Comparative leaf structure and its functional significance in Phyllospadix iwatensis Makino and Phyllospadix japonicus makino (Zosteraceae)

The leaf anatomy and ultrastructure of the Japanese Phyllospadix species have been studied. Subcuticular cavities, which sometimes contain fibrillar substances, are present in the leaf blades but are absent in the leaf sheaths. Blade and sheath epidermal cells have many chloroplasts, mitochondria, lipid droplets and much endoplasmic reticulum but no starch grains. Wall ingrowths are pronounced in the inner tangential wall of epidermal cells and are closely associated with mitochondria. Plasmodesmata are present between adjacent epidermal cells and between epidermal and mesophyll cells, suggesting both symplastic and apoplastic pathways for solute transport. Vascular bundles in both blade and sheath are represented by a single xylem lacuna and phloem tissue containing several nacreous-walled sieve elements. Fibre bundles are associated with the epidermis, vascular bundles and the leaf margin. The walls of fibre cells are thickened, but not lignified, suggesting they provide mechanical support and flexibility for leaves against wave action. The two Japanese Phyllospadix species can be distinguished by anatomical features including the leaf surface, shape of the epidermal cells, and distribution and size of fibre bundles.