Conservation of Cell Order in Desiccated Mesophyll ofSelaginella lepidophylla([Hook and Grev.] Spring)

Understanding of the basis of desiccation tolerance in mature plant tissues that survive extreme dehydration requires knowledge of the degree of cellular order in the dry state. Generally, aqueous fixatives have been used in ultrastructural studies of such material, and these are known to be inadequate in the preservation of dry material. Cryopreservation provides a more assured level of fixation fidelity than aqueous fixatives, particularly with dry material. Using freeze substitution and electron microscopy, we examined the ultrastructure of dry mesophyll cells of Selaginella lepidophylla ([Hook and Grev.] Spring). In this material the cells were condensed and had highly folded walls. The plasmalemma was bounded on both sides by layers of granular material, and the membrane was in close and continuous apposition to the walls. The conformation and position of organelles and their structure appeared to be influenced by being compacted within the shrunken cells, but the ultrastructural integrity of all organelles and cellular membranes, including mitochondria, chloroplasts and vacuoles, was maintained in the dry state. These cells had numerous small vacuoles clustered in aggregates, and the tonoplast membranes appeared to be coated on the internal side by a fine granular layer. The vacuoles contained osmiophilic material of varying degrees of condensation and had embedment holes suggesting the presence of salt crystals within the vacuoles. The general conclusions from these studies are that a critical level of cell order is maintained in the dry state in these desiccation-tolerant plants, and a high degree of effective packing and shape fitting of cellular constituents with the compaction forces of dehydration underlies this conservation of cell order.