Iris family Fossil range: 82 Ma late Cretaceous - Recent |
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Iris versicolor, a North American species. | |
Scientific classification | |
Kingdom: | Plantae |
(unranked): | Angiosperms |
(unranked): | Monocots |
Order: | Asparagales |
Family: | Iridaceae Juss. |
Subfamilies and tribes | |
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The Iris family or Iridaceae is a family of perennial, herbaceous and bulbous plants included in the monocot order Asparagales, taking its name from the genus Iris. Almost worldwide in distribution and one of the most important families in horticulture, it includes more than 2000 species. Genera such as Crocus and Iris are significant components of the floras of parts of Eurasia, and Iris also is well-represented in North America. Gladiolus and Morea are large genera and major constituents of the flora of sub-Saharan and Southern Africa. Sisyrinchium, with more than 140 species, is the most diversified Iridaceae genus in the Americas, where several other genera occur, many of them important in tropical horticulture.[1]
All members of Iridaceae have petaloid, soft-textured and colorful perianth in which the three tepals of the inner whorl and the three of the outer whorl are alike in structure, shape, and often in color. This type of corolla, a feature of all families of Asparagales as well as the related order Liliales, distinguishes them from other monocots, such as grasses, palms or reeds, in which the perianth is either reduced or with the members of one of both of the whorls firm-textured and dry and often brown or green. The character that sets apart Iridaceae from other plants of the Asparagales or the Liliales, is the male part of the flower, the androecium, which has three stamens—in most related families there are six. Another character that distinguish Iridaceae from most other members of the Asparagales is its inferior ovary, being superior in most families, with the exception of Orchidaceae. Apart from their flowers, the Iridaceae can usually be recognized by their characteristic leaves, sword-like and oriented edgewise to the stem with two identical surfaces. Such leaves are termed isobilateral and unifacial. In contrast, typical leaves – termed dorsiventral and bifacial – have upper and lower surfaces of different appearance and anatomy.
The Iridaceae originated in Antarctica-Australasia in the late Cretaceous, about 82 million years ago, although its subsequent radiation occurred elsewhere, notably in southern Africa and temperate and highland South America at the end of the Eocene or later. Currently, 66 genera are recognized which are distributed among 7 subfamilies and occur in a great variety of habitats. Most species are adapted to seasonal climates that have a pronounced dry or cold period unfavourable for plant growth and during which the plants remain dormant. As a result, most species are deciduous since their above-ground parts (leaves and stems) die down when the bulb or corm enters dormancy. The plants thus survive periods that are unfavourable for growth by retreating underground. Evergreen species are restricted to subtropical forests or savannah, temperate grasslands and perennially moist fynbos.
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In the Iridaceae the perianth is formed of two whorls of three tepals, all similar in structure, shape, and often color. Such a corolla differentiates Liliales and Asparagales from other monocots, where the number or size of the tepals are reduced, or where at least one whorl is papyraceous (firm and dry like a papyrus), and usually green or brown. The characters that differentiate Iridaceae, however, are its three stamen (related families such as Alliaceae and Amaryllidaceae have six) and an inferior ovary. Sword-like leaves parallel to the stem and with normally undifferentiated sides (termed "isobilateral" and "unifacial") are another distinctive feature.
