The life cycle of a fern is complex compared to that of a flowering plant, and the cycle involves both sexual and asexual reproduction. Some ferns are able to produce new plants entirely asexually, but usually a fern's reproductive process includes a sexual stage.
Spores vs. Seeds
Ferns differ from flowering plants in that they don't reproduce via seeds. Instead, most ferns produce tiny seedlike structures called spores, and when the spores germinate, they grow into new plants that begin the next stage in the life cycle. In flowering plants, seeds are produced sexually, which means they contain a mixture of genetic material from distinct sperm and egg cells that may come from different plants. Fern spores, however, are produced asexually, which means their genetic material is identical to that of the parent fern.
This asexual stage, though, is only the first step in a fern's life cycle. The next step involves sexual reproduction.
Gametophytes and Fertilization
Fern spores are dispersed by wind or water, and when they land in a place with the right growing conditions, which typically involves plenty of moisture and shelter from direct sunlight, the spores germinate and grow into tiny plants called gametophytes. Most ferns' gametophytes are very small compared to the size of the parent plants, and they may go entirely unnoticed as they develop.
Gametophytes develop sexual organs and produce sperm and egg cells. One gametophyte's egg cell may be fertilized by a sperm cell from a nearby gametophyte, or the little plant may fertilize itself. Even if the egg is self-fertilized, however, the new plant that grows from the fertilized egg contains genetic material from both a sperm cell and an egg cell. So this reproductive step is technically a sexual one.
Sporophytes and the Next Cycle
The new plant produced by the sexual reproductive stage is called a sporophyte, and the sporophyte is the conspicuous form of the fern that develops the plant's characteristic fronds, or leaves. The fronds sprout from the fertilized gametophyte; in some fern species, it may be as long as six months after fertilization before the first fronds unfurl.
When the sporophyte matures, it develops organs called sori on the underside of its fronds. The sori produce new spores, and the next asexual stage of the fern's life cycle begins.
Some species of ferns are able to reproduce through another asexual process called vegetative reproduction. In this process, new plants sprout from their parent plant's rhizomes, which are spreading stems that often grow underground. The new plants are genetically identical to the parent plant.
The Japanese shield fern (Dryopteris erythrosora) is a species that is able to reproduce asexually via spreading rhizomes. The species is a native of Asia, and in the United States it is perennial in U.S. Department of Agriculture plant hardiness zones 5 through 8.