan, Sundue & R.C.Moran

Description

by: M. Sundue, G. Rouhan and R.C. Moran

Description: Plants terrestrial, rhizomes erect, 2.7 cm diam., not forming large erect trunks, the scales 35.0 × 1.0-1.5(-2.3) mm, linear lanceolate, thin, brown, translucent, irregularly twisted, subentire, bearing few teeth distally; fronds up to 80 cm long, petioles sulcate; petiole bases scaly with scales similar to those of the rhizome, but shiny dark brown; laminae up to 41 cm long, lanceate to triangular; 2-pinnate-pinnatisect (rarely tripinnate) at base, 2-pinnate-pinnatifid to 2-pinntate-pinnatisect medially; basal pinnae up to 23 cm long, short-stalked, the stalk up to 1.5 cm long, slightly to strongly inequilateral with the basal basiscopic pinnules more elongate, the most basal basiscopic pinnules (to 15 cm long) 1.4-2.3 times the length of the most basal acroscopic ones, the pinnules on the acroscopic side not or only slightly reduced toward the base of the pinnae; pinnules sessile, the basal segments frequently overlapping the pinna rachis; rachises and pinna rachises puberulent to densely pubescent on both surfaces, non-glandular, the hairs erect, 0.2-0.5 mm long, to 5-celled, sparsely scaly, the scales up to 4.0 x 1.0 mm, lanceolate, entire, flat (non-bullate), shiny dark brown with clearer margins; costules abaxially pubescent, non glandular, the hairs erect to spreading, 0.2-0.5 mm long, to 5-celled, sparsely scaly, the scales similar to those of the pinna rachises but shorter, 1.5 × 0.5 mm, adaxially puberulent to pubescent, non glandular, the hairs erect to somewhat antrorse, 0.2-0.5 mm long; laminar tissue between veins glabrous on both surfaces; veins 3-6 pairs per pinnules lobe, visible on both surfaces, abaxially pubescent and sparsely glandular, the hairs erect, 0.2-0.5 mm long, usually 2-5-celled, the glands spherical, clear yellowish, shiny, short-stalked, ca. 0.1 mm diam., 1-2-celled, adaxially glabrous; lamina margins slightly recurved, sparsely ciliate, the cilia 0.3 mm long, 2- or 3-celled, fugacious indusia present, inconspicuous, 0.1-0.3 mm wide, provided with ca. 0.1 mm long sessile glands, and sometimes with 1-4 ca. 0.1 mm long setae.

Distribution: Amsterdam and St-Paul Islands, South Indian Ocean. Terrestrial in gullies, in shade of lava crevices and caves; observed along river and waterfall. Plants isolated, rare, 50-350 m.

Comments: Given some known mistakes in the labels by L'Isle, Tardieu-Blot (1954) postulated that this taxon -treated by her as Dryopteris aquilina (Thouars) C. Chr.- had been erroneously recorded from Amsterdam. Several recent and annotated collections attest the presence of this taxon on Amsterdam. The presence of this species on St. Paul however is still based on a single, collection (L'Isle 3). Until more recent collections can be made, some doubt remains as to the presence of the taxon on this Island. Megalastrum taafense resembles M. aquilinum from Tristan da Cunha in the cutting of the laminae, revolute margins, and fugacious indusia. However, M. taafense differs by having shorter hairs on the abaxial costules, and glabrous lamina tissue between the veins abaxially. It also lacks glands adaxially, whereas M. aquilinum sometimes has sessile sphaerical glands near the adaxial costae. The brown, non-bullate scales of the abaxial surfaces of the laminae, and the laminar surfaces glabrous between veins on both surfaces easily distinguish Megalastrum taafense from the 3 geographically closest tropical taxa: M. lanuginosum (Africa, Madagascar, Mascarene Islands, Comoros), M. canacae (Mauritius and La Réunion), and M. lanatum (La Réunion).

Etymology: From 'TAAF', the French acronym for the Territory of the French Southern and Antarctic Lands (French: 'Terres Australes et Antarctiques Françaises'). Amsterdam and Saint-Paul Islands, where the species is endemic, belong to this Territory.

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