Phenotypic Plasticity of the Hydrasperman Seed FernTetrastichia bupatidesGordon (Lyginopteridaceae)

Abstract

Fifty-nine stems of Tetrastichia bupatides, numerous frond segments, and roots from Oxroad Bay, Scotland, have recently been analyzed. They reveal that many characters originally used to describe this species exhibit a wider range of variation than previously suspected. These data suggest that ribs of the protostele are most commonly four in number but range from three to five. Phyllotaxy ranges from opposite decussate to two-fifths, and rachis forking can be equal or unequal. Secondary xylem could be produced by all stems regardless of size, and rachis trace morphology ranges from “butterfly” shaped, to U-, to broadly U-, to T-shaped. Pinnules are vascularized by a single trace that diverges from the outermost protoxylem strand of a pinna trace. These ranges of variation for many characters complicate identification of isolated specimens, but stems can be identified by a consistent mode of rachis trace production, and frond parts can be correlated by common histological characters and a continuous range of variation in trace configuration. These wide ranges of variation in numerous characters demonstrate that phenotypic plasticity could be far greater in some hydrasperman seed ferns than previously realized and the taxonomic diversity at Oxroad Bay may be lower than previously thought.