Plants that require water for fertilization are primarily the non-flowering plants, specifically the bryophytes (like mosses) and pteridophytes (like ferns). These groups of plants reproduce through a process known as flagellation, which involves the movement of sperm cells.
The sperm cells of these plants are motile and depend on a thin film of water to swim from the male gametophyte (the haploid structure producing sperm) to the female gametophyte (which contains the egg). In bryophytes, this water is often supplied by rain or dew, while in ferns, it is usually found in the surrounding habitat, allowing for successful fertilization. Without this water medium, the sperm would be unable to reach the egg, preventing fertilization from occurring.
This requirement for water is a key reason why these plants are usually found in moist, humid environments where water is readily available, as it directly impacts their reproductive success.