Starting with the “fern” as we recognize it (the sporophyte), the life cycle follows these steps:The diploid sporophyte produces haploid spores by meiosis, the same process that produces eggs and sperm in animals and flowering plants.Each spore grows into a photosynthetic prothallus (gametophyte) via mitosis. Because mitosis maintains the number of chromosomes, each cell in the prothallus is haploid. This plantlet is much smaller than sporophyte fern.Each prothallus produces gametes via mitosis. Meiosis is not needed because the cells are already haploid. Often, a prothallus produces both sperm and eggs on the same plantlet. While the sporophyte consisted of fronds and rhizomes, the gametophyte has leaflets and rhizoids. Within the gametophyte, sperm is produced within a structure called an antheridium. The egg is produced within a similar structure called an archegonium.When water is present, sperm use their flagella to swim to an egg and fertilize it.The fertilized egg remains attached to the prothallus. The egg is a diploid zygote formed by the combination of DNA from the egg and sperm. The zygote grows via mitosis into the diploid sporophyte, completing the life cycle.Before scientists understood genetics, fern reproduction was mystifying. It appeared as though adult ferns arose from spores. In a sense, this is true, but the tiny plantlets that emerge from spores are genetically different from adult ferns.Note that sperm and egg may be produced on the same gametophyte, so a fern may self-fertilize. Advantages of self-fertilization are that fewer spores are wasted, no external gamete carrier is required, and organisms adapted to their environment can maintain their traits. The advantage of cross-fertilization, when it occurs, is that new traits may be introduced into the species.