Pteridophytes (Ferns)

Transcription

1 George Yatskievych, Missouri Botanical Garden, St Louis, MO, USA Pteridophytes (vascular cryptogams or ferns and fern allies) comprise about species of primitive vascular plants; they do not produce flowers or seeds and reproduce instead via spores. They occur in most terrestrial habitats and also in some aquatic communities. Some species are very beneficial to humans, but the group also contains important species of weeds. Introduction Pteridophytes, also known as vascular cryptogams and ferns and fern allies, comprise about species of vascular plants that do not produce flowers or seeds, reproducing instead via the production of spores. Pteridophytes occur in most terrestrial habitats on earth and are also present in some aquatic communities. They are an important part of the ground vegetation in many forest communities and, with about one-third of the species growing on the trunks and branches of trees, they are also an important component of many epiphytic plant communities. Some species are very beneficial to humans, but the group also contains some of the most important weed species in the world. Life Cycle Pteridophytes are characterized by a life cycle that usually involves an alternation of two free-living generations sporophyte and gametophyte with the sporophyte the larger phase of the life cycle. Nonvascular plants like mosses and liverworts also have an alternation of generations, but in these organisms the gametophyte generation is generally the dominant phase. In seed plants, the gametophyte is no longer free-living but remains enclosed in tissues on the sporophyte and there is a progressive reduction in the size through various gymnosperm groups such that in flowering plants (angiosperms) the gametophyte generation is reduced to just a few cells in the germinating pollen grains and the ovules. The conspicuous phase of the pteridophyte cycle is the sporophyte, which is how most people observe the plants in nature. These are usually perennial. Sporangia are produced on the leaves of sporophytes (sometimes in specialized cone-like strobili). In true ferns, these are commonly on the leaf undersurface and are often clustered into discrete units called sori. Within each sporangium, specialized cells undergo a series of mitotic (structural) divisions followed by meiosis (sexual division) that results in production of spores with half as many chromosomes as in the original sporophyte. The more advanced ferns. Introduction. Life Cycle Introductory article. Reproductive Variations. Cytology Article Contents. Morphology and Anatomy. Systematics and Classification. Economic Importance usually have 64 spores per sporangium, but more primitive ferns and fern allies may have hundreds or even thousands. At maturity, the sporangium dries and ruptures, dispersing the spores into the air. When a spore lands on a suitable substrate, it germinates, the cells dividing and forming first a filament and eventually usually a heart-shaped gametophyte (sometimes other shapes in some groups) that is the same species as the sporophyte but appears very different. Gametophytes are often moss-like in appearance and are quite small, usually less than 1 cm wide at maturity, but are often fairly easily located in nature near adjacent sporophytes. Although in a few genera gametophytes can be long-lived, in most ferns their lifespan is usually much less than a year. They are the sexual phase of the life cycle in that they produce multicellular sex organs at maturity on the side away from the light. The more or less spherical antheridia (male gametangia) are produced among the rhizoids towards the base of the plant and at maturity they pop open to release motile flagellated spermatozoids. Archegonia (female gametangia) are usually produced at the opposite end near the notch region, and are flask-shaped structures containing a single egg cell. A film or droplet of free-standing water is necessary in order for the spermatozoids to swim to an archegonium of the same or a different gametophyte. The neck cells of the archegonium spread at maturity and the spermatozoid swims down the archegonial canal to fuse with the egg, effecting syngamy (fertilization) and forming a zygote with twice the number of chromosomes as the gametophyte. This zygote grows and develops into a new sporophyte, completing the cycle, while the maternal gametophyte withers away. In a typical pteridophyte, each gametophyte is potentially bisexual, producing both antheridia and archegonia. Because the eggs and spermatozoids of an individual all grew from a single spore and are thus genetically identical, potentially these plants can become self-fertilized in a way that renders the resultant sporophyte entirely homozygous (having only one kind of allele for each gene locus) for its entire makeup. Various mechanisms exist to promote cross-fertilization: the gametangia often mature at different times; the genome may have deleterious alleles that are ENCYCLOPEDIA OF LIFE SCIENCES 2002, John Wiley & Sons, Ltd. 1