Members of Iridaceae are herbs or, in a few cases, shrubs with woody caudex. They are almost all perennial (three Sisyrinchium species are annuals) that may be either evergreen or seasonal. The rootstock is a rhizome, bulb, or corm.[2] The leaves are found both at the base and on the stem, usually alternate, with the blade oriented parallel to the stem and thus sheathing it at the base. This results in the characteristic fan-like arrangement found in genera like Iris. This type of leaf lacks distinct upper and lower leaf surfaces. In many South African species the leaf has a thickened midrib and often variously thickened or winged margins that may also be crisped. In some species the leaves are needle-like with narrow longitudinal grooves. Species of Moraea are unusual in the family in having channeled leaves with a distinct upper and lower surface.[2][3]
Flowers may be either actinomorphic or zygomorphic. Almost all the parts are in threes, starting with two equal whorls of three usually large and showy petal-like tepals, distinct or fused in a tube. There are three stamens (rarely two), and their filaments are often partly to completely fused. Anthers have two pollen sacs opening toward the outside, or from their side, and usually along their length. The ovary is located below the tepals (except in Isophysis) with axile (rarely parietal) placentation in three locules. There is a single style branching into three at the top. Iridaceae do not present unisexual flowers, and all flowers have both a style and stamen. Most members other than Sisyrinchium produce nectar from nectaries at the base of the tepals, or on the gynoecium. Iridaceae species are usually pollinated by insects or birds.[1][2] The flowers are collectively arranged in two different types of inflorescences. Simple or branched spikes occur in all Crocoideae. In other subfamilies the basic inflorescence unit is a type of zig-zagging cyme called rhipidium, which is enclosed in enlarged, opposed, bracts called spathes.[1][2]
The fruit is a dry capsule, usually splitting along three sides spontaneously at maturity. It is very variable in shape and texture, from firm to cartilaginous, occasionally woody. In most genera they are tetrahedral or variously angled and without obvious adaptations for dispersal.[3] Seeds are also varied in shape.[4] Winged seeds adapted to wind dispersal characterize Gladiolus and Tritoniopsis and also occur in some species of Hesperantha. Globular seeds with shiny coats that are relatively long-lived occur in several genera of Crocoideae. Chasmanthe aethiopica has fleshy seeds adapted to dispersal by birds and several other species that grow in more wooded places – like Chasmanthe and some freesias – have reddish or black seeds that mimic fleshy seeds. They have a hard endosperm, with reserves of hemicellulose, oil, and protein, and a small embryo.[2][5][6]
Members of the Iridaceae are nearly worldwide in distribution, but remain rare in tropical lowlands and at high latitudes. The family is best represented in Southern Africa, especially the winter-rainfall region in the southwest. Other centers of diversity are temperate South and Central America (with several small genera) and the Mediterranean region (Iris and Crocus). About 2000 species are distributed among some 65 genera are recognized worldwide, just over half of them from Southern Africa, where 38 genera are known. In the Cape Floral Region alone, 707 species and 27 genera are recorded.[1][3]
The family prefers open, seasonal habitats. In Africa, the montane grasslands of eastern South Africa, Swaziland and Lesotho, and the Succulent Karoo and fynbos of the Northern and Western Cape have the most species. Fewer species occur in savannas or the semi-arid central karoo, and very few in forests.[3] The species grows in a variety of soils, derived from basalt, clay, dolerite, granite, limestone and sandstone, as well as rarer rocks like serpentine. Most species favor loamy soils, often among rocks where drainage is good, but some grow in marshes and others in pure sand. Species grow from just above the high tide mark to over 3,000 metres (9,843 ft) above sea level.[3]
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The family name is based on the genus Iris, the largest and best known genus in Europe. Iris dates from 1753, when it was named by Swedish botanist Carl Linnaeus. Its name derives from the Greek goddess, Iris, who carried messages from Olympus to earth along a rainbow, whose colors were seen by Linnaeus in the multi-hued petals of many of the species.[3] The family name is attributed to Antoine Laurent de Jussieu's 1789 Genera Plantarum, secundum ordines naturales disposita juxta methodum in Horto Regio Parisiensi exaratam, and is a conserved name, so that even if an earlier name were to be discovered for the family, Iridaceae would remain valid.[7]
The family has been accepted in all major classification systems of the 20th century. The Cronquist system treated it as part of the order Liliales of the subclass Liliidae,[8] the Takhtajan system placed it in an order Iridales, together with Isophysidaceae and Geosiridaceae treated as single-genus families,[9] and the Thorne system treated it as part of the order Orchidales in its own suborder, Iridineae.[10] The Angiosperm Phylogeny Group in 1998 and 2003 (APG and APG II, respectively) system of flowering plant classification organizes flowering plants into a "selected number of monophyletic suprafamilial groups" and placed Iridaceae in the order Asparagales, which was part of a clade called "Non Commelinoid Monocots".[11][12]
