Cyperaceae

▪ plant family

Introduction

      sedge family of monocotyledonous flowering plants, a division of the order Poales. The Cyperaceae are grasslike herbaceous plants found especially in wet regions throughout the world.

Distribution and abundance

      The Cyperaceae, among the 10 largest families of flowering plants, contain about 5,000 species and, depending on the classification used, between 70 and 115 genera. The members are distributed throughout all the continents except Antarctica. Although there is a large number of species in Arctic, temperate, and tropical regions, the diversity of genera is far greater in tropical regions. Many species of sedges that occur in northern latitudes have circumpolar distributions. Species occurring in tropical or warm temperate regions, except for those that are widespread agricultural weeds, are generally confined to a single continent.

 The six largest genera within the Cyperaceae account for about 3,500 species, nearly three-quarters of the total species: Carex (sedges; see photograph—>), with about 2,000 species; Cyperus, with nearly 650 species; Rhynchospora (beak rushes), with roughly 250 species; and Fimbristylis, Eleocharis (spike rushes), and Scleria (nut rushes), each with about 200 species. Other large genera are Bulbostylis, with approximately 100 species; Schoenus, also with about 100 species; and Mapania, with up to 80 species.

      Each of the six large genera except for Carex are primarily found in warm temperate or tropical regions, with only a few cold temperate species. Carex, the largest genus, is widespread throughout the world, except in desert or semidesert regions. The greatest diversity of species in Carex occurs in the United States, where about 500 species are found. About the same number occur in eastern Asia, from Siberia through China to Japan. In the tropics, Carex is uncommon in lowlands, although often diverse in montane regions. Eleocharis also has a number of species in cold temperate or even Arctic regions, although the great bulk of its 200 species are confined to warmer areas. Other large genera are basically distributed throughout the tropical regions of the Earth, with only a few outlying species in cold temperate regions. Schoenus is basically found in the Old World, with only a few of its species located in the New World.

      Most of the remaining genera of Cyperaceae are quite small, almost all with less than 30 species, and many with only a single species. The limits of these genera are also somewhat unsettled, with the circumscription and limits of such well-known genera as Cyperus and Scirpus (bulrush) (bulrushes or clubrushes) being somewhat controversial. In some classifications, for example, up to 300 species have been included in broad definitions of Scirpus; however, as so defined, Scirpus is extremely diverse and includes several different embryo types (defined by shape and relative orientation of the parts) and a number of fundamentally different morphological forms. In this article, Scirpus is recognized in a narrow sense to include only the leafy bulrushes (about 40 species) in order to more accurately reflect the evolutionary relationships within the family. Genera recognized here that are sometimes included in Scirpus are Bolboschoenus, Isolepis, Schoenoplectus, and Trichophorum (club rushes). Cyperus is recognized here in a broad sense, although the genus Kyllinga is not included with it. All Cyperus have a uniform embryo type and a relatively consistent morphology.

      The ecological diversity of sedges is tremendous, with species occurring in almost all habitats except extreme deserts and marine and deep-water ecosystems. A majority of sedges, however, are plants of sunny, moist to wet habitats such as fresh and salt marshes, pond and lakeshores, meadows, bogs, fens, wet prairies and savannas, and moist to wet tundra. Species preferring moist, sunny sites may also occur in man-made habitats such as ditches and canal banks. Many species of sedges also occur in understories of diverse types of forest, both temperate and tropical. Some are adapted to specialized habitats, including sand dunes, freshwater lakes and streams, and cliffs. Epiphytes are very rare among the sedges, occurring primarily in the South American genus Everardia.

Economic and ecological importance

      Few sedges are crop plants. Of these, by far the most important are the Chinese water chestnut (Eleocharis dulcis) and chufas or tiger nuts, cultivars of the yellow nut sedge (Cyperus esculentus) grown primarily in Africa. In both species, the edible parts are underground tubers. In boreal regions and mountainous areas, species of Carex are often important pasture and rangeland plants and may even be managed, as, for example, meadows of Carex lyngbyei in Iceland.

