Structure of mucilaginous epidermal cell walls in Passerina (Thymelaeaceae)

Leaves of Passerina are inversely ericoid. Adaxial epidermal cells are relatively small; abaxial ones are large and tanniniferous. Mucilaginous epidermal cells are usually present in many Thymelaeaceae, including Passerina , mainly in the abaxial epidermis. They are unequally divided by a periclinal wall-like septum into two separate compartments: (1) the outer, adjacent to the cuticle, containing mostly tanniniferous substances and (2) the inner, containing mucilage. This type of epidermis has often been incorrecdy described as uni-, bi- or multiseriate. Transmission electron microscopy revealed mucilage, characterized by microfibrils, embedded between die innermost wall-like septum and outermost layers of the inner periclinal cell wall. As accumulation of mucilage increases, the innermost (adjacent to the cell contents) layer of the original periclinal cell wall is pressed against the cytoplasm, thus forming a clearly demarcated cellulose periclinal wall which divides the epidermis cell into two compartments, the inner wiuh mucilage and the outer comprising the cell lumen. Existing controversy is critically discussed. Our observations confirm the authenticity of mucilagination in epidermal cell walls.