Ferns are simple plants, but they have somewhat complicated life cycles. In the scientific language of botany, the life cycle of a fern is described as the alternation of the sporophyte and the gametophyte. Relatively primitive plants, ferns take two completely different forms in alternating generations. Though they have leaves and a vascular system like more highly evolved plants, they do not reproduce with flowers and seeds.
Instead, a fern produces spores, and the spores grow into organisms that produce more ferns. The organisms that produce the spores are called the sporophytes, and almost anyone would recognize most sporophytes as ferns. Each successful spore, however, grows into a small unobtrusive organism called a gametophyte, which produces and nurtures a new generation of delicate ferns.
Ferns, sporophytes, bear sori, cases that hold sporangia, on the underside of their leaves. Often they are visible as raised round dots. The sporangia, in turn, bear clusters of tiny dust-like spores. When they mature, the spores are released from the sori, to take root and grow.
Each fern produces many spores, because so much can happen to each one. When the spores drop from the fern, each must land somewhere that it is able to strike root, because they will become genuine plants, with rhizoids that need to draw up water and minerals from the earth.
A successful spore germinates and becomes an inconspicuous growing plant, the gametophyte. The gametophyte feeds itself by converting sunlight to energy in its chloroplasts, just as other plants do. Therefore it must have sunlight, in the correct measure.
When the gametophyte is mature, it produces egg cells and sperm cells. The sperm cells fertilize the eggs. Because the eggs and sperm of one plant often do not mature at the same time, the sperm often fertilize the eggs of another gametophyte. The surroundings must be moist enough for the sperm cells to swim, however. Otherwise, they will never be able to reach the egg.
A fertilized egg is the first stage of the sporophyte, the next generation of the fern line. As it grows within the gametophyte, the fern embryo differentiates into cells that will become the fronds, the stem, the root, and the foot, which takes nourishment from the gametophyte until the sporophyte grows strong and mature enough to live without its progenitor.
Then the gametophyte dies or is absorbed, and the sporophyte, the fern, continues to grow until it is mature enough to produce spores. The life cycle of the fern continues.