2 fatal to homozygous individuals (genetic load); and there may exist mating factors that prevent successful selffertilization. Some ferns also produce pheromones known as antheridiogens, in which the first spore to germinate at a site becomes a female gametophyte and exudes a substance causing later-germinating spores to develop into male gametophytes. Reproductive Variations Unusual gametophytes In some ferns and fern allies, including some clubmosses (Lycopodiaceae), whisk ferns (Psilotaceae), and grape ferns (Ophioglossaceae), the gametophytes are not surfacedwelling and green. Instead, they are subterranean and nonphotosynthetic, often appearing as pale brown or yellow fuzzy cylinders or pads of tissue. These gametophytes are mycotrophic; that is, they receive their nutrients from soil-borne fungi that establish connections with their rhizoids. However, although such gametophytes are usually slow-growing, they usually produce normal gametangia and otherwise complete their life cycles in the typical fashion. Heterospory Other ferns and fern allies, including spike mosses (Selaginellaceae), quillworts (Isoetaceae), and aquatic ferns (Azollaceae, Marsileaceae, Salviniaceae), depart from the typical life cycle in producing two different types of sporangia. One of these produces numerous microscopic microspores that germinate to produce male gametophytes. The other sporangial type produces many fewer and much larger megaspores (usually visible to the naked eye), which grow into female gametophytes. In both spore types the gametophytes are reduced in structure and develop mostly within the ruptured spore wall. Vegetative reproduction Many pteridophytes supplement their sexual cycles with various forms of vegetative reproduction. This may be as simple as the fragmentation of a creeping rhizome into smaller pieces that become established as separate plants. Horsetails (Equisetaceae) growing along rivers and streams are frequently spread over long distances in this fashion by flooding. Other species develop specialized structures to effect vegetative propagation. Some ferns produce stolons, which are specialized long, spreading stems that root at their tips and form new plants. Others produce buds or bulbils on their leaves that can germinate to form new plantlets. Still others produce roots where their fronds come into contact with soil. A few species produce specialized underground structures, such as tubers and similar offsets. Some species have sporophytes or gametophytes that produce gemmae, which are specialized relatively undeveloped fragments of plants that break off and are dispersed, eventually germinating to form new plants. In a few species of filmy ferns (Hymenophyllaceae), shoestring ferns (Vittariaceae), and other families, the ability to produce sporophytes has been lost and the plants exist only as colonies of gametophytes spreading through the production of tiny air-dispersed gemmae. Apogamy Apogamy is a widespread and important mechanism of reproduction in pteridophytes more than in any other group of plants. Apogamous ferns, which frequently occur in environments with seasonal extremes of heat, cold and/ or drought, avoid the necessity for sex. In the sporangia of such plants, a mechanism during the series of cell divisions results in the production of spores with the same genetic constitution as the sporophyte plant (meiosis does not result in a reduction in chromosomal ploidy). These diplospores grow into gametophytes that produce new sporophytes directly from meristematic tissues near the notch region. The environmental advantages of apogamy include the faster development of the gametophyte and the release from the requirement of standing water for fertilization to take place. Interestingly, many apogamous ferns continue to produce antheridia with functional spermatozoids, which can be released and fertilize eggs on nearby gametophytes of related sexual species. Once formed, such hybrids are always apogamous and thus able to reproduce themselves. Cytology Pteridophytes characteristically have high chromosome numbers. The highest chromosome number recorded for any organism is in an adder s tongue fern, Ophioglossum reticulatum, with about 1260 pairs