Evidence that abaxial leaf glands are the sites of salt secretion in leaves of the mangrove Avicennia marina (Forsk.) Vierh

summary Pieces of leaf from the halophyte Avicennia marina (Forsk,) Vierh. were freeze‐substituted and sectioned with special precautions to keep specimens always dry. Spot energy dispersive X‐ray microanalysis and X‐ray mapping showed that vacuoles of water parenchyma and palisade mesophyll cells had much more sodium and chlorine than the cytoplasm of the same cells. Potassium and phosphorus were also found in the vacuoles of the palisade mesophyll cells but in the water parenchyma cell vacuoles, sodium and chlorine generated most of the X‐ray counts. This is consistent with a function for the water parenchyma cells of vacuolar salt storage. No sodium and chlorine were detected in the peltate cells of the abaxial epidermal hairs, and there was very little sodium in the vacuoles of their basal cells, potassium apparently being the major cation. Salt crystals were not detected in substomatal cavities. Abaxial epidermal glands contained sodium and chlorine in the collecting cell, stalk cell, secretory cells and the subcuticular cavity. Material just outside the glands gave extremely high X‐ray counts for sodium and chlorine. We conclude that the material was salt that had been secreted, and that the glands are the secretory apparatus on the abaxial surface of the leaves.