Abstract
The potassium content of open and closed stomata has been investigated in 46 fern species using the Macallum-test. About the half of the species showed substomatal ion-absorbent structures as they were recently described by S tevens and M artin (1977). Others showed a staining behaviour as it is known from many angiosperms. In the latter case there were higher amounts of potassiwn in open stomata than in closed ones. The occurrence of substomatal ion-absorbent structures could be correlated with diacytic stomata. A diffuse distribution of staining deposits occurred only in polocytic and pericytic stomata, where one subsidiary cell surrounds more than three fourths or the total circumference of a stoma. It may bc supposed that the substomatal ion-absorbent structures are involved in exchange processes between the guard cells and the two polar neighbouring cells.
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