Fern, any one of a large class of flowerless, seedless plants. There are about 12 families, 260 genera, and at least 10,000 species. About 250 species of ferns grow in the United States, chiefly in Florida and other southeastern states. A few species, however, grow north of the Arctic Circle. Ferns are used in floral pieces and as house plants.
Ferns vary in size from tiny mosslike growths to tree ferns sometimes 60 feet (18 m) tall and one foot (30 cm) or more in diameter. Ferns are among the oldest of land plants. Fossils of ferns have been found in rocks of the Devonian period some 370 million years old. During the Carboniferous and Permian periods, which followed the Devonian, ferns became one of the chief types of land plants. Scientists believe ferns and seed fernsextinct plants that had both seeds and fernlike leavessupplied much of the vegetable matter that became coal.
The tree ferns of the tropics have woody trunks topped with clusters of feathery leaves, or fronds. Most ferns have no trunks; the fronds grow directly out of the ground from rhizomes, or rootstocks, modified stems that grow horizontally underground.
A fern does not have flowers nor does it produce seeds. The life cycle of a fern is shown in the illustration above. Like all plants, ferns undergo an alternation of generationsone generation reproduces asexually, the next sexually. The plants that reproduce asexually are called sporophytes; those that reproduce sexually, gametophytes. The mature fern is the sporophyte. Small circular growths called sori appear on the underside of the fronds, on modified leaves, or on separate stalks of the sporophyte, depending on the species. Each sorus is made up of many spore cases called sporangia. Each sporangium contains many one-celled reproductive structures called spores.
When ripe, the sporangia break open, releasing the spores. If a spore falls in a spot where conditions are right for growth, it develops into a small heart-shaped plant, the gametophyte, or prothallium. The gametophyte is less than 0.4 inch (one centimeter) wide. On the underside of the gametophyte are many tiny, hairlike structures called rhizoids, which absorb water and nutrients. The gametophyte's sexual organs, containing male and female gametes (sex cells) are also on its underside.
A film of water from rain or dew causes the sex organs to swell and open. The male organ, or antheridium, releases sperm cells. These cells swarm about the neck of the female organ, or archegonium, which contains the egg. One sperm cell enters the archegonium and fuses with the egg, producing a zygote.
The zygote divides into many cells, forming an embryo. After being nourished within the gametophyte and undergoing more cell division, the embryo develops into a new sporophyte. The gametophyte withers away soon afterward.
Ferns make up the division Pterophyta, or Filicinophyta. Among the many genera of ferns are the tree ferns (Cyathea), the maidenhairs (Adiantum), the sword ferns (Nephrolepis), wood ferns (Dryopteris), the walking leaf (Camptosorus), spleenworts (Asplenium), the bracken (Pteridium), the filmy ferns (Trichomanes and Hymenophyllum), and the ostrich ferns (Pteretis).