fern

fern, class of non-flowering, herbaceous vascular plants that possess true roots, stems, and complex leaves and that reproduce by spores. The number of known extant fern species is about 10,500, but estimates have ranged as high as 15,000, the number varying because certain groups are as yet poorly studied and because new species are still being found in unexplored tropical areas. The ferns constitute an ancient division of vascular plants, some of them as old as the Carboniferous Period (beginning about 358.9 million years ago) and perhaps older. Their type of life cycle, dependent upon spores for dispersal, long preceded the seed-plant life cycle. Another informal name for the group, monilophytes, has gained currency in modern botanical literature.

The ferns are extremely diverse in habitat, form, and reproductive methods. In size alone they range from minute filmy plants only 1–1.2 cm (0.39–0.47 inch) tall to huge tree ferns 10 to 25 metres (30 to 80 feet) in height. Some are twining and vinelike; others float on the surface of ponds. The majority of ferns inhabit warm, damp areas of the Earth. Growing profusely in tropical areas, ferns diminish in number with increasingly higher latitudes and decreasing supplies of moisture. Few are found in dry, cold places.

Some ferns play a role in ecological succession, growing from the crevices of bare rock exposures and in open bogs and marshes prior to the advent of forest vegetation. The best-known fern genus over much of the world, Pteridium (bracken) is characteristically found in old fields or cleared forests, where in most places it is often succeeded by woody vegetation.