Are the endexines of pteridophytes, gymnosperms and angiosperms structurally equivalent?

Abstract

Substructures have the same size and configuration, like wire-wound springs, in all parts of the exine of Artemisia (tectum, bacules, foot layer and the endexine), and similar substructures are also present in all parts of the exospore of Lycopodium. Substructures like these are illustrated here for pollen exines of Poa, Betula (endexine), and Fagus as well. The generalized radial orientations of these substructures are at right angles to the “lamellations” common in endexines and in exospores. In the ectexine of angiosperm pollen the substructures are organized into unit structures (tufts) that are 70–200 nm or more in diameter. The tufts are structurally like plasmodesmata of tapetal cells and like viscin threads. In pollen of angiosperms and gymnosperms there is at some stages of development a white line at the junction between units of the ectexine and the endexine. The exine substructures cross the ectexine-endexine junction, and it can be assumed that exine units are continuous across these zones. I would say that endexine operates like the exospore of pteridophytes. According to my interpretation, the endexine opens up and closes off large irregular channels functioning as a pumping system. I consider the arrangement, size and appearance of endexine and exospore substructures to be similar in pteridophytes ( Lycopodium), gymnosperms and angiosperms. Even if they are shown definitively to be structurally the same the distinctive term “exospore” ought to be retained because of the development and exclusive nature of the exospore.