Endospory

Endospory is a mode of spore development in which the germinating spore contains a developing gametophyte within its wall or inside the sporangium, rather than producing a free-living gametophyte outside the spore. This term contrasts with exospory, where the spore germinates into a separate, independent gametophyte after release. Endospory is especially associated with the megaspores of seed plants, where the female gametophyte develops inside the spore within the ovule’s integument.

In seed plants, megasporogenesis occurs within the megasporangium of the ovule. From the surviving megaspores, the megagametophyte forms in situ within the spore. The mature megagametophyte typically contains the egg apparatus and nourishes itself in connection with the surrounding nucellus and other sporophytic tissues. The retention of the developing gametophyte inside the spore reduces dependency on external conditions for fertilization and early development and closely integrates gametophyte development with seed formation.

Endospory is a hallmark of the evolution toward the seed habit and is widespread in gymnosperms and angiosperms. It also appears in some extinct or primitive pteridophyte lineages. The contrast with exospory highlights a fundamental difference in life-cycle organization: endosporic development confines the female gametophyte within the spore, whereas exosporic development produces a free-living gametophyte outside the spore.