      Throughout the world many species of Cyperaceae have regional importance in weaving mats, baskets, screens, and even sandals because of their strong, fibrous stems and leaves. Genera used for these purposes include Carex, Cyperus, Eleocharis, Schoenoplectus, and, in Malaysia, the species Rhynchospora corymbosa. Cyperus tegetum is cultivated in India for this purpose, but the other genera are gathered from wild stands. Indigenous peoples on Lake Titicaca, in the Andes, use tatora (Schoenoplectus tatora) to construct small boats called balsas for traversing the lake.

      Papyrus (Cyperus papyrus) was used in ancient Egypt for making paper and for constructing boats; it apparently was the bulrushes referred to in the biblical story of the infant Moses. Papyrus is still of local importance in Africa as a fuel source and is cultivated throughout the tropics and in conservatories in temperate regions as an ornamental for ponds. Several other species of Cyperus also are cultivated as ornamentals, including Cyperus involucratus (umbrella plant), a native of Africa, and, less commonly, Cyperus albostriatus and C. isocladus.

      A number of species of Carex, often those forms with variegated leaves, are cultivated as ornamentals in temperate gardens, especially along the shores of streams and ponds, as edgings, in rock gardens or woodland gardens, or as ground covers. The most significant of these are variegated forms of several Japanese species, Carex conica, C. morrowii, and C. phyllocephala, which are grown as edging or accent plants. The variegated form of the European C. ornithopoda serves the same function. The European Alpine species C. baldensis, with decorative white heads, and the variegated form of C. firma, with shiny, short, stiff leaves, are sometimes grown as ornamentals in rock gardens. Yellow-leaved forms of the large European species C. riparia and C. elata are popular waterside plants. Several New Zealand Carex, especially C. buchananii, C. comans, C. flagellifera, and C. secta, have wiry, brown or copper-coloured leaves and are grown in mild temperate climates as accent plants under the collective name New Zealand bronze sedges. Species of woodland Carex, notably the eastern North American C. grayi and C. plantaginea, the eastern Asian C. siderosticta, and the European C. pendula, are also grown as edgings or accent plants in shaded areas. Carex baccans, a Southeast Asian tropical species with fleshy, red to purplish fruiting structures in large clusters, is sometimes grown in greenhouses.

      Species of other genera of Cyperaceae also are cultivated. A variegated form of the great bulrush, Schoenoplectus lacustris, is occasionally grown in ponds. The dwarf, tufted Isolepis cernua is commonly grown as a greenhouse edging or potted plant. The monotypic eastern North American Cymophyllus fraseri, with white inflorescences, is sometimes grown in shady wildflower gardens, and Eriophorum (cotton grasses) and the whitetop sedges, Rhynchospora section Dichromena (genera with a large number of species may be divided into sections), are popular plants of bog gardens.

      Some large, fast-growing wetland sedges, especially Schoenoplectus lacustris and some Carex species are grown in pond and tank treatments of domestic wastewater because of their capacity to absorb excess nutrients (notably phosphorus and nitrogen).

      A substantial number of sedges are economically important because they are weeds. Purple nut sedge (Cyperus rotundus), arguably the world's worst weed, infests crops throughout tropical and warm-temperate regions of the world. The yellow nut sedge (Cyperus esculentus), also a weed, is a serious threat in cooler climates, causing considerable crop loss. Many other species of Cyperus grow in bare soils in areas that have had their vegetative cover removed or disrupted and also occasionally invade cultivated sites. A few Cyperus species, as well as species of Fimbristylis and Schoenoplectus, are significant weeds of rice paddies. Several coarse species of Carex, including wolf tail (C. cherokeensis) in the southeastern United States and Australian sedge (C. longebrachiata) in New Zealand, are serious pasture weeds that substantially reduce the grazing quality of lands with their tussocks of harsh, inedible foliage.

      The ecological importance of sedges is extraordinary. They are significant, often dominant components of many types of wetlands (wetland) throughout the world, both tropical and temperate. They are thus of crucial importance to primary productivity and to many aspects of the continuous circulation of water in the Earth's atmosphere (the hydrologic cycle)—i.e., evaporation, transpiration, condensation, precipitation, and runoff. Their fruits, and sometimes their shoots and tubers, often produced in quantity, are important food for many aquatic and amphibious animals. Large stands of sedges are also critical as cover for many wetland animals.

