Structural changes of lamellar cells in leaves of the moss Polytrichum formosum Hedw. during winter freezing and thawing processes

The lamellar cells of winter leaves of Polytrichum formosum were studied by electron microscopy of thin sections, and the results were compared with those for spring and summer plants. Pigment content and photosynthetic activity were also analysed. The lamellar cells of winter plants, when grown at temperatures above zero, had a similar ultrastructure to those of spring and summer plants. Their pigment content and photosynthetic activity were also similar. On the other hand, the lamellar cells of frozen plants (examined at temperatures about –5 °C) were desiccated, i.e. deformed, the cytosol was shrunken, and the ribosomes and mitochondria were aggregated. The thylakoid system of their chloroplasts was irregularly arranged and the grana thylakoids were compressed. The desiccation of the cells was soon annulled when the ambient temperature rose above zero.