Ferns are the biggest and most diverse group of non-flowering plants, with about 11,000 species. Most species live in the tropics and many are epiphytes on trees. They typically have large feathery leaves or fronds which are tightly coiled when young, unwinding as they expand. Some ferns are large enough to be called tree ferns but they do not develop a trunk structure, the stalks of the fronds sprouting directly from a rhizomous rootstock. The reproduction of ferns is complex. Spores are produced on the undersides of the fronds. If these fall in a suitable damp place they develop into small heart-shaped structures called prothalli. After a further sexual stage, a new plant grows from a female prothallus.
Order of tropical ferns which date back to the carboniferous era. It is considered to be more primitive than the Polypodiopsida as the spore-bearing sporangium forms a whole layer of cells rather than being grouped into discrete piles called sori. There are about 6 genera and 200 species. The tree fern, Psaronius which is now extinct, was a member of this group.
Order of water ferns containing just three genera. They are heterosporous which means that they produce large megaspores and small microspores, corresponding to female and male spores. The genus Marsilea with about 50 species grows in mud, with its leaves resembling four-leaved clover floating in the water above. Its drought-resistant sporocarps can survive many years while still remaining viable.
Order of ferns with just three living genera, two of which are widespread in the northern temperate regions, Botrychium, the grape ferns and Ophioglossum, the adder’s tongues. Their gametophytes are subterranean and tuberous.
This is much the largest order of ferns with about 35 families, 320 genera and 10,500 species. Members are homosporous, meaning the spores they produce are all alike. These spores are borne in sporangia, which are ofter clustered together in heaps called sori. The spores germinate to produce free-living gametophytes which are inconspicuous and grow in moist places. These then produce male and female reproductive structures. When they have been fertilized, new plants grow from the female gametophytes.
Order of small, floating ferns with just two genera. Salvinia has leaves in whorls of three on the floating rhyzome. Two are small, undivided leaves covered with hairs for buoyancy, but the third is much larger and bears the sporangia. It resembles a mass of white threads that hang down into the water. Azolla is a tiny fern that floats in the paddy fields of Southeast Asia and is of considerable economic importance. There are pouches in its floating leaves that house the cyanobacterium Anabaena azollae. These bacteria fix nitrogen from the air and provide the rice fields with a valuable source of fertility.