The Effects of Centrifugation on Asymmetric Cell Division and Differentiation of Fern Spores

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Abstract
We have investigated the effects of centrifugation on spore polarity, asymmetric cell division, and rhizoid differentiation in the sensitive fern Onoclea sensibilis L. Centrifugation at 10000 g for 30 min produces a random orientation of spores with stratification of the cell contents. After centrifugation at most early stages of development, the nucleus retains its normal pattern of migration from the centre of the ellipsoidal spore to the proximal face and then to an end of the spore, without regard to the orientation of stratification. This indicates that the polarity of the spore is stable to centrifugation. As long as the nucleus migrates to an end of the spore and asymmetric cell division occurs, the small cell differentiates into a rhizoid. The arrangement of large cytoplasmic organelles appears to have no influence on nuclear migration, asymmetric cell division, or rhizoid differentiation. The only period during development when centrifugation blocks asymmetric cell division is immediately preceding and during mitosis and cytokinesis. Spores centrifuged at this stage do not complete nuclear migration, and symmetric cell division results, with neither cell differentiating into a rhizoid. The source of the stable polarity of the spore is discussed.