Zostera capensis Setchell II. Fine structure of the cavities in the wall of leaf blade epidermal cells

Summary The ultrastructural morphology of cavities in the outer walls of leaf blade epidermal cells of the marine angiosperm Zostera capensis is described. The cavities usually occur beneath the cuticle and are generally spherical to ovoid in shape. They are not bounded by a membrane although their margins are occasionally more electron-dense than the wall that surrounds them. In young epidermal cells the subcuticular cavities penetrate partially into the overlying cuticle. In more mature cells, flask-shaped extensions of the cavities penetrate through the cuticle. The actual opening through the cuticle appears to be slit-like. It is extremely small and is filled with an electron-dense material. While the cavities in the youngest epidermal cells examined generally lack contents, those of more mature cells contain mainly fibrillar material. In addition to fibrillar material, electron-dense amorphous or crystalline-like material maybe present within the cavities. The occurrence of the latter types of material on the cuticle surface, as well as within the wall below the cavities, suggests that these materials are probably excreted from the epidermal cells via the cavities. Structural similarities between the subcuticular cavities and the subcuticular spaces in the outermost secretory cells of salt glands on the leaves of certain halophytes are pointed out, and some suggestions are offered as to the nature of the material apparently being excreted from the cavities.