WHICH FUNCTION HAVE THE HYALOCYSTS OF THE LEAVES OF DICRANACEAE

The function of the hyalocysts of Leucobryum and other Dicranaceae (Pa raleucobryum, Campylopus) is still dubious. As possible explanations, (1) water storage, (2) protection of the enclosed chlorocysts against desiccation, (3) a role for gas exchange, (4) a role for nutrient upta ke and (5) an increased evaporation for the concentration of nutrients are discussed. The fact that predominantly species of wet habitats po ssess hyalocysts, which do not need to store water, seems to indicate that water storing is not the main function of this anatomical structu re. The observation that species with hyalocysts of mesic habitats com pete with species without hyalocysts without having an advantage seems to indicate that such structures were evolved under different ecologi cal conditions than today. Protection of the enclosed chlorocysts agai nst desiccation works only for a short period when the water stored in the hyalocysts is already evaporated but the chlorocysts are not yet dried out. It is therefore also not a sufficient explanation for its f unction. Air bubbles observed in the hyalocysts are not a result of ph otosynthesis as suggested in the literature but a result of dehydratio n. Cation exchange was so far regarded as a method for nutrient uptake typical for Sphagnum. Cation exchange experiments with Sphagnum palus tre, S. fimbriatum, Leucobryum glaucum, Dicranum scoparium and Mnium h ornum revealed that all these species have about the same cation excha nge capacity regardless their different structures of their leaves. Th us the special anatomy of Sphagnum leaves does not increase the cation exchange. Evaporation of a Leucobryum tuft is not higher than that of a water table. However, the amount of water stored is 2.3 times highe r than that of Mnium hornum and the desiccation takes 3 times as long. During desiccation, the leaves undergo three phases with the hyalocys ts filled with water, with air bubbles and with air, which provide dif ferent conditions for gas exchange. It is not yet known, whether the p rolongation of the period of desiccation balances or succeeds the net photosynthesis reduced by the diffusion of gas through the water fille d hyalocysts as compared with other mosses. The function of hyalocysts may perhaps be explained as a combination of the different explanatio ns mentioned above. The enormous water uptake made possible by the hya locysts principally extends the time of desiccation by factor three. P hotosynthesis is reduced as long as the hyalocysts are filled with wat er but is suboptimal when the hyalocysts are filled with air bubbles a nd optimal when the empty hyalocysts protect the chlorocysts from desi ccation. This could finally result in a higher net-photosynthesis as c ompared with other mosses growing in the same habitat.