A cryo-scanning electron microscope study of the water relations of the remarkable cell wall in the moss Rhacocarpus purpurascens (Rhacocarpaceae, Bryophyta)

The functional properties of the unique trilamellate walls in Rhacocarpus purpurascens revealed by cryo-scanning electron microscopy are interpreted as a supreme adaptation to exposed habitats that experience frequently alternating periods of drought and heavy precipitation The porous outer layer ensures rapid water uptake and retention whilst its highly hydrophobic cuticle-like layer prevents waterlogging Thus R purpurascens is unlikely to suffer from depressed metabolism like that recorded in waterlogged Sphagnum Water retention by the middle lamellar stratum extends the periods of protoplast hydration and active metabolism under drying conditions Comparisons between the wall layering in Rhacocarpus and papillate moss walls indicate that whilst both are adaptations associated with rapid water uptake the presence of papillae accelerates water loss and hence the shutting down of stressed metabolism under drying conditions Unlike the intermittent soaking and dry conditions experienced by Rhacocarpus taxa with surface waxes are found either in habitats with constantly flowing aerated water or beneath underhangs where waterlogging would depress gaseous exchange