Morphology and evolution of epiphytic Davalliaceae scales

Abstract

The scales of most members of the Davalliaceae and related epiphytic, secondary-hemiepiphytic, and climbing ferns consist of a stalk and a shield, are dense, and are persistent on the long-creeping rhizomes. They differ from the sessile, mostly deciduous scales of the related terrestrial eupolypod ferns. The stalked scales first developed two-dimensionally and later produced the proximal part of the shield from near the base to comprise a peltate or pseudopeltate shield on a stalk. Anatomical observations showed variations in the cellular and histological features and the degree of stalk insertion into a rhizome concavity. A character-phylogenetic tree of scales suggested an evolutionary linkage between the stalked scale and the epiphytic life form in the lineage leading to Davalliaceae and Polypodiaceae. The elaborated scales of most species of the obligate-epiphytic Davalliaceae were similar to the sunken-stalked, peltate, absorbing, epiphyllous trichomes of some angiosperm epiphytes. These results suggest that the stalked scales may have significant roles such as protection against desiccation, external water storage, and absorption of water and nutrients.