Morphological and anatomical plasticity of a rare amphibious species of Eriocaulaceae (Poales, Monocotyledons)

Phenotypic plasticity is common in aquatic monocots, but is sometimes mistaken with heteroblasty. In Eriocaulaceae, aquatic and amphibious species may present interesting adaptive morphological features. However, Paepalanthus saxicola var. pilosus is unusual by having two leaf morphotypes and scapes that can be partially submerged or completely exposed. This taxon also presents an uncertain identity due to its incomplete morphological description. Here, we studied the morphology and anatomy of both leaf morphotypes and of scapes of P. saxicola var. pilosus to understand if the morphological differences in leaves correspond to phenotypic plasticity or heteroblasty; and the ecological and systematic significance of its leaves and scapes. Besides, we describe its morphology to evaluate its identity. The short curved leaves show supporting tissue and three vascular bundles, while the long filiform ones lack supporting tissue and have a single vascular bundle. All specimens share anatomical characteristics of scape. We raised this taxon to the species level, giving it a replacing name, Paepalanthus amphibius. We observed that P. amphibius is the first confirmed report of heterophylly in Eriocaulaceae. The anatomy of its short leaves and scapes reflects adaptations to a xeric environment, while the anatomy of the long leaves shows characteristics adapted to an aquatic habit. Possibly, such degree of plasticity is underreported in the family, so we stress the importance of seasonal studies to understand the morphology and taxonomy of the amphibious species.