How do microspores develop into male gametophytes in Selaginella?

In the genus Selaginella, the development of male gametophytes is characterized by heterospory, where the sporophyte produces distinct microspores that give rise to male gametophytes.12

The process follows these stages:

• Endosporic Development: The development of the male gametophyte, or microgametophyte, begins through cell division within the microspore wall while the spores are still contained within the microsporangium.32
• Maturation: These initial divisions may occur before the spores are shed from the microsporangium. Final development typically takes place on the soil after the spores are released.32
• Structure: The resulting microscopic male gametophyte is composed essentially of a single antheridium, which produces biflagellate sperm.2
• Nutrition: Because these gametophytes lack chlorophyll, they rely entirely on nutrients stored within the spore for their development.2