SILICON DEPOSITION AND ANATOMICAL STUDIES IN THE INFLORESCENCE BRACTS OF FOUR PHALARIS SPECIES WITH THEIR POSSIBLE RELEVANCE TO CARCINOGENESIS

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Abstract
Summary: Inflorescence bracts of four grasses, Phalaris canariensis, P. minor, P. paradoxa and P. arundinacea, were investigated using light microscopy, scanning electron microscopy and electron‐probe microanalysis to determine their anatomy and silicon depositional patterns.The heaviest silicon accumulation detected was associated with the outer (abaxial) epidermis of the lemma. Preferential deposition sites were located in the cell walls of the macrohairs, the walls and lumina of the long cells, and external to the epidermis, forming a thin layer of extracellular sheet or cuticular silica. Occasional silicification of internal tissues occurred in two forms, either as elongated, pointed fibres of the dendriform type, resulting from the infilling of lumina and pits of hypoderm fibres, or as rows of spongy mesophyll cells and intercellular spaces.Dimensions and silicon levels, recorded for the various siliceous structures indicated that the macrohairs and the friable sheets formed by silicification of the abaxial epidermal cells were a potential source of sharp, elongated siliceous fibres falling into the critical size range for classification as being carcinogenic. The seeds of all four Phalaris species are known contaminants of the cereal crops of the Middle East, and have been implicated as factors in the high incidence of oesophageal cancer in north‐east Iran.