Centrifugation of living pollen mother cells of Lilium henryi has permitted the modifications of complex walls developed on the four daughter microspores. This experimental approach has not only provided an insight into the mechanism by which patterning is established, but has also identified the stages in cell development when wall determinants are present. These determinants can be first detected late in the meiotic prophase, but are not necessarily active at the cell surface at this early stage. The fact that treatment during cytokinesis may affect subsequent wall formation, combined with the obvious interference of the colpus with the spore patterning, indicates pattern determination to be a lengthy process, commencing during late prophase and reaching completion in the young tetrad. The study of walls formed on enucleate cell fragments suggests that neither the spindle nor the cytoskeleton is intimately involved in formation of the basic reticulate patterning. It is proposed that material is inserted into the mosaic of the plasma membrane and is reorganized biophysically to form contiguous ‘plates’, which then modify the properties of the membrane in such a way that the cytoplasmic protrusions that constitute the first stages of wall formation only develop in the interstices between them. Changes in the plasma membrane have been detected over this period and it seems that a population of coated vesicles is closely involved in these events. Examination of the cytoskeleton of the young spore suggests that microtubules play a critical role in the positioning of the colpus, and reveals ways by which only small changes in this skeleton could give rise to multicolporate grains.

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