Re-evaluation of a neglected layer in pollen wall development with comments on its evolution

This study focuses on one particular layer of the pollen wall, which develo ps below the endexine in the free microspore stage and prior to the initiat ion of the intine. This membranous-granular layer (MGL) has been described by different terms in the literature and has often been interpreted either as part of the endexine, or the intine. During ontogeny, however, the granu lar material shows a development that is clearly distinct, both in timing a nd mode of formation, from the endexine as well as the intine. Its chemical composition is also characteristic; the MGL resists acetolysis. Our ontoge netic observations from four dicot and one monocot species are used to illu strate the systematically widespread occurrence of this wall layer, its ult rastructure and histochemistry, and its comparable nature throughout angios perms.