Ferns

There are about 11,000 species of ferns in the world. Ferns are found throughout the world, from the tropics to the subarctic region. The greatest species diversity is in the tropical and subtropical region from southern Mexico to northern South America.

In temperate North America, most ferns are terrestrial plants and grow in woodlands. However, in the tropics, many ferns grow as epiphytes. Epiphytes are plants which rely upon other plants, such as trees, for physical support, while obtaining their nutrition from organic debris and rain water that falls through the forest canopy.

Ferns can be found in very different habitats throughout the world. Some species are free-floating aquatic plants, some species grow in moist woodlands, and a few species grow in arid or semiarid regions. Most species require some rainfall because their sperm cells must swim through a fluid to reach the egg cells.

Interestingly, sperm cells of the resurrection fern (Polypodium polypodioides) swim through a fluid exuded by the fern itself to reach the female's egg. This species is widely distributed in semi-arid and arid regions of the world, such as central Australia, central Mexico, and central Africa. The resurrection fern is sold in some garden shops as a brown, dried-out, and curledup ball of fronds. When this dried-out fern is soaked in water, it rapidly expands and becomes green in a day or so, attesting to the remarkable desiccation tolerance of this species.

At the other extreme are water ferns of the genus Azolla, which grow free-floating on fresh water. Azolla is particularly interesting because it has special pockets in its leaves which apparently have evolved to accommodate symbiotic cyanobacteria of the genus Anabaena. These cyanobacteria transform atmospheric nitrogen (N₂) to ammonia (NH₃), a chemical form useful to plants. This process is called nitrogen fixation. Many Asian farmers encourage the growth of Azolla and its associated Anabaena in their rice paddies to increase the amount of nitrogen available to their rice plants.

Many species of ferns can act as alternate hosts for species of rust fungi that are pathogenic to firs, economically important timber trees. The rust fungi are a large and diverse group of fungi, which have very complex life cycles, often with four or five different reproductive stages. In the species of rust fungi that attack ferns, part of the life cycle must be completed on the fern, and part on the fir tree. These parasitic fungi can usually be eradicated by simply eliminating one of the hosts, in this case, either the fern or the fir tree.