Polypodiophyta. The ferns are terrestrial or sometimes epiphytic or rarely aquatic homosporous or rarely heterosporous vascular plants comprising about 350 genera and 12,000 species. The free-living sporophytic plant body consists commonly of a rhizome with adventitious roots, and an aerial portion comprising 1-several, commonly tufted leaves, or rarely an aerial stem crowned with leaves. The leaves are typically large, megaphyllous, simple to highly pinnately compound, and occasionally dimorphic with fertile and sterile leaves or portions of leaves differentiated. Like other lower vascular plants, a small number of ferns have thick-walled sessile eusporangia derived from more than one initial cell and containing hundreds of spores, however, most of the ferns have narrowly stalked, thin-walled leptosporangia derived from a single superficial initial cell and containing 128 or fewer spores. Sporangia are generally grouped in clusters called sori that vary greatly in size, shape, protective structures called indusia, and position on the margins or lower surfaces of leaves. Leptosporangia have a hygroscopic mechanism that catapults spores from the spore chamber. The gametophyte is typically free-living, photosynthetic, often heart-shaped, usually about 5 mm or less in diameter and bisexual with archegonia produced around the notch and antheridia produced at the base. Multiflagellated sperm produced in an antheridium must travel through a film of water in order to reach the egg of an archegonium and initiate the zygote that may develop into a new sporophyte individual.

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