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Why do ferns need water for fertilization?

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tree fern — Fern Tree fern (*Cyathea medullaris*).

Ferns require free water for fertilization because their reproductive process relies on motile sperm.1

During the sexual phase of the fern life cycle, the gametophyte produces sperm within organs called antheridia. These sperm are equipped with spiral bands of cilia—hairlike structures that enable them to swim. When the archegonium (the egg-producing organ) is ready, it releases simple organic acids that act as a chemical signal. The sperm must swim through a film of free water to reach the archegonium, guided by these chemical cues, to penetrate and fertilize the egg.1

While many ferns depend on this moisture-reliant process, some species have adapted to drier environments by utilizing apogamy, a modified life cycle that bypasses fertilization entirely, thereby removing the need for swimming sperm.2