Fern

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Fern is a common name for any of a division of cryptogamous (spore-producing) plants. The fern phylum contains about 150 genera; estimates of the number of species range from 6000 to 15,000. Ferns are found throughout the world. Most grow in damp, shady places, although certain species grow on dry ground, soil, or rocks. Some ferns, in fact, grow only in rocky places—in fissures and crevices of cliff faces and in boulders. Others grow as epiphytes, or air plants, on trees. Fossil remains of ferns have been found in rocks of the Lower Devonian Period. The phylum thus appeared on earth earlier than any seed-bearing plants. Ferns are among the oldest land plants.

Ferns vary in size from those only a few centimeters high to the tree ferns of tropical regions, which may reach a height of 24 m (80 ft). Tree ferns have woody trunks without branches, topped with clusters of feathery leaves, or fronds. Most ferns, however, have no trunks, and fronds grow directly from a short underground stem.

The reproductive cycle of ferns consists of two generations, one asexual and one sexual. The asexual, or sporophyte, generation represents the fern plant as it is commonly known. On the underside of the leaves, groups of spore cases, or sporangia, form. These can be observed as small brown spots, called sori. There are two major groups of ferns, leptosporangiate and eusporangiate. In leptosporangiate ferns, the sporangium develops from the outer derivative of a single epidermal cell, is slender-stalked, and produces fewer than 64 spores. In eusporangiate ferns, the sporangium develops from the inner derivatives of several epidermal cells, is sessile, or thin-stalked, and produces more than 256 spores. In many species all leaves are photosynthetic and bear sori. In others some leaves are sterile and photosynthetic, while other fertile leaves are nongreen and bear only sori. Upon drying out, the sporangium breaks open and in many cases catapults the spores into the wind.

When a spore falls in a place that has the proper conditions of heat and moisture, it begins to germinate, developing into a small, sexual fern plant, or gametophyte, called the prothallium. The prothallium, which in no way resembles the asexual fern plant, is a small, flat, heart-shaped structure with a number of rhizoids growing on its underside. Also on the underside of the prothallium are the sexual organs: the female organ, or archegonium, and the male organ, or antheridium. In certain species these organs are carried on separate prothallia. Fertilization of the eggs carried in the archegonia takes place only in the presence of water, which causes both organs to swell and open. Only one sporophyte plant develops from each prothallium, and after development has begun, the parent prothallium dies.

Scientific classification: Ferns make up the division Filicinophyta.

See also Maidenhair Fern.
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