FERN

any of several nonflowering vascular plants that possess true roots, stems, and complex leaves and that reproduce by spores. They belong to the vascular plant division Filicophyta, having leaves with branching vein systems and the young leaves usually unrolling from a tight fiddlehead, or crosier. The number of fern species is usually placed at approximately 12,000, but estimates range from as low as 9,000 to as high as 15,000, the number varying because certain groups are as yet poorly studied and 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 360 million years ago) and perhaps older. Their type of life cycle, dependent upon spores for dispersal, long preceded the seed-plant life cycle. any of several nonflowering vascular plants that possess true roots, stems, and complex leaves and reproduce by spores. Though ferns were once classed with the primitive horsetails and club mosses, botanists have since made a clear distinction between the scalelike, one-veined leaves of these plants and the more complexly veined fronds of the ferns, which are more closely related to the leaves of the seed-bearing vascular plants. There are approximately 10,00012,000 known species of ferns in a wide variety of sizes and shapes. Many are small, fragile plants, like those in the family Ophioglossaceae, which produce a single frond each year, or like the filmy ferns (Hymenophyliaceae), which have fronds only one or two cells thick. The genus Cyathea, on the other hand, has a treelike form with a trunklike stem up to 24 m (80 feet) tall capped by a thick crown of fronds. Ferns were prominent during the Carboniferous Period (360 to 286 million years ago), so dominating the first 25 million years that it is sometimes called the Age of Ferns. The remains of these massive forests contributed to the formation of the Earth's coal beds. The life cycle of the fern is divided into two morphologically distinct phases: sporophyte and gametophyte. The sporophyte generation is the mature, fronded form familiar in greenhouses and gardens, while the gametophyte strongly resembles a moss or liverwort. The gametophyte generation begins with the germination of a spore, a single microscopic cell produced by a mature sporophyte. Spores are generally scattered widely by the wind. They can survive long periods of dryness, but, once the proper conditions arise, they germinate to produce a tiny heart- or ribbon-shaped structure. This gametophyte is haploid; that is, it possesses a single set of chromosomes. The typical green gametophyte uses chlorophyll to manufacture its own food. Lying flat on the earth, it penetrates the soil with hairlike cells called rhizoids that absorb water and minerals. It then produces the same microscopic reproductive structures found in mosses and liverworts: antheridia (sperm producers) and archegonia (egg producers). The antheridia produce unicellular, flagellated sperms that are released in the presence of water. These sperms then try to enter the narrow cylinder of the archegonium to combine with the egg embedded in the tissues of the gametophyte. The union of the haploid egg and haploid sperm produces a diploid cell, that is, a cell with two sets of chromosomes. This begins the sporophyte generation. As the fertilized egg grows, the gametophyte shrivels and dies. The first cellular differentiation occurs when the sporophyte develops an absorbing organ, the foot, to draw moisture and minerals from the soil. Two growth axes soon appear, one that eventually develops into the main root and another that forms the first leaf. Successively larger and more complicated leaves grow from the base of this first leaf, and their bases ultimately form the stem. Once the stem is established, new fronds appear as tightly wound crosiers (also called fiddleheads) that slowly unfurl to produce a mature leaf. The cuticle, a microscopic waterproof layer, covers the exposed parts of the fern to help the plant retain moisture. The cuticle is punctured (especially on the leaves) by many slitlike openings called stomata. Each stoma is surrounded by guard cells that open or close the stoma. If a plant has sufficient water and light, the stomata open to bring in carbon dioxide for food-manufacturing cells inside the plant; if moisture and light are lacking, the stomata close to prevent the fern's interior from drying. The spores of a mature fern are contained in cases called sporangia found on the leaves of the plant. Diploid-spore mother cells line the inside of a sporangium. Each undergoes meiosis to produce four haploid daughter spores. The wall of the sporangium has an annulus, a row of cells that contract when exposed to dry air, tearing open the sporangium; they then suddenly