The large, leafy fern sporophyte alternates with a small (3–4 mm), flat green gametophyte—called a prothallus—in the typical life cycle (see Figure , Polypodium, a leptosporangiate modern fern). The sporophytes of ferns are independent, divided into leaves, stems (rhizomes), and roots, and have vascular tissues whereas the gametophytes are small, photosynthetic thalli that live anchored to the ground with rhizoids. Many are heart‐shaped and only one‐cell layer thick. The gametangia are sunken or protrude from the underside of the gametophyte. Fern sperm have several flagella (hundreds in some species). When the sperm are released through a pore at the tip of the antheridium, they swim in a film of external water to the opening at the top of the archegonium and down the neck to the egg where fertilization takes place. The zygote divides within a few hours after fertilization and is supplied at first with nutrients (and perhaps hormones) through an absorbing foot attached to the gametophyte. A tiny sporophyte with a rhizome, adventitious roots along its surface, and a juvenile green leaf soon pushes out from under the prothallus, establishes independence, and the prothallus whithers and dies.
Ferns, among the vascular plants, are second in number to the flowering plants and have adapted to all manner of habitats. Some are aquatic, some live in deserts or on dry rock cliffs, a few persist in the cold arctic desert, but mostly the modern ferns live in the tropics. Here the ferns attain some of the glory of their past. Some tree fern trunks exceed 70 feet in height, and their 15‐feet‐long leaves share canopy space with the angiosperm tree crowns. Tropical ferns grow vigorously as epiphytes on and over everything in the understory. The smallest ferns are aquatics that float on subtropical and tropical ponds with modified leaves a centimeter or less in size.