Anatomy of the hinge region of the petiole of prostrate leaves of the wintergreen fern Polystichum acrostichoides (Dryopteridaceae)

The reorientation of leaves of Polystichum acrostichoides to a prostrate position is achieved by forming a hinge region near the base of the petiole. Anatomical details of the Distal Transition Zone (DTZ), Folding Zone (FZ), and Proximal Transition Zone (PTZ) of the hinge region were studied using light microscopy. Data showed that the vascular bundles are intact throughout the hinge region. Towards the FZ, the DTZ and PTZ have an increasing number of cortical cells that underwent deformation and protoplast degeneration, and cell nuclei are apparent in strongly deformed cortical cells. In the FZ, the petiole is compressed, having a network of cell walls of dead cortical cells with patches of turgid cortical cells and largely intact endodermis surrounding the vascular bundles (VBs). The helical secondary wall thickenings of the protoxylem and scalariform thickenings of the metaxylem may contribute to the flexibility of the VBs. This study provided evidence for the deformation and degeneration of cortical cells, which is responsible for the loss of turgidity of the hinge region while their cell walls may provide cushioning during bending and insulation for the living tissues under the snowpack, illustrating a unique strategy of ferns to adapt to a cold environment.