of chromosomes. The base chromosome number (x) in various genera is quite variable: for example, Asplenium (x 5 36), Botrychium (x 5 45), Osmunda (x 5 22), Pellaea (x 5 29), Polystichum (x 5 41); and Pteridium (x 5 52). Exceptions occur in a few ferns, including most aquatic genera (Salvinia, x5 9). Some fern allies also have low chromosomal base numbers (Selaginella, x5 mostly 7 10). Several theories have been advanced as to why ferns have so many chromosomes. Among the most intriguing is that of palaeopolyploidy. Botanists have long known that polyploidy (the development of extra sets of chromosomes over the basic diploid level) is very widespread and common in pteridophytes. Both autopolyploidy and allopolyploidy have been documented in numerous genera. 2

3 Allopolyploidy involves hybridization between species, resulting in a sterile hybrid that regains its fertility by doubling its chromosome number during spore production in some sporangia. Autopolyploidy involves the same doubling of chromosome number during spore production but without a hybridization event. With polyploidy so pervasive among pteridophytes, geneticists have long been puzzled that most fern species are apparently functionally diploid, having only two alleles for each gene locus. In the last two decades, gene-silencing the selective shutting off of duplicate copies of genes found in polyploid species has been documented in several fern genera. This has given rise to the hypothesis that, over time, ferns undergo regular rounds of polyploidy (which increases their chromosome numbers) followed by gradual diploidization of the genomes (selective silencing of the extra gene copies to return the species to a functionally diploid level). Evidence for this mechanism is circumstantial, but it seems likely to function in at least those cases that have been better studied thus far. Morphology and Anatomy Sporophytes Stems Most ferns have specialized stems called rhizomes that are positioned at the level of the substrate or somewhat buried. Rhizomes vary greatly in size, thickness and orientation. Most commonly, they are horizontal and creeping, but many species have short upright rhizomes. In some groups, notably the tree ferns, specialized stems are trunk-like and may be 20 m or more tall. These modified stems produce only adventitious roots and are usually covered with dense scales or hairs, at least towards the growing tip. Other types of stems occur in some primitive ferns and in most fern allies. Grape ferns (Ophioglossaceae) usually have somewhat tuberous stems. Horsetails (Equisetaceae) have both rhizomes and fluted or ridged aerial stems. Quillworts (Isoetaceae) have very short stout stems with the nodes very close together (corms). Most clubmosses (Lycopodiaceae) have relatively unspecialized stems Rhizomes are structurally simpler than those of most seed plants in that they do not produce secondary growth (wood). Even the tree ferns have only primary growth, and a thick mantle of interwoven roots is produced to help with structural support. In most ferns, the vascular system of the stem is in the form of a hollow cylinder interrupted (with gaps) where traces branch off to the leaves. In crosssection, most fern rhizomes thus appear as an irregular ring of vascular bundles. In some primitive ferns and fern allies, the vascular system is a solid uninterrupted cylinder. Leaves Pteridophytes exhibit an amazing variety of leaf morphologies. In most fern allies and a few primitive ferns, the leaves are reduced and scale-like, needle-like or grass-like, with at most a single vein. In most true ferns, however, the leaves are the dominant organ of the sporophyte and can be extremely complex in their pattern of division. In a few genera (especially in the Gleicheniaceae), the leaves are indeterminate in growth; that is, they continue to elongate at the tip, often reaching several metres in length and clambering over surrounding vegetation. The petiole (stipe) of fern leaves may be circular, angled, or U-shaped in cross-section, and is sometimes hairy or scaly. There are one to several vascular strands, and the number and position of these in the petiole are often diagnostic for individual families or genera. In most ferns, the development of the leaf follows a pattern known as circinate vernation. This produces a characteristic fiddlehead or crozier as the leaf uncurls. In a few genera, this pattern has become modified so that the unfurling leaf produces a hook-like structure. The fern allies and the grape ferns (Ophioglossaceae) do not exhibit circinate vernation but expand by unfolding or in an indefinite pattern. The