All seagrass pollen is trinucleate regardless of whether it is granular or filamentous, and its nuclei are located centrally. Posidonia australis contains an elongated vegetative nucleus invariably in close apposition to the two ovoid sperm cells (Fig. 12K and L). These cells are bounded by irregular periplasm containing vesicles and their cytoplasm resembles that of the surrounding vegetative cells (Ducker etal., 1978).
The pollen from both granular and filamentous types is released from the submerged anthers and 'searches' through currents and waves (Verduin etal., 1996) for a submerged stigma. Cox (1988) and Ackerman (1995) discussed the two or three-dimensional 'search' by the pollen, particular in intertidal seagrass populations (see also Ackerman, Chapter 4). It is possible that pollination can also take place at the water surface in some intertidal seagrass genera such as Halophila, Phyllospadix and Zostera, because their mature reproductive organs often float at the water surface and pollen appears to be released during the highest tidal range.
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