The Iridaceae differentiated in the late Cretaceous, about 82 million years ago, and diverged from the next most closely-related family, the Doryanthaceae, during the Campanian. The Tasmanian Isophysis is the only extant member of the clade sister to the remainder of the Iridaceae, from which it may have diverged 66 million years ago, in the Maastrichtian. The Iridaceae originated in Antarctica-Australasia, although its subsequent radiation occurred elsewhere, notably in southern Africa and temperate and highland South America at the end of the Eocene or later.[13]
Paleopolyploidy was important in the early diversification of Iridaceae since many genera have basic chromosome numbers which are derived from ancestors with fewer chromosomes. The appearance of polyploidy is important in Northern Hemisphere genera, especially Iris and Crocus, but has an unusually low frequency in Africa, the center of diversity for the family. Changes in basic number, frequent in a few genera, are the result of Robertsonian translocations which led to a reduction in the number of chromosomes.[a] In all but a few possible examples, accompanying morphological specialization suggests that reduction in the number of chromosomes (dysploid reduction) is involved in gradual changes in base number. Major dysploid series are restricted to a few genera, such as Romulea, the related Crocus as well as Gladiolus and Lapeirousia (all members of subfamily Crocoideae), and Iris, Moraea, and Sisyrinchium (Iridoideae).[14] Complex translocation heterozygotes have been discovered in three species of Homeria (currently included in Gladiolus), a genus from South Africa. In these species, almost every chomosome of the plant has exchanged an arm with another chromosome, which results in a chain of translocated chromosomes. These unusual species, H. tenuis (2n = 10, 9), H. flavescens (2n = 9), and H. pallida (2n = 8) are the only aneuploids in the genus; H. pallida also has forms with 2n = 12. The basic chromosome number in Homeria is x = 6 and most species are diploid, a few other polyploid. The genus comprises mostly large-flowered outcrossing species, but self-compatibility and autogamy occur in several, including the three with complex heterozygosity.[15]
Several studies based on morphology and DNA sequence information have shown that the Iridaceae are a monophyletic group included in the order Asparagales.[16][17][18][19] Phylogenetic analyses of sequences of five plastid DNA regions, rbcL, rps4, trnL–F, matK, and rps16, confirm most aspects of the traditional classification of the Iridaceae in four subfamilies and the evolutionary patterns that they imply, importantly the sister relationship of Isophysidoideae to the remainder of the family and the monophyly of Iridoideae. However, it was shown that subfamily Nivenioideae as traditionally defined is paraphyletic: Crocoideae was consistently found nested within it, sister to the core Nivenioideae (the woody genera Klattia, Nivenia, and Witsenia).[1] This clade is sister to Aristea, which in turn is sister to the Madagascarian Geosiris, and then to the Australasian Patersonia.[13][20] This has led to a shift in classification.
Based on the morphology, anatomy, embryology and chromosome numbers, the family traditionally was divided into four subfamilies (Isophysidodeae, Nivenioideae, Iridioideae and Ixioideae)[21][22] but the results from DNA analysis suggest that several more should be recognized. In fact, the genera Aristea, Geosiris, and Patersonia are now considered as separate subfamilies: Aristeoideae, Geosiridoideae and Patersonioideae, respectively, rendering Nivenioideae and Crocoideae monophyletic.[13][23] Only Iridoideae and Crocoideae are currently subdivided, each into several tribes.
Up to 66 genera have been recognised in the family, with a total of around 2,000 species worldwide. The Afrotropic ecozone, and in particular South Africa, have the greatest diversity of genera. DNA sequence information coupled with some associated morphological features provided evidence that several previously recognised New World Tigridieae genera could not retain a generic rank. These are Ainea, Colima, Fosteria, Rigidella and Sessilanthera, which are currently included in Tigridia. The Bolivian genus Cardenanthus was subsumed in Mastigostyla, and Onira and Kelissa were included in the temperate South American genus Cypella.[1] For the same reason, Tamia was included in Calydorea and Tucma in Ennealophus.[1][24]
The genera of Iridaceae, their number of species and geographic distribution are given below.[1][21][22][25][26]
Members of the Iridaceae occur in a great variety of habitats. About the only place they do not grow is in the sea itself, although Gladiolus gueinzii occurs on the seashore just above the high tide mark within reach of the spray. Most species are adapted to seasonal climates that have a pronounced dry or cold period unfavorable for plant growth and during which the plants remain dormant. As a result most species are deciduous. Evergreen species are restricted to subtropical forests or savannah, temperate grasslands and perennially moist fynbos. A few species grow in marshes or along streams and some even grow only in the spray of seasonal waterfalls.[3]
