Protoplast Rotation, Cell Division and Frustule Symmetry in the Diatom Navicula bacillum

Navicula bacillum frustules, like those of many other naviculoiddiatoms, exist in two forms, cis and trans, which occur in theratio 1 cis: 2 trans. During interphase the protoplast bearsa constant relation to the orientation of the hypovalve, suchthat the nucleus lies on the same side as the first-formed halfof the hypovalve. Immediately before mitosis the plastid andnucleus migrate around the cell until the nucleus lies on theopposite side from its interphase position, where it then divides.After cytokinesis the new valves form with their primary halveson the same side as the recently divided nuclei. This and the180° rotation of the protoplast during the cell cycle explainthe 1 cis: 2 trans ratio. Navicula bacillum Ehrenb., cell symmetry, diatom systematics, plastid structure, protoplast rotation