Megalastrum Holttum
Descripción
Stipes scaly, especially near their bases; blades 2–4-pinnate, catadromous at the bases of proximal pinnae, the surfaces without cylindrical glands; proximal pinnae more developed basiscopically; veins simple or furcate, ending on adaxial surface of laminae in clavate tips well back from the blade margins, basal basiscopic veins of the distal pinnules arising from the costae, not from the costules; indument adaxially usually of stout, hyaline, acicular, falcate, septate hairs on rachises, costae, costules, and lesser axes, abaxially the midribs with similar or shorter hairs, also with linear to ovate, often marginally toothed scales, blades lacking short-segmented hairs of the type found in Ctenitis; indusia absent (present in a few spp. outside Mexico); spores echinate or with low, subparallel wings; x=41.A
Forma de vida
TerrestreA
Distribución
México (país) Nativo y no endémicoA
Categoría IUCN
No incluidaB
Categoría NOM-059-SEMARNAT-2010
No incluidaC
Discusión taxonómica
In the neotropics, Megalastrum comprises about 40 species, and the species of South America are especially poorly known and virtually unstudied since the monograph by Christensen (1920). In addition, there is one species in Africa and five species in Madagascar and the Mascarene Islands.
Megalastrum was relatively recently segregated from Ctenitis on the basis of the venation and trichome type (Holttum, 1986; Smith & Moran, 1987). The venation is characterized by the basal basiscopic veins of the ultimate segments arising from the penultimate costae, not from the midribs of the segments themselves; individual veins terminate well before the blade margins in clavate tips, as viewed adaxially. Distal segments of the pinnae tend to be strongly adnate, even decurrent basiscopically. Species lack ctenitoid hairs (short, twisted, often reddish, septate hairs) but have, on the midribs adaxially, stiff, antrorse, acicular hairs like those found on cyatheoid and thelypteroid ferns.
The relationships of Megalastrum are certainly with the dryopteroid genera, as indicated by the base chromosome number of x=41, but beyond that they are uncertain. Unpublished molecular studies by Cranfill (pers. comm.) indicate that the relationship with Ctenitis is not particularly close, rather Megalastrum may be related to Lastreopsis (the species of which also often bear acicular hyaline hairs) and Rumohra. Further studies are needed.
A
Megalastrum was relatively recently segregated from Ctenitis on the basis of the venation and trichome type (Holttum, 1986; Smith & Moran, 1987). The venation is characterized by the basal basiscopic veins of the ultimate segments arising from the penultimate costae, not from the midribs of the segments themselves; individual veins terminate well before the blade margins in clavate tips, as viewed adaxially. Distal segments of the pinnae tend to be strongly adnate, even decurrent basiscopically. Species lack ctenitoid hairs (short, twisted, often reddish, septate hairs) but have, on the midribs adaxially, stiff, antrorse, acicular hairs like those found on cyatheoid and thelypteroid ferns.
The relationships of Megalastrum are certainly with the dryopteroid genera, as indicated by the base chromosome number of x=41, but beyond that they are uncertain. Unpublished molecular studies by Cranfill (pers. comm.) indicate that the relationship with Ctenitis is not particularly close, rather Megalastrum may be related to Lastreopsis (the species of which also often bear acicular hyaline hairs) and Rumohra. Further studies are needed.
A