A new interpretation of the structure of the megaspore membrane in some gymnospermous ovules

SUMMARY A light and electron microscope investigation of the tapetum during stages in the ontogeny of a variety of gymnospermous ovules reveals that eventually this tissue degenerates and forms a thick acetolysis-resistant membrane (the tapetal membrane) between the embryo sac and the nucellus. This membrane is not considered part of the megaspore membrane in the sense that that term is used in free-sporing heterosporous plants. In the mature ovules of the genera investigated the true megaspore membrane is present as a thin, acetolysis-proof membrane closely associated with the peripheral prothallial cells and internal to the thicker tapetal membrane. This megaspore membrane can be related to a somewhat more elaborate structure that is present at the earlier stages of development. There is strong evidence to suggest that the events in the tapetum of the ovule are to some extent paralleled in the tapetum of the pollen sac.