      The vast papyrus marshes of Africa are almost totally dominated by Cyperus papyrus. The huge areas of peatlands in the Northern Hemisphere are frequently dominated by species of Carex, Eriophorum, and Trichophorum. Both salt- and freshwater marshes in temperate and boreal zones have a large sedge component and are often dominated by species of Bolboschoenus, Carex, Cladium (twig rush), Cyperus, Eleocharis, Schoenoplectus, and Scirpus. Many sedges are important emergent and floating mat aquatics in lakes and streams worldwide, including species of Carex, Cladium, Cyperus, Eleocharis, Rhynchospora, and Schoenoplectus. Some species are even submerged aquatics, although only a few, notably Websteria confervoides, are obligately so.

      Sedges not only are important components of stable wetland communities but also play a major role in wetland succession. Many species of annual and perennial sedges, especially in the genera Cyperus, Eleocharis, Fimbristylis, and Schoenoplectus, are the first colonizers on the bare soil of newly created wetlands. In mature, stable wetlands, these species are replaced by the longer-lived perennial sedges. Seeds of sedges may arrive in new wetlands by dispersal, typically by birds. In many species, however, especially those of wetlands that undergo cyclic periods of drying, dormant viable seeds persist in the wetland soil as a seedbank. The wetland vegetation is thus rejuvenated from the seedbank under appropriate conditions rather than relying entirely on dispersal to the site.

      In temperate deciduous forests (forest) worldwide, but especially in eastern North America and eastern Asia, many species of Carex occur in the understory and often dominate the local vegetation. Tropical forests and woodlands also have many Cyperaceae in the understory, but they are primarily species of Cyperus, Hypolytrum, Mapania, Rhynchospora, and Scleria.

      Other habitats in which sedges are important are temperate and tropical grasslands (grassland) and savannas (savanna), especially in moist sites. In dry or seasonally dry grasslands or savannas, especially in sandy soils or thin soils over rock, genera such as Abilgaardia, Bulbostylis, and some species of Cyperus, Lagenocarpus, Scleria, and, in Australia, Gahnia and Lepidosperma are important. Moist-to-wet zones along lakeshores and streambanks essentially worldwide also may have an important sedge component, as do various moist Alpine habitats, including paramo—the distinctive Central and South American alpine grasslands dominated by bunch grasses.

Characteristic morphological features

      Although the Cyperaceae are similar in appearance to grasses (family Poaceae) and placed in the same order, there is a growing body of evidence that suggests that the closest relatives of Cyperaceae are the rushes (family Juncaceae). Rushes share with sedges a number of specialized anatomic and developmental features. Both families have chromosomes with a very peculiar structure. The centromeres, the point of attachment of the spindle fibres during meiosis, are not localized at one point near the middle but rather are distributed diffusely over the length of the chromosomes. Both the Cyperaceae and the Juncaceae have pollen that is dispersed as tetrads, although in the Cyperaceae three of the four nuclei produced by meiosis degenerate to leave only one functional nucleus in the tetrad. Both families also have tristichous phyllotaxy (leaves in three ranks).

      Grasses (grass) differ from sedges in many features, most obviously in their sheaths and the arrangement of the leaves on the stem. In the grasses leaf initiation begins on one side of the stem and the leaf margins grow around the stem from both sides of the centre of initiation until they encircle the stem, the margins overlapping when they meet. For the sedges, growth is as in grasses except that the leaf margins fuse when they meet around the stem on the opposite side of the centre of initiation. In grasses, the leaves are arranged alternately and in two ranks (distichous); that is, successive leaves arise on opposite sides of the stem, creating two vertical rows of leaves. In the sedges, three vertical rows are created (tristichous), with successive leaves developing at 120 degrees around the circumference of the stem.

      The stems of Cyperaceae are often triangular and mostly solid, whereas those of grasses are never triangular and are usually hollow except at the nodes. In spikelets of Cyperaceae the individual flowers are subtended by a single scale, whereas the individual flowers of the grasses usually are subtended by two scales. Grasses have localized centromeres and single-celled pollen grains.

