An Ultrastructural Study on the Developmental Phases and Silicification of the Glumes of Phalaris canariensis L.

In the glume of Phalaris canariensis L. silicon deposition takes place in the macrohairs, papillae, prickle hairs and silica cells of the abaxial epidermis before panicle emergence. Early in their development the macrohairs have large vacuoles and thin walls. At maturity the walls become thickened and are major sites of silica deposition. Dry ashing reveals a helical pattern within the hair walls. Distinct papillae and prickle hairs were first observed one week before panicle emergence. Here silicification was initially confined to outer tangential walls, but by two weeks after emergence their cytoplasmic contents had broken down, and the lumina were filled with siliceous granules. Cork-silica twin cells were also present in the abaxial epidermis. By panicle emergence the silica cells were infilled, but the cork cells retained their cytoplasmic contents. The long cells of the abaxial epidermis were initially thin walled, but thickening occurred in the outer tangential wall, this being complete by one week after emergence. These cells remained relatively unsilicified throughout. After panicle emergence the adaxial epidermal cells, and their associated parenchyma cell layers began to lose their cellular contents and collapse. This process was complete two weeks after emergence when the collapsed walls formed a thin internal layer between the two epidermi. Electron opaque granular material, containing several elements, but predominantly calcium, was present between the collapsed cell walls. The results are compared with those for the lemma, and silica deposition mechanisms are also discussed.