leaf blade (lamina) varies from entire to highly divided, with pinnate, pedate and palmate patterns of division in various species, but most commonly is one or more times pinnately compound. The continuation of the petiole as the central axis of the leaf blade is known as the rachis, to which the pinnae (primary divisions or leaflets) are attached. The pinnae may themselves be entire or one or more times compound. The ultimate divisions of the leaf are called pinnules, which may be entire or lobed. Venation of the leaves can be quite complex, with several orders of successively finer midveins (costae) and lateral veins. The venation of the pinnules may be unbranched or branched with free to variously anastomosing veinlets. Fern leaves may be glabrous or variously covered with hairs and/or scales. In some species, the leaves are glandular and sticky. Other species secrete a powdery farina, usually on the leaf undersurface, which may be white or bright yellow or orange. The leaves also vary greatly in thickness and texture. The thinnest leaves occur in the filmy ferns (Hymenophyllaceae), in which the leaves are often only two cell layers thick. The production of thick, leathery leaves or leaves with dense vestiture of hairs, scales, glands or farina is generally explained as adaptation to droughty habitats and/or areas of high sunlight. Sporangia In fern allies and a few primitive ferns, relatively large sporangia are produced either in complex cone-like strobili at the stem or branch tips or in the axils at the bases of leaves. Some fern species produce dimorphic leaves, with vegetative (trophophyll) and fertile (sporophyll) leaves 3

4 having different morphologies. In other ferns, the leaf is divided into specialized fertile and vegetative regions. However, in most ferns, the sporangia are produced on the undersurface of normal leaves. The positional patterns and other details of sporangia on the leaf undersurface are often diagnostic for particular families or genera and are a principal tool in fern classification. At one extreme, the sporangia may entirely cover the leaf undersurface (acrostichoid). In contrast, in some primitive ferns, the sporangia are sparsely scattered along some veins. In most ferns, however, the sporangia are grouped into discrete lines or clusters known as sori. Sori may be circular to linear, positioned along the margin or towards the midvein (costa), surficial or in a groove or channel, etc. In some cases, the developing sori are protected by a recurved leaf margin (false indusium), a covering of deciduous scales, or a more permanent small flap of tissue, the indusium. Indusia vary greatly in shape, size, texture and persistence, ranging from umbrellashaped to globose to linear. In the water ferns (Azollaceae, Marsileaceae, Salviniaceae), the sporangia become enclosed in hardened capsular structures called sporocarps that are formed either from modified leaflets or from modified indusia. The sporangia themselves are usually positioned on a somewhat thickened vein ending or along a portion of a vein. In most cases, the sporangium consists of a stalk, of varying length and cell number, and a multicellular capsule. In most ferns, the capsule is differentiated into thin-walled cells and an annulus, a ring or region of cells with only some of the walls thickened. The annulus functions in spore release. Spores Spores are the main structures by which ferns are dispersed to form new populations. As such, in most ferns, they are relatively impervious, long-lived and metabolically inactive. Although the majority of spores produced fall within a few metres of the parental sporophyte, the spores of some ferns have been recovered from air currents in the stratosphere during high-elevation atmospheric sampling studies and ferns are among the most successful colonists of highly isolated oceanic islands. Although the life of most spores is measured in terms of months or a few years, in some cases fern spores have been induced to germinate after more than a hundred years of storage. In a few groups scattered throughout the ferns and fern allies, the spores are relatively thin-walled, green and photosynthetically active, and relatively short-lived, reflecting an adaptation to rapid establishment of new plants following dispersal. Developmentally, spores are the direct products of meiosis, which begins with a single spore mother cell undergoing two separate rounds of division, and yields a tetrad of products that breaks apart into four individual spores. The spatial relationship between the plane of division of the two rounds of meiotic division affects the shape and