      A majority of sedges have the morphological appearance of grasslike herbaceous perennials with fibrous roots, triangular stems, and three-ranked, linear leaves. A significant number are annuals, especially those of weedy or seasonal habitats. Many species have rhizomes (rhizome) of varying lengths; in a number of species, these rhizomes are important food storage organs and may even be tuberous. In many species, these rhizomes form extensive underground systems that are very important in local vegetative dispersal. In those species adapted to dunes or other sandy sites, the rhizome systems play an important role in dune formation and soil stabilization. Some species, especially in the genera Eleocharis and Schoenoplectus, have round stems, and a few species such as Eleocharis quadrangulata have four-angled or polygonal stems. Most sedges have solid stems or stems with only a small, irregular cavity, but a few, such as the three-way sedge (Dulichium arundinaceum), have hollow stems.

      Sedges range in size from tiny plants less than 1 centimetre (0.4 inch) high found in a number of genera, such as Eleocharis, Lipocarpha, and Abildgaardia, to the giant papyrus, which can attain a height of 5 metres (16 feet). Some species of Scleria, for example, the African S. boivinii, are scrambling vines up to 10 metres long. Unlike grasses, which have extensively exploited the woody habit in the bamboos, very few sedges are woody. A few species of Gahnia have woody stems, and the remarkable West African Microdracoides squamosus is a woody shrub up to 1.5 metres tall with a form resembling a miniature Joshua tree.

      All sedges have sheathing leaves, usually with blades; but members of a substantial number of genera, including Caustis, Eleocharis, Lepironia, Schoenoplectus, and Trichophorum, may be bladeless or nearly so. The sheaths are uniformly closed except in the small African genus Coleochloa. As in grasses, many genera have a small flap of tissue or fringe of hairs called a ligule that extends from the top of the sheath; the blade elongates above the ligule.

      Leaf blades of sedges are highly variable in form. Most are linear, less than 1.5 centimetres wide, and flat or folded at the main veins. In a number of genera, especially those of dry or seasonally dry habitats, the leaf blades are stiff and circular in cross section or are strongly rolled inward. Sedges adapted to shady habitats, especially species of tropical forests and the deciduous forests of eastern North America and eastern Asia, may have expanded (broad) blades; this includes a number of species of Carex in temperate forests and species of Hypolytrum, Mapania, and Scleria in tropical woodlands. In some instances, including species of Carex such as the Southeast Asian C. scaposa and species of Mapania, the leaves may be contracted into a false petiole. The leaf of the remarkable genus Cymophyllus from the eastern United States appears to be a broad, flat blade without a sheath, midrib, or ligule; however, the apparent blade is evidently an expanded and opened bladeless sheath.

      The flowers (flower) in Cyperaceae are highly reduced in size and complexity and are either unisexual or bisexual. The perianth (the calyx and corolla or the tepals) is either absent altogether or represented by up to six (though sometimes one) hairlike to stiff, sometimes barbed bristles. In Eriophorum, the bristles are extremely long, white to russet, and up to 50 in number. A few sedges, including most species of Fuirena (umbrella grass) and Oreobolus, have small scales instead of bristles. The flower usually has 3 stamens, although sometimes as few as 1 or 2 or very rarely (in the Australian genus Evandra) as many as 20. The single unilocular ovary has a terminal style with two or three stigmatic branches or very rarely one or up to as many as eight styles (in Evandra). Underlying each flower is a chaffy floral bract.