spring back into their original shape, violently discharging the spores. In most ferns all leaves bear sporangia and are called sporophylls. In some ferns the sporophylls are specialized and are easily distinguished from the purely vegetative leaves (trophophylls). Ferns are popular as houseplants because of their attractive foliage and easy handling. In some areas their foliage and rhizomes are eaten, and an ether extract of the rhizomes of the male fern (Dryopteris filix-mas) is used by veterinarians to expel parasitic worms. Additional reading F.O. Bower, The Ferns (Filicales): Treated Comparatively with a View to Their Natural Classification, vol. 1, Analytical Examination of the Criteria of Comparison, vol. 2, The Eusporangiatae and Other Relatively Primitive Ferns, and vol. 3, The Leptosporangiate Ferns (192328), is a classic work of comparative morphology and systematics that emphasizes the need, now being realized, for a broad spectrum of comparative data. A comprehensive summary of paleobotanical knowledge is provided in Thomas N. Taylor, Paleobotany: An Introduction to Fossil Plant Biology (1981). The American Fern Society and the British Pteridological Society assemble the record of current research in the field in their publications American Fern Journal (quarterly), Fiddlehead Forum (bimonthly), The Fern Gazette (annual), and Pteridologist (annual).The abundance and diversity of pteridophytes are the focus of Hermann Christ, Die Geographie der Farne (1910), still an important broad treatment of fern distribution; John T. Mickel, How to Know the Ferns and Fern Allies (1979), the first manual to cover all of North America, with keys, brief descriptions, and illustrations; Rolla M. Tryon and Alice F. Tryon, Ferns and Allied Plants (1982), a good summary of the genera of tropical American pteridophytes with descriptions, maps, discussions, and many illustrations; John T. Mickel and Joseph M. Beitel, Pteridophyte Flora of Oaxaca, Mexico (1988), the best illustrated and most comprehensive pteridophyte manual for Latin America; and R.E. Holttum, A Revised Flora of Malaya: An Illustrated Systematic Account of the Malayan Flora, Including Commonly Cultivated Plants, vol. 2, Ferns of Malaya (1954), a well-illustrated enumeration and description of ferns that presents many of the author's ideas of systematic relationship.Life cycle and habitats are discussed in A.F. Dyer, The Experimental Biology of Ferns (1979), a series of essays on ecology, cytogenetics, reproduction, chemistry, and development; A.F. Dyer and Christopher N. Page (eds.), Biology of Pteridophytes (1985), a collection of symposium papers on a broad range of topics; F. Gordon Foster, Ferns to Know and Grow, 3rd rev. ed. (1984), a well-known book of horticulture with many helpful tips on cultivation; Barbara Joe Hoshizaki, Fern Growers Manual (1975), a good introduction to horticulture with encyclopaedic information on the species in cultivation; and Christopher N. Page, Ferns: Their Habitats in the British and Irish Landscape (1988), with excellent illustrations of habitats and ecology.Studies of form and function include K.R. Sporne, The Morphology of Pteridophytes: The Structure of Ferns and Allied Plants, 4th ed. (1975), a concise summary of ideas on fern structure; B.K. Nayar and S. Kaur, Gametophytes of Homosporous Ferns, The Botanical Review 37:295396 (1971), a thorough summation of the knowledge of the haploid generation of ferns, with an extensive bibliography; John T. Mickel, The Home Gardener's Book of Ferns (1979), a useful compilation of information on fern morphology, diversity, and cultivation; and Lenore W. May, The Economic Uses and Associated Folklore of Ferns and Fern Allies, The Botanical Review 44:491528 (1978), a summary of the diverse uses to which ferns have been put.For the origin and evolution of ferns and fern allies, see I. Manton, Problems of Cytology and Evolution in the Pteridophyta (1950), a milestone in the biology of ferns containing, for the first time, accurate data on chromosomes in relation to evolution and systematics; Richard A. White (ed.), Taxonomic and Morphological Relationships of the Psilotaceae: A Symposium, Brittonia 29:168 (1977), a series of papers on structure, relationships, and fossil history; and J.D. Lovis, Evolutionary Patterns and Processes in Ferns, Advances in Botanical Research 4:229439 (1977), an outstanding summary of the knowledge of fern phylogeny and classification. Also see appropriate sections of Robert F. Scagel et al., An Evolutionary Survey of the Plant Kingdom (1965); Ernest M. Gifford and Adriance S. Foster, Morphology and Evolution of Vascular Plants, 3rd ed. (1989); and David W. Bierhorst, Morphology of Vascular Plants (1971), which provides detailed treatments of vascular plants together with theory and interpretation.Nomenclature for the taxonomy of pteridophytes is provided in Edwin Bingham Copeland, Genera Filicum: The Genera of Ferns (1947), a valuable treatment of the classification and characteristics of ferns, containing many of the author's original correlations. Other works on classification include R.L. Hauke, The Taxonomy of Equisetum: An Overview, New Botanist 1:8995 (1974); J.A. Crabbe, A.C. Jermy, and John T. Mickel, A New Generic Sequence for the Pteridophyte Herbarium, The Fern Gazette 11:141162 (1975), a list of pteridophyte genera in a phylogenetic sequence; and Benjamin llgaard, A Revised Classification of the Lycopodiaceae s. lat., Opera Botanica 92:153178 (1987), a clear, detailed discussion of the taxonomic characters, genera, and species groups of the family, and Index of the Lycopodiaceae (1989), a listing of all the names, references, and type (original) specimens. Warren H. Wagner, Jr. Ernest M. Gifford John T. Mickel Cytogenetics Chromosome numbers and polyploidy The study of chromosomes, hybrids, and breeding systems has revealed much of value in understanding ferns. The chromosomes of ferns tend to have high base, or x, numbers, ranging from approximately 20 to 70, with the majority between 25 and 45. The familiar genus Osmunda, for example, has x = 22, Pteris has 29, Asplenium 36, Dryopteris 41, Botrychium 45, and Pteridium 52. Among homosporous ferns, exceptions to the rule of high chromosome numbers are rare; in one species of filmy fern (Hymenophyllum peltatum), x = 11, the lowest number reported. Among heterosporous ferns, however, the situation is conspicuously different, and all have low base numbers (Marsilea, x = 10, 13, or 19; Salvinia, x = 9; Azolla, x = 22). The explanation for the difference traditionally adopted by cytologists is that the high numbers in homosporous ferns arose from polyploidy, the repeated duplication of whole sets of chromosomes. Indeed, some workers regard homosporous ferns as nearly 100 percent polyploid. An alternative hypothesis should also be considered, however; namely, homosporous ferns were primitively high numbered, and heterosporous ferns derived their low numbers through reduction. Apparently the ferns do not need all their chromosomes; recent evidence has shown a considerable degree of gene silencing. The base chromosome numbers (indicated by the symbol x) have been used for classification purposes. Commonly, the base number is uniform for a genus or family, or it ranges around a given number. More rarely, the number varies drastically, as in the genus Thelypteris, which has x numbers ranging from 27 to 36, or Lindsaea, with x numbers from 34 to about 50. So much variation in the chromosome base number suggests that the genus concerned may be unnatural or that it may be very ancient, with intermediate numbers having disappeared (e.g., Dennstaedtia), or that it is in a state of active evolution (Thelypteris). Simple polyploid seriesmultiples of the base numberare prevalent among ferns, and a few species are reported to have forms or races that are diploid (with two times the base number of chromosomes), tetraploid (four times), and hexaploid (six times). For example, the fragile fern, Cystopteris fragilis, has races with the number of chromosomes per nucleus in the sporophyte generationrepresented by 2nequal to two, four, and six times the base number of x = 42; or 2n = 84, 168, and 252. Species with both diploid and tetraploid forms are common, especially among widespread, abundant ferns. In most cases the cytological races are differentiated on quantitative characters, especially the sizes of such cells as spores, epidermal guard cells (cells next to stomates), and hair cells. Hybridization In certain temperate fern genera, such as spleenworts (Asplenium), wood ferns (Dryopteris), and holly ferns (Polystichum), hybridization between species (interspecific crossing) may be so frequent as to cause serious taxonomic problems. Hybridization between genera is rare but has been reported between closely related groups. Fern hybrids are conspicuously intermediate in characteristics between their parents, and simple dominance of single characters is unusual. Occasionally, when the interspecific crosses involve strongly different characteristics, the hybrid displays an irregularity in expression of these characteristics, often involving marked asymmetry. The majority of hybrids are sterile and reproduce, if at all, only by vegetative propagation. Reproduction in sterile fern hybrids is also accomplished by the process of apogamy, in which spores possessing the same chromosome complement as the sporophyte