Ultrastructural development of cork-silica cell pairs in Avena internodal epidermis

In this paper we report on the microscopic and ultrastructural changes that characterize the primary stages of differentiation of cork-silica cell pairs in Avena internodal epidermis. Cork-silica cell pairs arise from symmetrical division of short cell initials in the internodal intercalary meristem. The daughter cells appear essentially alike ultrastructurally following this division. They then embark on two remarkably different pathways of differentiation. The first changes involve differential wall thickening and cell expansion. Well after these changes have occurred, the nucleus in the upper cell of the pair (future silica cell) breaks down, whereas that in the lower cell (future cork cell) remains intact The upper cell then becomes filled with fibrillar material and osmiophilic droplets, presumably breakdown products of membranes and organelles. The membranes, organelles, and nucleus remain unaltered in the cork cell. Finally, silica is deposited as long chains of silica bodies in the lumen of the silica cell, whereas no detectable silica occurs in the cork cell. Possible bases for these divergent pathways of cellular differentiation in cork-silica cell pairs are discussed.