An ultrastructural study on the development of silicified tissues in the lemma of Phalaris canariensis L

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Abstract
Silica deposition in the lemma of Phalaris canariensis L. takes place after panicle emergence in both the outer epidermal long cells and the macrohairs covering the outer epidermis. Before panicle emergence, the macrohairs have large vacuoles and thin walls. The first indication of silica deposition occurs at emergence when a thin layer of electron-opaque siliceous material appears at the outer edge of the wall. The hair wall becomes considerably thickened during the two weeks after emergence, and silica is eventually deposited in the whole of this thickened wall. At maturity the hairs are highly silicified, and the cytoplasm breaks down to leave a small central lumen. The outer epidermal cells are relatively thin-walled one week before emergence. By emergence, however, the inner tangential wall becomes greatly thickened, occupying most of the cell volume. During the week following emergence granular siliceous material is deposited in the wall surrounding the pre-existing lumen. This deposition is continued in the same area, until by maturity a silica aggregate is formed. The results are compared with those of previous studies, and discussed in relation to silica deposition mechanisms.