THE STRUCTURE OF MUCILAGE-PRODUCING CELLS AND CELL WALLS OF LEAF TRACHEAL ELEMENTS (COMMUNICATION AND DISTRIBUTION)
A question was put about possible functional role of mucilage cells in leaves. The leaves of more than 77 species of various families of dicotyledons and gymnosperms were studied by transmission electron microscope. The structure and type of mucilage cells as well as the structure of tracheal elements in vascular bundles were studied. Mucilage-producing cells are mucilage-secreting or mucilage-containing cells. Mucilage is excreted beyond the plasmalemma in the former and stored in vacuole in the latter. Mucilage passes through the cell wall and accumulates on its surface or in the intercellular space in the case of mucilage secreting cells of the 1st type (MSC1). Mucilage accumulates in periplasm in mucilage secreting cells of the 2nd type (MSC2). In epiderm, MSC2 are divided by cell wall («inner cell wall») in 2 chambers containing either protoplast or mucilage. MSC2 usually contain one mucilage layer, but cells with two mucilage layers separated by the cell wall were found too; the latter ones were found in sun leaves. Usually MSC2 are located over palisade cells in adaxial epiderm while MSC1 are under vascular bundle in mesophyll. Sometimes mucilage appears in intercellular spaces after breaking the inner cell wall of MSC2. Frequently it happens in stress conditions of vegetation. In plants of the same species grown in the south compared with those in the north areas the number of MSC increase, mucilage appears in intercellular spaces near vascular bundle. According to our data, the structure of primary cell wall in different species as well as in the plants of the same species grown under different conditions can vary significantly. Two variants of the primary cell wall structure were found in the studied species: dense and loose structure. Loose cell walls have different structure, too: fibrillar and granular. Localization of mucilage near vascular bundles and the loose structure of vessel cell walls let us to propose the role of mucilage in water balance in leaves. MSC1 and MSC2 have different functional roles. MSC1 are located on the phloem side of the leaf as well as epistomatal mucilage plugs, and can possibly prevent the excessive water loss during stomatal transpiration in species with loose cell wall in vessels. MSC2 (accumulating mucilage in periplasm) participate in two processes. On one hand, they protect leaves from exceeding solar radiation by dispersing the light, and/or improve the thermal resistance of the leaves. On the other hand, these cells in upper epidermis may take part in regulation of cuticular transpiration under stressed water conditions due to outflow of mucilage into intercellular spaces through brakes in cell walls (as it usually was found near xylem composed of vessels with loose cell walls) in species with loose cell wall in vessels. The occurrence of mucilage near vascular bundles containing vessels with loose primary cell walls may prevent water loss from the leaves due to water-retaining capacity of mucilage.