markings of the resulting spores. Two main types are recognized. Trilete (tetrahedral) spores vary from nearly spherical to somewhat three-angled and have a three-branched scar where each spore was attached to the others in the tetrad. Monolete (bilateral) spores are ellipsoid to bean-shaped and have a linear attachment scar along one side. The attachment scar is usually where the spore ruptures during germination. As it matures, each spore develops two or three outer protective layers, the relatively thin endospore, the thick perispore, and in some cases an outermost exospore. The exospore and also portions of the perispore actually develop from materials produced by the inner sporangium wall and deposited over the spore, rather than by the spore itself. Mature spores vary greatly in size and surface sculpturing. Spores of some Marattiaceae are only about 15 mmin diameter, whereas the megaspores of some Selaginella species may approach 1 mm. The surface sculpturing is often diagnostic for various species, genera and/or families, ranging from smooth to wrinkled, spiny and/or with wing-like ridges. Gametophytes Upon germination of spores, cell divisions produce first a filamentous structure. In most ferns, this subsequently continues to divide in two or more planes and eventually differentiates into the mature gametophyte. The typical fern gametophyte is a flat, heart-shaped structure, with two lobes and an intervening notch at one end and the other end narrowed or rounded. It can vary in size from a few millimetres to about 1 cm in size. A number of variations exist but are not widespread, including filamentous, strapshaped and irregularly lobed gametophytes. In pteridophytes with subterranean mycorrhizal gametophytes, these can mature to various shapes, but most are either tubular or cushion-shaped. Gametophytes are moss-like in that they lack vascular tissue and roots. Slender hair-like structures called rhizoids function to absorb water and nutrients and act to anchor the gametophyte to the substrate. The gametangia (sex organs) generally are formed on the side of the gametophyte away from the light (except in subterranean gametophytes). The antheridia are positioned among the rhizoids and are more or less spherical structures consisting of a jacket of cells enclosing the spermatozoids. The archegonia are usually positioned near the notch on a slightly thickened pad of tissue. They are flask-shaped and somewhat sunken into the tissue. The neck of the archegonium consists of four columns of cells that separate at maturity, opening a canal and exposing the egg cell in the base for fertilization by the spermatozoid. 4

5 Systematics and Classification Classification of pteridophytes remains somewhat controversial. The terms pteridophytes, ferns and fern allies and vascular cryptogams continue to be used informally by botanists who wish to avoid becoming enmeshed in the technical details of competing systems of fern classification. Although many of the groups of ferns and fern allies are distinctive and have been recognized since antiquity, the relationships among these groups and the taxonomic level at which they should be recognized still has not been fully resolved. In recent years, a consensus has begun to emerge, and molecular phylogenetic studies involving mostly the comparison of various gene sequences have helped to refine theories of pteridophyte evolution and taxonomy. In general, pteridophytes have a long fossil record and the main lineages trace their origins to the first vascular land plants. Pteridophytes were dominant plants in the swamps of the Carboniferous Period more than 300 million years ago, which gave rise to the world s major coal deposits. In some groups, such as the horsetails (Equisetaceae), the relatively few modern species are the remnants of formerly much more diverse lineages. On the other hand, some modern fern species have existed for long times, as shown by the fossils indistinguishable from the modern sensitive fern (Onoclea sensibilis) dating back to the Palaeocene Epoch more than 60 million years ago. In recent years, a fundamental shift in our understanding of primitive pteridophyte classification has occurred. Traditionally, three or four main groups had been recognized. These included the clubmosses and related groups (lycophytes), the horsetails (sphenophytes, sometimes called arthrophytes), the true ferns (filicaleans), and sometimes the whisk ferns (psilophytes). The psilophytes, an unusual group with structurally relatively simple plants, were considered the most primitive group of extant vascular plants by some botanists and true ferns