New structure associated with stomatal complex of the fern Polypodium vulgare

Abstract

THE opening movements of stomata are generally considered to result from the guard cells gaining an osmotic advantage over their immediately adjacent epidermal cells resulting in the uptake of water into the guard cells and their consequent increase in volume. Fujino¹ and Fischer² independently proposed that active potassium transport between the guard and epidermal cells could be the mechanism by which the cationic fraction of the osmotic imbalance could be achieved. In recent years many investigators have produced corroborative evidence for this potassium shunt³ using Macallum's histochemical stain⁴. Further support for the hypothesis has been obtained using electron probe microanalysis^5–8 and potassium-sensitive microelectrodes⁹. Despite these studies, no clear-cut picture has emerged concerning either the reservoirs of the ions at either end of the flux or the intercellular ionic pathway although the findings of Heller et al.¹⁰ implicate the vacuole as the ion reservoir within the guard cells as do those of Penny and Bowling⁹. There is no current evidence of where or, indeed, whether, potassium is sequestered after it is shunted out of the guard cells during stomatal closure, but it has been suggested that the differentially thickened cell walls of the stomatal complex could serve as storage sites^11,12. We have recently discovered some new morphological features in the stomatal complex of the common polypody, Polypodium vulgare L., which we believe represent the site of potassium accumulation in this species.