Why do ferns and other lower vascular plants require water for fertilization?

Ferns and other lower vascular plants require water for fertilization because their male gametes, known as antherozoids, are motile. These antherozoids are produced in structures called antheridia and possess whiplike hairs, or flagella, which they use to swim.12

In the life cycle of these plants, the male gametes must travel from the antheridia to the archegonia, the flask-shaped structures that house the female gametes (eggs). Because these plants lack the specialized pollen-delivery systems found in seed plants, the antherozoids rely on a film of external water to swim toward the archegonia to effect fertilization. Once the male gamete reaches the egg, it enters through the neck of the archegonium to complete the fertilization process, forming a zygote that will develop into the next sporophyte generation.12