The above-ground parts (leaves and stems) of deciduous species die down when the bulb or corm enters dormancy. The plants thus survive periods that are unfavorable for growth by retreating underground. This is particularly useful adaptation for growth in areas like grasslands and fynbos which regularly have fires in the dry seasons— the plants are dormant and their bulbs or corms are able to survive underground. Veld fires clear the soil surface of competing vegetation and fertilize it with ash. With the arrival of the first rains, the dormant corms are ready to burst into growth, sending up flowers and stems before they can be shaded out by other vegetation. Many Iridaceae species which grow in grassland and fynbos flower best after fires and some fynbos species will only flower in the season after a fire.[3]
The family has a very diverse pollination ecology. Although the majority of species of African Iridaceae are pollinated by Hymenoptera (mostly bees), the remaining species are pollinated mainly, or solely, by insects in the orders Coleoptera (beetles), Diptera (short- and long-proboscid flies) and Lepidoptera (butterflies and moths), or by passerine birds (Nectarinidae). It is now known that pollination systems are predominantly specialized: plants rely on a single species or a few ecologically analogous species for pollination. By contrast, generalist species, which are pollinated by a range of pollinators from at least three pollinator groups, are rare among southern African Iridaceae. In consequence, almost all genera of any size exhibit a range of pollination syndromes, with similar patterns of floral variation having developed repeatedly within different genera.[32]
Most significantly, the diversity of pollination systems increases primarily with floral complexity and secondarily with genus size. Thus, Aristea (approximately 56 species), which has radially symmetric, mostly blue flowers, has three different pollination systems, whereas Sparaxis (15 species), with both zygomorphic and secondarily radially symmetric flowers, in a variety of colors, exhibits five different pollination systems, and Gladiolus, with a similar array of floral types but over two hundred species, exploits seven different pollination systems, some of which have evolved multiple times. As is usual in predominantly specialist pollination systems, floral attractants and rewards correlate closely with pollinator profile, resulting in the development of distinct floral syndromes. Attractants are primarily perianth pigmentation, complemented by a range or floral odors in many species, but flower shape and tepal orientation, in particular functional floral symmetry, may be equally important for some pollinators. The reward to visitors in the majority of species is nectar, but in others it is pollen, and one species offers non-volatile oil. In the case of hopliine beetles (Scarabaeidae, Hopliinae), flowers provide a stable platform on which to congregate, and the value of pollen, which beetles sometimes consume, as a reward is uncertain.[32]
Several species of Iridaceae are endangered or threatened by extinction due to habitat degradation or loss and a restricted geographical distribution. According to the IUCN, the following species are endangered: Babiana longicollis, Cyanixia socotrana (as Babiana socotrana), Gladiolus pole-evansii, Moraea garipensis, Moraea graniticola and Moraea hexaglottis.[33]
Several cultures have used species of Iridaceae as food, ornamental, condiment or medicinal plants. The Navajo, the largest Native American tribe of North America, used decoctions of Iris missouriensis as an emetic.[34] Pieces of the rhizome of the same species were used to relieve toothaches,[35] or earaches.[36] The mashed roots of Iris versicolor were applied to wounds, presumably as an antiseptic,[37] and the infusions of dry roots of the same species were used to calm pain.[38] Sisyrinchium acre was used in Hawaii in different ways. Leaves or leaf-sap were used as a dye, to give the blue color to tattoos. The use of the leaves, macerated with salt, sugar and other spices was recommended to clean the skin and cure skin diseases.[39] Iris ensata was used in India as anthelmintic and diuretic, and, mixed with other species, to treat venereal diseases.[40] Belamcanda chinensis has a long history of use as medicinal plant in China. Apparently, it was very effective in controlling bacterial, viral and fungal diseases, and in reducing fever and inflammations.[41] Another popular member of the family is Crocus sativus; the spice saffron – obtained from the stigma – has been used for centuries in folk medicine as an antispasmodic, aphrodisiac, expectorant, narcotic and sedative.[42]
Iridaceae are of considerable economic importance in ornamental horticulture and the cut-flower industry, especially Iris, Gladiolus, and Freesia. Several other genera (e.g., Crocus, Dietes, Sparaxis, Tritonia, Watsonia) are cultivated in gardens in both tropical and temperate areas. Moraea and Homeria are poisonous and pose significant problems in cattle- and sheep-raising areas, notably in southern Africa.