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Ferns

Ferns, like the more familiar seed plants, have stems, roots, and large, highly veined leaves. Ferns do not reproduce by seeds, however, and have several other distinctive features. The leaf of a fern is called a frond and, in many species, the green blade is divided into segments called pinnae. The leaves of most ferns have a distinctive juvenile stage called a fiddlehead, where all the segments are curled in a manner resembling the end of a violin's neck. Most ferns have underground stems called rhizomes and the only parts of the fern plant visible above ground are the leaves. Some tropical ferns, called tree ferns, have erect, unbranched stems up to 20 meters tall with all of the fronds arising from the tip. Ferns are perennial plants and some may grow for many years, but, as they lack annual growth rings, their age is not easily determined. However, in 1993, researchers using molecular genetic markers found some individual bracken fern (Pteridium aquilinum) plants more than 1 kilometer across. Researchers estimated that these ferns took more than 1,180 years to grow to this size, possibly putting them among the oldest living plants on Earth.

Ferns and seed plants are similar in having two kinds of plants present in their reproductive life cycle, but overall ferns reproduce very differently than seed plants. The familiar fern plant, described in the preceding paragraph, is the sporophyte (spore-bearing phase). Fern fronds bear organs known as sporangia. Inside each sporangium certain cells undergo reduction division, or meiosis, which yields haploid spores that have one set of genes for the fern. All of the cells in the sporophyte fern plant itself are diploid, having two sets of genes. The sporangia of most ferns are very small, scalelike, and contain only sixty-four spores, but some ferns have large sporangia containing hundreds of spores. A typical fern sporophyte plant may produce up to one billion spores per year. When the sporangia open, the spores are shed into the air and dispersed. While most fern spores land within one hundred meters of the fern producing them, some may be spread very far. Fern spores have been recovered from the upper atmosphere in samples collected by airplanes and weather balloons.

When the single-celled fern spore lands on a suitable substrate, it may undergo mitotic cell division and develop into a very different kind of plant. The plant that grows from a fern spore, called the prothallus, is barely visible to the naked eye. It resembles a tiny, heart-shaped ribbon and lacks any stems, roots, leaves, or internal food- or water-conducting tissues. Reproductively, this small, independent fern plant is critical because it bears the sex organs. Although the basics of sexual reproduction in ferns were discovered in the nineteenth century, many crucial details are still being clarified. A single fern gametophyte may produce both sperm-bearing sex organs, called antheridia, and egg-bearing sex organs, known as archegonia, but frequently an individual gametophyte has only one type of sex organ. Fertilization occurs when a sperm swims to unite with an egg to form a diploid zygote, which then develops into the sporophyte.

Whether the individual gametophyte plants in a population are bisexual or unisexual is very important because it is basic to determining the degree of genetic variation possible in the sporophyte generation produced. A single bisexual gametophyte can fertilize its own eggs and produce a new sporophyte plant, but such a sporophyte would be highly inbred because both the sperm and egg producing it would be genetically identical. Most ferns control the sexual expression of the individual plants in a gametophyte population so that each plant is either male or female. Thus, fertilization usually requires two gametophytes that are close enough for sperm to swim in water between them. Receptive archegonia secrete a sperm attractant to help the sperm find its way. When genetic material from two different gametophytes is mixed in the zygote, the sporophyte that develops has more genetic variation than one arising from a single gametophyte. If the entire fern sporophyte population is reproduced this way, it may be more likely to survive because some of its members may have inherited the traits needed to endure unforeseen changes in its environment. On the other hand, a distinct survival advantage arises when a single fern spore, dispersed a long distance, can produce a sporophyte from one gametophyte, because this permits rapid colonization of distant, favorable habitats.

Although factors regulating fern spore germination and development are fairly well known from laboratory studies, relatively little is known about how ferns actually reproduce in their environments. Most fern spores germinate readily on moist soil. Germination often requires red light that is absorbed by a pigment in the spore. Calcium ions are important to germination, and red and blue light control the pattern of gametophyte development. Fern spores are known to persist in the soil, forming spore banks. These factors and many more interact in complex ways in the field. Ecologically, most fern species are found in habitats where moisture is readily available, permitting gametophytes to grow and sperm to swim to eggs in water. Those concerned with preserving a rare fern species at a site must understand that if the locality does not provide safe sites for the independent gametophytes, with their distinct ecological requirements, the species cannot reproduce and the sporophytes will eventually die off.

Moist, tropical mountain forest communities contain the largest number of fern species. Of the approximately 12,000 fern species worldwide, about 75 percent are tropical. The flora of North America, north of Mexico, contains about 350 fern species, whereas southern Mexico and Central America have about 900. A few ferns are occasionally eaten. However, some, like bracken fern, contain poisons or carcinogens. Ferns are present in most plant communities but dominant in few.