are produced (normal spores have only half the chromosome number as the parent plant cells). These unreduced spores (with the 2n number of chromosomes) are viable and germinate into normal-appearing gametophytes that may or may not form sex organs. The hybrid gametophytes do not, however, undergo normal sexual fusion. Instead, the meristematic (cell-producing) region of the prothallium simply buds off a new sporophyte, and there is a direct conversion from gametophyte to sporophyte generation. In most fern hybrids the spore mother cells are unable to form bivalents (chromosome pairs) at meiosis, and reduction division results in irregular, deformed, and inviable spores. In the sporangia of most apogamous ferns, however, automatic doubling of chromosomes occurs by endomitosis (duplication of chromosomes without formation of two nuclei), and each of the spore mother cells has a restitution nucleusone with doubled chromosomes. In these doubled sporangia there are, therefore, only 8 spore mother cells rather than the usual 16, and they undergo meiosis, producing viable diploid spores. Apogamous ferns are known in a number of genera of higher ferns in various families, including Adiantum, Asplenium, Cheilanthes, Dryopteris, Pellaea, Polypodium, and Pteris. Besides apogamous hybrids, there are numerous demonstrated or suspected allopolyploid hybrids, which are believed to have originated by doubling of the chromosomes of sterile crosses. These are intermediate in their characteristics between well-known parental species and behave like normal, divergent species, alternating sporophytes with gametophytes and undergoing normal meiosis and fertilization. Genera with frequent hybridization often exhibit a variety of chromosome numbers that are multiples of the generic base number. One of the best examples is the tropical genus Anemia, with the base number of 38 and species with 76, 114, 152, 190, and 266. Both apogamous and allopolyploid hybrids may enjoy wide geographic ranges and occur in as great abundance as normal species. Both types of hybrids are also capable of creating additional hybrids by backcrossing (to the parent species) or by crossing with other species. In apogamous ferns it is assumed that the sperm are generally viable and capable of fusing with eggs of other, normal species. In total, hybridssterile, apogamous, and allopolyploidmay make up as many as 25 percent of the different kinds of ferns in a given flora. Curiously, in spite of the high number of ferns that are epiphytic (growing on trees), nearly all the fern hybrids are terrestrial or epipetric (growing on rocks); hybridization is very rare among epiphytes. The reason for this phenomenon is not yet clear; it could be simply that the mosses and decaying leaves on tree trunks and branches may keep the individual gametophytes apart, whereas on muddy banks gametophytes of different species may be in close proximity. Form and function Spore The fern sporea single living cell, usually protected by a thick wallis the main source of population dispersal, being readily carried by wind. Ferns display a wide diversity of spore types in terms of shape, wall structure, and sexuality, and these types prove to have great value in determining taxonomic relationships. The full functional significance of the different types, except on the grossest scale, is not yet fully understood; for example, the minute differences in sculpturing of the outer wall surface do not, in the present state of knowledge, appear to have functional significance. Shape The basic spore shape among ferns is tetrahedral; the proximal face (the one facing inward during the tetrad, or four-cell, stage following reduction division, or meiosis) is made up of three sloping planes, and the distal, or outer, face consists of a single rounded surface. The tetrahedral structure is commonly obscured in so-called globose spores, the walls of which are thin and soft. Typically, the wall is composed of exospore (outer spore layer) only, there being no additional jacket, or perispore. The wall may be either unsculptured and smooth or provided with a variety of sculptured patterns. The tetrahedral spore is formed by simultaneous division of the products of the spore mother cell. In contrast, the bilateral spore type of many fern species is formed by successive cell divisions of the spore mother cell. Where the tetrahedral spore possesses a triradiate scar on the proximal facecorresponding to its contact with three other spores in the tetradthe bilateral spore has only a narrow, linear scar running parallel to the long axis. Most bilateral spores in ferns are bean-shaped and jacketed by a perisporial layer, a distinctive covering of the outer wall.