      The flowers are arranged along a shortened axis, called a rachilla, that arises from the central stem of the plant. From a few to many flowers are arranged along the rachilla (rarely only a single flower), forming the spikelet, the basic unit of a sedge inflorescence. Spikelets are arranged into inflorescences of variable size and form: from small, tight heads in many genera to panicles, the usual form of the inflorescence; panicles as long as one metre or more can be found in some species of Cladium or Gahnia. Leaflike bracts often also underlie the major branches of the inflorescences. The reduction of an inflorescence to a single spikelet has occurred repeatedly in different evolutionary lines, usually in conjunction with a reduction in the size of the plant or as an adaptation to extreme habitats, or both. In some sedges, the spikelets are reduced to the point where they simulate a single flower (a pseudanthium), and these highly reduced flowerlike spikelets may then also be arranged, as if they were true flowers, into structures that simulate spikelets formed from true flowers. These structures are called pseudospikelets and become the basic units of compound inflorescences. The subfamily Mapanioidieae, an important tropical group, contains a number of examples of sedges with pseudospikelets.

      The vast majority of sedges are wind-pollinated (pollination) and have adaptations reflecting this fact, including abundant pollen production, nonsticky pollen, exserted anthers, and open inflorescences. In the few genera of Cyperaceae that are insect-pollinated, the inflorescences are contracted into dense heads, and the scales are usually white or sometimes yellow. If leaflike bracts are present in the inflorescence, these also may be white or yellow and are usually clustered together beneath the heads and arrayed radially at right angles to the stem. The coloration attracts the pollinator, and the arrangement of the flowers permits access to the most pollen per visit. Whitetop sedges (Rhynchospora section Dichromena), which occur from the southeastern United States to South America, are the best-known examples of insect-pollinated sedges. Species of Ascolepis and Ficinia in Africa, a number of tropical and subtropical species of Cyperus and Kyllinga, and even Cymophyllus fraseri in the eastern United States and Carex baldensis in the Alps have white heads (sometimes yellow or orange in Ascolepis and Ficinia) and are in all likelihood primarily insect-pollinated.

      Fruits of sedges are most commonly achenes (achene) (nutlets), but in a few genera, notably Mapania and Scirpodendron, are single-seeded fleshy fruits called drupes (drupe). In many instances, the achenes have no obvious dispersal (dispersion) mechanism and are probably eaten and dispersed by birds and small mammals. In Carex, the achenes are enclosed in a sac called a perigynium, a modified tubular bract. The perigynium may tightly envelop the achene or it may be inflated like a bladder, flattened and scalelike, or even fleshy and edible. Many woodland species of Carex have food bodies (elaiosomes) at the base of the perigynium for ants, which disperse the perigynia. Species of Lepidosperma also have elaiosomes. In some species of Cyperus, the achenes are partly enclosed by the corky rachilla; at maturity the rachilla breaks apart to produce many rachilla segments and their attached achenes, which are then dispersed by water. When present, bristles may be barbed and cling to animal fur to disperse the nutlets, as in Rhynchospora, or they may be long and silky and act as parachutes for wind dispersal, as in Eriophorum and some species of Scirpus and Trichophorum.

Evolution and classification

      Although fossil sedges are known from as early as the Eocene, they are as yet of little use in interpreting evolution in Cyperaceae because they are both fragmentary and apparently closely resemble modern groups. The rather uniform morphology of the nonreproductive parts of the plant body as well as the highly reduced flowers make deduction of evolutionary patterns from living sedges difficult. Thus, most theories on the evolution of the Cyperaceae at this point are derived from studies of the morphology and development of the spikelets.

      Current systems of classification of the Cyperaceae divide it into two to five subfamilies. A division of the family into two subfamilies would result in the subfamily Cyperoideae with usually bisexual flowers and the subfamily Caricoideae with unisexual flowers, but many botanists consider this to be a rather arbitrary division. Four subfamilies are recognized in this article. The Cyperoideae, the largest subfamily including about 70 genera and 2,400 species, has usually perfect flowers in simple spikes with often numerous spirally arranged or two-ranked scales. The Caricoideae, the next largest subfamily, has 2,100 species dispersed among only 5 genera and is characterized by unisexual flowers with the female in single-flowered spikelets enclosed by a bract. The subfamily Sclerioideae has about 14 genera and 300 species; its flowers also are unisexual, but its fruit is not enveloped by a similar bract. The smallest subfamily, the Mapanioideae, has about 170 species in 14 genera. The highly reduced, unisexual flowers are grouped together tightly in such a way as to simulate a single flower (pseudanthium).

