Structural Adaptations of Desiccation-Resistant Fern Ceterach officinarum Willd. (Aspleniaceae)

摘要

In this study, structural adaptations of the desiccation-resistant homosporous fern Ceterach officinarum Willd are revealed at the organ and cellular-tissue levels. The largest role is played by features that contribute to the maintenance of cell hydration. Xero-heliomorphic signs at the organ level are nanism, area reduction, and leatheriness of fronds and dense cover of trichomes and scales. At the cellular-tissue level, water is retained due to the relatively thick blade of fronds, their small cells, large number of cells per unit area and pycnomorphy, high values of the specific surface density of the fronds (SSDF), dorsiventrality of the mesophyll, and a relatively dense network of veins per unit area of the frond. At the biochemical and functional level, it is done by increasing the concentration of osmotically active substances, lowering osmotic pressure, and the accumulation of water-retaining chemical compounds (catechins and tannins); at the phytocenotic level it is due to facultative bryophily. Sciomorphic features at the cellular-tissue level are hypostomaticity, a relatively small number of stomata per unit area of frond, tortuous seams of epidermal cells, and minimal values of the ratio of cell surface to volume. Mesomorphic features are the differentiation of the mesophyll into columnar and spongy tissue, a thin cuticle layer on the surface of the epidermis, and the presence of stomata in the lower epidermis. Another adaptation mechanism that determines the rhythm of fern development is the ability to inactivate life processes and fall into cryptobiosis under conditions of water stress and high temperatures. Unlike poikilohydric ectohydric bryophytes, the sporophytes of this fern, resistant to dehydration, can withstand drying out (and are, to this extent, poikilohydric), but are endohydric. In short, the poikilohydricity of fern sporophytes is achieved on a different structural basis, which is more evolutionarily advanced in comparison with thalloid plants. However, the frequent state of cryptobiosis, especially under conditions of alternating wet and long dry seasons, leads to an imbalance in carbon metabolism. Apparently, poikilohydricity is not the most perfected and productive way to combat drought.