The largest group of living seedless vascular plants—and probably the most familiar—are the ferns with about 12,000 species, over two‐thirds of which are tropical. Ferns are an ancient group. Spores and leaf impressions of plants that lived 400 million years ago in the Middle Devonian have been found, but almost all of these early types (grouped simply as “preferns”) were extinct by the Permian. The group to which most modern ferns belong, the Filicales, first appeared in the Lower Carboniferous, 300 million years ago.
The ferns are an extremely diverse group, and there is no single characteristic that defines them. The following features are present in most:
- Leaves, called fronds, are megaphylls. Most are compound with a rachis and numerous pinnae (or compound once again with pinnules). Almost all have circinate vernation—they are coiled (circinate) tightly in “shepherd's crook” or crozier fashion over the growing tips. These unroll as they mature (growth from the base to the tip like this is termed acropetal). The croziers are called fiddleheads and are eaten by some people, although many species are toxic.
- Stems, for the most part, are rhizomes that grow at, or just under, the ground surface. They have only primary tissues. “Tree” ferns have erect, thick trunks, the bulk coming from roots clustered around the small true stem. The more primitive species have a protostele, most have siphonosteles, and some have complex dictyosteles.
- Roots are simple, uncomplicated and arise adventitiously along the rhizomes near the base of the fronds.
- Sporangia are located, for the most part, on the undersides of ordinary leaves in clusters called sori (singular, sorus). In early ferns, and some living ones, sori occur on specialized, exceedingly unleaf‐like leaves. In many ferns a small leaf outgrowth called an indusium covers each sorus. Two types of sporangia exist:
- Eusporangia: These sporangia are thick‐walled and open by splitting transversely. They produce thousands of spores.
- Leptosporangia: These thin‐walled, delicate sporangia are only one or a few layers thick. They have an area, the annulus, where cell walls are thickened. When the annulus cells dry out at maturity, the sporangium splits and, like a catapult, throws out the spores. (Spores are few—128 at most, but commonly 64.)
- Ferns are divided into two groups based on the kind of sporangium they possess. The more primitive are the eusporangiate, and the more advanced the leptosporangiate.
- Ferns have the highest number of chromosomes known in vascular plants.
- Most modern ferns are homosporous (two orders of water ferns and some extinct ferns are heterosporous).
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.