Comparative rhizome anatomy of some species of Ceradenia L.E. Bishop and Zygophlebia L.E. Bishop (Polypodiaceae, formerly Grammitidaceae) from Madagascar

Abstract

Rhizome anatomy of 2 species of Ceradenia and 2 species of Zygophlebia, mainly Malagasy endemics, was studied in detail, and compared with preliminary results previously achieved in 2 other Zygophlebia species from Madagascar. Stele architecture was reconstructed in each species, with two sketches (a cross section and a splitting-out diagram), thus providing anatomical features and allowing a discussion about their relevance for distinguishing the two genera. Especially, we emphasize that Ceradenia differs from Zygophlebia by lacking any accessory ventral gap, but that character has a value only in combination with the occurrence of whitish waxy hairs in the sori. On the other hand, histology provides few correlated results, due to a narrow sampling, but tracheids lumen appears in transverse section sinuate in Ceradenia, rounded  in Zygophlebia. Rhizotaxis was analyzed, revealing a more or less helical insertion of roots, along meridians in definite number, mainly crowded at the ventral rhizome side, and whose divergence angle seems to be little altered by environmental constraints. Phyllopodial divergence angles are constant too. All these geometrical data may be useful for characterizing species, or species groups, but not at a generic level. They appear however not tightly correlated to rhizome size and might have an adaptive significance.