      The most primitive type of spikelet is found in the genus Scirpus (bulrush) (bulrushes) and its relatives in the subfamily Cyperoideae. Scirpus has many-flowered spikelets with all but the topmost bracts bearing flowers. From this basic type, the remaining, more advanced members of the family can be derived by a reduction in the number of flowers, by the sterilization of flowers, and by the evolution of unisexuality in the loss of stamens or pistils within a flower.

      The Scirpus spikelet is not the only type found in the subfamily Cyperoideae. The spikelet found in Cyperus and several related, smaller genera is similar, but the lowermost bract does not bear a flower. Spikelets characteristic of Rhynchospora and its allies and Cladium and its allies are derived by a reduction in the number of flowers per spikelet and a sterilization of lowermost or uppermost flowers, as well as by the conversion of some bisexual flowers to staminate only; in Rhynchospora, for example, male flowers are above the perfect flowers, and in Cladium male flowers are below.

      The caricoid spikelet, characteristic of Carex and its satellite genera in the subfamily Caricoideae, is a highly specialized, reduced spikelet with uniformly unisexual flowers. The female flowers are always solitary and borne within a perigynium derived from the partly or wholly closed first leaf (prophyll) of the spikelet axis. Male flowers occur in the axils of bracts either on the rachilla extending beyond the female flower or on axes bearing perigynia. The evolutionary relationships of the Caricoideae are unclear. The spikelet of the Caricoideae can, in theory, be derived from the Scirpus-type by a reduction in flower number and conversion to unisexual flowers, and its relationships are probably with the Cyperoideae subfamily. The uniformly unisexual flowers, however, have also prompted botanists to align it with the subfamily Sclerioideae, which has unisexual flowers as well.

      The subfamily Mapanioideae has a pseudanthium, or false flower, composed of a single terminal female flower surrounded by a number of naked stamens, each of which is subtended by a bract with the lowest two stamens situated opposite each other. In the genus Hypolytrum, only the two opposite stamens and their subtending bracts are present beneath the female flower. The pseudanthium is thought to be derived from a hypothetical prototype inflorescence in which a single female flower terminates an axis along which numerous male flowers are situated in the axils of bracts. The inflorescence then underwent reduction so that the male flowers were represented by only a single stamen each and the axis between them became greatly shortened. The reduction in the inflorescence was so extreme that it strongly resembles a flower—hence the term pseudanthium. This subfamily is highly specialized and entirely tropical.

      The subfamily Sclerioideae also has inflorescences with terminal female flowers but the branches housing the male flowers below the female flowers are slightly more extended, though still short. This subfamily may have originated from plants with branched inflorescences similar to the hypothetical prototype ancestral to the subfamily Mapanioideae; however, the male flowers immediately below the terminal female flower were reduced to bracts, while branches below this point on the inflorescence have lost their terminal female flower and bear a few male flowers and one or a few distal bracts. In the tribe Bisboekelereae, the bracts below the terminal female flower are united to form a perigynium somewhat similar to that of the subfamily Caricoideae, though evolved independently. The subfamily Sclerioideae is almost entirely tropical, with only a few species of Scleria occurring in the temperate zone.

      Some authorities have proposed a hypothesis of evolution in the Cyperaceae that is essentially the reverse direction of evolution to that presented above—that is, that unisexual flowers were the primitive condition. They suggest that inflorescences of the subfamily Mapanioideae, with their pseudanthia composed of tiny, unisexual flowers, represent the primitive condition in the Cyperaceae. The apparent flowers of other genera of Cyperaceae are thus also pseudanthia, and the seemingly simple, bisexual flowers of, for example, Scirpus, with three stamens and a single ovary, is actually a pseudanthium composed of three male flowers each with one anther and one female flower of a single pistil.

      This hypothesis is based on the observation that unisexual flowers in the Cyperaceae apparently have no vestiges of the other sex. Also, it assumes that the broad-leaved tropical lowland species of the subfamily Mapanioideae are the most primitive representatives of the family. This hypothesis is thought by many, including Arthur Cronquist, to introduce unnecessary complexity in floral morphology for which there is little support.

Anton A. Reznicek

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