How does alternation of generations work in lycophytes?

Lycophytes exhibit an alternation of generations characterized by two distinct, multicellular phases: the spore-producing sporophyte and the sex-cell-producing gametophyte. In these vascular plants, the sporophyte is the dominant, independent generation.123

The cycle follows two primary patterns based on spore production:

• Homosporous (e.g., Lycopodium): These plants produce one type of spore. The resulting gametophytes are bisexual, bearing both sperm-producing antheridia and egg-producing archegonia. Depending on the species, these gametophytes may be photosynthetic and surface-dwelling or subterranean and dependent on fungi for nutrition. Fertilization occurs when flagellated sperm swim through water to reach an egg.13
• Heterosporous (e.g., Selaginella, Isoetes): These plants produce two types of spores: small microspores and large megaspores. Development is endosporic, meaning the gametophytes develop largely within the spore wall. Microspores produce male gametophytes (microgametophytes), while megaspores produce female gametophytes (megagametophytes) that eventually bear archegonia.13

In all cases, the zygote develops into an embryo that eventually grows into an independent sporophyte, completing the cycle.12