Pollen wall formation in Abies concolor and a discussion on wall layer homologies

Development of the pollen wall in Abies concolor (Gord. & Glen.) Lindl. is documented using ultrastructural techniques. Pollen wall formation is initiated by the deposition of a microspore surface coat and the formation of callosic protrusions in the proximal and lateral areas, outside the plasma membrane of the microspores enclosed in tetrads. The alveolar ektexine is then elaborated on these structures. Endexine formation starts with the appearance of tripartite lamellae to the inside of the microspore surface coat around the entire microspore. Both the ektexine and endexine are elaborated while the microspores are enclosed in tetrads by the callose wall. These results are utilized in assessing homologies between various wall layers in gymnosperm and angiosperm pollen. In considering criteria of homology, pollen wall development is discussed as a causal sequence and this discussion concludes that the ektexine and endexine in gymnosperm pollen are homologous with the ektexine and endexine in angiosperm pollen.