Megalastrum aquilinum (Thouars) Sundue, Rouhan & R.C.Moran

Description

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

Lectotype: (here designated): Tristan da Cunha, without date, du Petit-Thouars, L.-M. A. s.n. (P-: P00713304; duplicate P- P00713305)

Description: Plants terrestrial; rhizomes not seen; fronds 0.5-1.0 m long; basal petiole scales ca. 4.0 × 0.2 cm, linear-lanceolate, light brown, translucent, minutely denticulate, twisted, the cells elongate; laminae 25-50 cm long, 3-pinnate-pinnatifid at base, 2-pinnate-pinnatifid medially; basal pinna 18 cm long, inaequilateral, short-stalked to sessile; pinnules sessile, the basal segments frequently overlapping the pinna rachis, the largest pinnule of the basal pinna 6 cm long; pinna rachises abaxially glandular, scaly, and densely pubescent, the glands sessile, yellow, hyaline, 0.1 mm wide, the hairs 0.2-0.7 mm long, 3- or 4-celled, hyaline, acicular, the cross walls reddish, the scales 1.5-2.0 × 0.5 mm, ovate to lanceolate, light brown, translucent, entire, the cells isodiametric, adaxially glandular, scaly, and densely pubescent, the glands and scales similar to those of the abaxial side, the hairs 0.2-0.3 mm long, 2- or 3-celled, reddish, blunt-tipped; costules abaxially densely glandular, scaly, and pubescent, the glands like those of the pinna rachises, the scales 1.0-1.5 × 0.3-0.5 mm, otherwise similar to those of the pinna rachises, the hairs 0.5-0.7 mm long, 5- or 6-celled, otherwise similar to those of the abaxial pinna rachises, adaxially the costae densely glandular and pubescent, the hairs 0.5 mm long, 4-celled; laminar tissue between the veins abaxially slightly pale, glandular, glabrous or with scattered erect hairs 0.2 mm long, adaxially darker in color, glabrous; veins not visible on either side of the lamina, abaxially glandular, pubescent, and provided with scattered 0.2 mm long uniseriate scales, the hairs 0.4-0.6 mm long, 4-celled, adaxially with glands along the segment costa, otherwise glabrous; lamina margins recurved, glabrous or very sparsely ciliate, the hairs 0.2-0.3 mm long, 2- or 3-celled, acicular, restricted to the proximal portions of the ultimate lobes; indusia present or rarely absent, fugacious, 0.1-0.4 mm wide, provided with ca. 0.1 mm long sessile glands, and sometimes with 1-4 ca. 0.1 mm long setae; n = 41 (Manton and Vida 1968).

Distribution: Endemic to Tristan da Cunha, Nightingale, Inaccessible, and Gough Islands in the south Atlantic Ocean, where it reportedly to grows in dense thickets beneath groves of Phylica arborea Thouars (Rhamnaceae), which is one of the few trees native to the island (Wace and Holdgate 1958); 50-600 m.

Comments: In addition to the type at P, a du Petit-Thouars collection at B is also annotated by Willdenow as type material (BGBM virtual herbarium, image ID: 260189, barcode: B-W 19721-01 0; Röpert 2000) but it is not clear that this collection should be regarded as such. Although some of du Petit-Thouars' collections were distributed to B (Stafleu and Cowan 1976), the sheet in question does not bear a label indicating that it was in fact part of du Petit-Thouars' herbarium. The status of the B specimen is critical though, because it is a different species, which we have identified as belonging to Pseudophegopteris. The protologue provided by du Petit-Thouars (1808) is brief and might pertain to either a species of Megalastrum or Pseudophegopteris. The provenance of the collection at B provides some evidence of the name's application. Pseudophegopteris is not known from Tristan da Cunha (Christensen 1940; Holttum 1969, 1974; Tryon 1966; Wace and Holdgate 1958; Wace 1960; Wace 1961), suggesting that the specimen at B was collected elsewhere and mislabeled. The B sheet is most likely P. aubertii (Desv.) Holttum, a species named by Desvaux in honor of du Petit-Thouars. Alternately, it could be P. cruciata (Willd.) Holltum, a species that is doubtfully distinct from P. aubertii (Holltum 1974). After departing from Tristan da Cunha, Du Petit-Thouars later traveled to Madagascar and the Mascarenes (Williams 2003) where P. aubertii is present. It seems likely that the specimen at B was collected there. Consequently, the B material is in conflict with the protolog because it was not collected on Tristan da Cunha. Considering these points and Recommendation 9A.5 of the ICBN (McNeill et al. 2006), we chose to designate the specimen at P as the lectotype for this species. Christensen (1940) and Tryon (1966) remarked on the similarity of Megalastrum aquilinum to M. spectabile from Chile and Argentina, and suggested long distance dispersal as a means to explain this apparent amphi-atlantic distribution. Morphologically, the plants have similar lamina cutting, densely pubescent axes provided with short erect hairs, brown lanceolate scales, and sub-glabrous to puberulent abaxial lamina surfaces between the veins. Megalastrum aquilinum differs from M. spectabile most notably by its entirely glabrous adaxial lamina surfaces, the presence of fugacious indusia (sometimes absent, see below), very sparsely ciliate lamina margins with hairs that are often restricted to the proximal portions of the ultimate segments, thicker laminae, and recurved lamina margins. These last two characters, which were also cited by Christensen (1940), may be a response to wind damage, but are consistent in all of the material that we have seen. Christensen reported (1940) that he did not find an indusium in M. aquilinum. We have seen one specimen annotated by Christensen (Christophersen 2545), and confirm that it does not have any trace of an indusium, but all other specimens that we have seen do have small fugacious indusia (0.1-0.4 mm wide) bearing sessile glands and setae.

Etymology: From the Latin aquilinum, like that of an eagle. Du Petit-Thouars chose this specific epithet because he found the "root" (likely the rhizome) similar to the one of "Pteris aquilina" (= Pteridium aquilinum). This is a strange finding because the "roots" are probably not similar at all. Although we have not seen the rhizome of M. aquilinum, it is probably erect, like all other species in the genus, not creeping as in Pteridium.

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