with reduced simplified structure by others. Recent anatomical and molecular studies have shown that the latter interpretation is probably correct the Psilotaceae are primitive ferns whose stems, leaves and sporangia have become simplified over time. These same studies have yielded an even more fundamental conclusion. The lycophytes are apparently the most primitive group of extant vascular plants. The lineage leading to the seed plants (gymnosperms and angiosperms) has its origins within the pteridophyte lineage before the divergence of both the true ferns and the horsetails. Thus, the most recent hypothesis of pteridophyte evolution advocates the existence of two fundamental groups, the lycophytes and the remaining ferns and fern allies. There are a number of relatively primitive fern families, many of which are represented by relatively few modern species but some of which have extremely long fossil records. The greatest diversity of modern species exists among the most advanced fern groups, and the number of families to be accepted and the relationships among these families are the topic of intensive systematic research at present. Table 1 summarizes current hypotheses concerning pteridophyte classification, from most primitive to most advanced. An estimate of the number of extant genera and species, in parentheses, follows each family name, and also the common names of selected well-known examples. Economic Importance Relatively few species of pteridophytes are economically important. Perhaps the best-known current use is horticultural, as garden plants, house plants and specimen plants in conservatories and greenhouses. One species, Ruhmora adiantiformis is often called florist s fern; its finely divided but thick and leathery leaves resist wilting and are used in cut flower arrangements. Another horticultural practice has been the use of chunks of the dense rotresistant root mantles covering the stems of tree ferns (known as orchid bark) as a substrate for growing orchids and other plants that are epiphytic in nature. However, this has caused the decline and endangerment of numerous tree fern species, with the result that commercial trade in tree fern products is now strictly regulated by international law and trade treaties. A number of ferns have been used in handicrafts. Petioles of some members of the climbing fern family, Schizaeaceae, as well as other groups, are used in some tropical countries for colour designs in basketry and bracelets. Pteridium (bracken) leaves have been used to make a green dye. The rhizomes of the tree fern Cibotium, which are covered with dense, long, golden hairs, have been fashioned since antiquity into animal statue curios sometimes known as vegetable lamb of Tartary. One group of pteridophytes with an extensive history of use is the clubmosses (Lycopodiaceae). The microscopic spores of these fern allies contain nonvolatile oils that made them useful as dry industrial lubricants. They have also been used to keep latex items such as surgical gloves and condoms from sticking together, but this practice has been mostly discontinued since it was discovered that the spores caused skin irritation and allergic reactions in some people. Other uses of the spores have been in flash powder for photography and in fingerprint powder used in forensic investigation. Various ferns are also eaten as food, with the young foliage usually steamed as a vegetable or dried and used as an additive in stews and sauces. Several species are eaten, including Diplazium esculentum (which is cultivated for this purpose in parts of Asia), but the commercially most important species in the western hemisphere is Matteuccia struthiopteris, the ostrich fern, whose fiddleheads are a 5

6 Table 1 Summary of the classification of extant pteridophyte families Lycophytes (Division Lycopodiophyta) (4 extinct orders 1 3 extant orders (each with 1 family)) Lycopodiaceae (4/380), clubmosses Isoetaceae (1/130), quillworts Selaginellaceae (1/800), spikemosses Ferns (Division Pteridophyta) Eusporangiate ferns (3 extant orders, each with 1 family) Psilotaceae (2/12), whisk ferns Ophioglossaceae (3 8/80), grape ferns Marattiaceae (4/100), giant ferns Sphenophytes (3 extinct orders 1 Equisetales) Equisetaceae (1/15), horsetails Leptosporangiate ferns (1 extinct order 1 about 10 extant orders) Primitive isolated groups (6 orders (each with 1 modern family except that Cheiropleuriaceae is in Dipteridales)) Osmundaceae (3/20), cinnamon and royal ferns Hymenophyllaceae (2 or 3/650), filmy ferns Stromatopteridaceae (1/1) Gleicheniaceae (4/150), scrambling ferns Cheiropleuriaceae (1/1) Dipteridaceae (1/8) Schizaeaceae (5/200), climbing ferns, curly grass fern Heterosporous aquatic groups (Order Marsileales, also called Hydropteridales) Marsileaceae (3/70), water clovers Azollaceae (1/6), mosquito ferns Salviniaceae (1/11), water spangles Tree ferns (Order Cyatheales) Loxomataceae (2/2) Plagiogyriaceae (1/11) Matoniaceae (2/4) Metaxyaceae (1/2) Dicksoniaceae (5/28 33), tree ferns Lophosoriaceae (1/1) Hymenophyllopsidaceae (1/8) Cyatheaceae (14 20/ ), tree ferns Advanced groups (Order Polypodiales) Lindsaeaceae (5/200) Dennstaedtiaceae (14/350), cup ferns, hay-scented fern, bracken Pteridaceae (35 45/1150), cliff brakes, lip ferns, goldback ferns, maidenhair ferns, shoestring ferns Aspleniaceae (1/800), spleenworts Thelypteridaceae (1 35/900) Blechnaceae (8/250), chain ferns Dryopteridaceae (including Tectariaceae) (30/950), wood ferns, shield ferns, halberd ferns, ostrich fern, sensitive fern Woodsiaceae (including Athyriaceae) (10 13/500), fragile ferns, lady ferns, oak ferns Lomariopsidaceae (6/550), paddle ferns, tongue ferns Davalliaceae (14/120), rabbit s foot ferns, Boston fern Polypodiaceae (including Grammitidaceae) (35 45/1050), polypodies, staghorn ferns common sight in markets of the northeastern United States in late spring. Formerly, Pteridium aquilinum (bracken) was quite important in some cuisines, particularly in parts of eastern Asia. However, medical studies have linked this species to stomach cancer and its use has declined. Perhaps the most economically valuable species of pteridophyte is Azolla, a genus of tiny floating aquatic ferns. For centuries farmers in parts of eastern Asia jealously guarded strains of this plant, which they used to inoculate rice paddies in the spring for markedly increased yields. During the Vietnam War era, this practice came to 6

7 the attention of western scientists. They discovered that hollow chambers in Azolla leaves contain symbiotic cyanobacteria (Anabaena azollae) that are able to convert atmospheric nitrogen into the nitrate form that serves as a major plant nutrient. Thus, the fast-growing plants of Azolla acted as a living source of fertilizers. During the past few decades, millions of dollars have been spent to locate superior strains of this fern and to make the process more efficient, in an attempt to increase rice production in developing countries. A few ferns have had negative economic impacts because of their weediness. Two of the best examples include Salvinia and Pteridium. Salvinia molesta (Kariba weed, giant salvinia) is a floating aquatic fern that is weedy throughout the warmer parts of the world. In some situations it can form a mat several inches thick on the surface, that prevents light and oxygen penetrating into the water. In places such as New Guinea, this fern has at times threatened to destroy local fishing economies, and it is being carefully eradicated where found outside of its natural range in southern Brazil. Pteridium aquilinum (bracken) is a coarse fern with an immense creeping rhizome capable of reaching lengths of 400 m. The plant quickly invades open habitats, competing vigorously with other plants. Because the plants are toxic to livestock, bracken has ruined the pasturage on large acreages of land, especially in parts of Europe. Further Reading Camus JM, Gibby M and Johns RJ (eds) (1996) Pteridology in Perspective. Kew, UK: Royal Botanic Gardens. Galston AW (1975) The water fern rice connection. Natural History 84(12): Hoshizaki BJ and Moran RC (2001)Fern Grower s Manual, revised edn. Portland, OR: Timber Press. Kramer KU and Green PS (eds) (1990) Pteridophytes and Gymnosperms; vol. 1 in Kubitzki K (ed.) The Families and Genera of Vascular Plants. Berlin: Springer-Verlag. May LW (1979) The economic uses and associated folklore of ferns and fern allies. Botanical Review (Lancaster) 44: Perring FH and Gardiner BG (eds) (1976) The biology of bracken. Botanical Journal of the Linnaean Society 73: Pryer KM, Schneider H, Smith AR et al. (2001) Horsetails and ferns are a monophyletic group and the closest living relatives to seed plants. Nature 409: Sheffield E, Wolf PG and Haufler CH (1989) How big is a bracken plant? Weed Research 29: Tryon RM and Tryon AF (1982) Fern and Allied Plants, with Special Reference to Tropical America. New York: Springer-Verlag. Wolf PG (ed.) (1995) Use of molecular data in evolutionary studies of pteridophytes. American Fern Journal 85: Wolf PG, Sipes SD, White MR et al. (1999) Phylogenetic relationships of the enigmatic fern families Hymenophyllopsidaceae and Lophosoriaceae: evidence from rbcl nucleotide sequences. Plant Systematics and Evolution 219: