The microscopy of P-protein filaments in freeze-etched sieve pores : Brownian motion limits resolution of their positions.

Intact vascular bundles from Nymphoides peltata (S.G. Gmel.) O. Kuntze, shown to have translocated carbon-14, were freeze-fractured and etched for electron microscopy. The interpretation of freezefractured and etched sieve pores and P-protein filaments seen in them is discussed. The entire widths of most of the sieve pores seen contained filaments separated by less than 100 nm. Their arrangement indicates too high a resistance to flow for pressure flow alone to drive translocation at known rates; pumps would be necessary at places along sieve tubes. However, calculations are presented to show that during the time taken to fix pores, by fast freezing or chemically, the filaments in them could rearrange and move further by Brownian and other motion than the distances between filaments which we need to measure. These calculations show that it is not possible, by microscopy alone, to answer the outstanding question "How are filaments arranged in translocating sieve pores?" with enough certainty to tell us whether pressure flow is adequate to explain translocation where filaments are present. The calculations are relevant also to microscopy of other cell structures which may move.