A wide variety of growth forms are present:shrubs are most common (e.g.Coffea,Psychotria), but members of the family can also betrees (e.g.Cinchona,Nauclea),lianas (e.g.Psychotria samoritourei), orherbs (e.g.Galium,Spermacoce). Some epiphytes are also present (e.g.Myrmecodia). The plants usually containiridoids, variousalkaloids, andraphide crystals are common. The leaves are simple, undivided, and entire; there is only one case of pinnately compound leaves (Pentagonia osapinnata[3]). Leaf blades are usually elliptical, with a cuneate base and an acute tip. In three genera (Pavetta,Psychotria,Sericanthe), bacterial leaf nodules can be observed as dark spots or lines on the leaves. Thephyllotaxis is usually decussate, rarely whorled (e.g.Fadogia), or rarely seemingly alternate resulting from the reduction of one leaf at each node (e.g.Sabicea sthenula). Characteristic for the Rubiaceae is the presence ofstipules that are mostly fused to an interpetiolar structure on either side of the stem between the opposite leaves. Their inside surface often bears glands called "colleters", which produce mucilaginous compounds protecting the young shoot. The "whorled" leaves of the herbaceous tribeRubieae have classically been interpreted as true leaves plus interpetiolar leaf-like stipules. The inflorescence is acyme, rarely of solitary flowers (e.g.Rothmannia), and is either terminal or axillary and paired at the nodes. The 4-5-merous (rarely pleiomerous; e.g. six inRichardia[4]) flowers are usuallybisexual and usually epigynous. The perianth is usuallybiseriate, although the calyx is absent in some taxa (e.g.Theligonum). Thecalyx mostly has the lobes fused at the base; unequal calyx lobes are not uncommon, and sometimes (e.g.Mussaenda) one lobe is enlarged and coloured (a so-called "semaphyl"). Thecorolla is sympetalous, mostly actinomorphic, usually tubular, mostly white or creamy but also yellow (e.g.Gardenia spp.,Mycelia basiflora), and rarely blue (e.g.Faramea calyptrata) or red (e.g.Alberta magna,Ixora coccinea). Thestamens are alternipetalous and epipetalous.Anthers are longitudinal in dehiscence, but are poricidal in some genera (e.g.Rustia,Tresanthera). Thegynoecium is syncarpous with an inferiorovary (rarely secondarily superior, e.g.Gaertnera,Pagamea[5]).Placentation is axial, rarely parietal (e.g.Gardenia);ovules are anatropous to hemitropous, unitegmic, with a funicularobturator, one to many per carpel.Nectaries are often present as a nectariferous disk atop the ovary. The fruit is aberry,capsule (e.g.Oldenlandia),drupe (e.g.Coffea,Psychotria), orschizocarp (e.g.Cremocarpon). Red fruits are fairly dominant (e.g.Coffea arabica); yellow (e.g.Rosenbergiodendron formosum), orange (e.g.Vangueria infausta), or blackish fruits (e.g.Pavetta gardeniifolia) are equally common; blue fruits are rather exceptional save in thePsychotrieae and associated tribes. Most fruits are about 1 cm in diameter; very small fruits are relatively rare and occur in herbaceous tribes; very large fruits are rare and confined to theGardenieae. The seeds areendospermous.[6][7]
Rubiaceae have acosmopolitan distribution and are found in nearly every region of the world, except for extreme environments such as the polar regions and deserts. The distribution pattern of the family is very similar to the global distribution of plant diversity overall. However, the largest diversity is distinctly concentrated in the humid tropics and subtropics. An exception is the tribeRubieae, which is cosmopolitan but centered in temperate regions. Only a few genera are pantropical (e.g.Ixora,Psychotria), many are paleotropical, while Afro-American distributions are rare (e.g.Sabicea). Endemic rubiaceous genera are found in most tropical and subtropical floristic regions of the world. The highest number of species is found inColombia,Venezuela, andNew Guinea. When adjusted for area, Venezuela is the most diverse, followed byColombia andCuba.[8]
The Rubiaceae consist of terrestrial and predominantly woody plants. Woody rubiaceous shrubs constitute an important part of the understorey of low- and mid-altitude rainforests. Rubiaceae are tolerant of a broad array of environmental conditions (soil types, altitudes, community structures, etc.) and do not specialize in one specific habitat type (although genera within the family often specialize).[citation needed]
Although most Rubiaceae species are hermaphroditic,outbreeding is promoted throughsequential hermaphroditism and spatial isolation of the reproductive organs. More complex reproductive strategies include secondary pollen presentation,heterostyly, and unisexual flowers.[citation needed]
Secondary pollen presentation (also known as stylar pollen presentation or ixoroid pollen mechanism) is especially known from theGardenieae and related tribes. The flowers are proterandrous and the pollen is shed early onto the outside of the stigmas or the upper part of the style, which serve as a pollen receptacle. Increased surface area and irregularity of the pollen receptacle, caused by swellings, hairs, grooves or ridges often ensure a more efficient pollen deposition. After elongation of the style, animals transport the pollen to flowers in the female or receptive stage with exposed stigmatic surfaces. A pollen catapult mechanism is present in the generaMolopanthera andPosoqueria (tribePosoquerieae) that projects a spherical pollen mass onto visitinghawk moths.[9]
Heterostyly is another mechanism to avoid inbreeding and is widely present in the family Rubiaceae.[10] The tribes containing the largest number of heterostylous species areSpermacoceae andPsychotrieae. Heterostyly is absent in groups that have secondary pollen presentation (e.g.Vanguerieae).
Unisexual flowers also occur in Rubiaceae and most taxa that have this characteristic aredioecious. The two flower morphs are however difficult to observe as they are rather morphologically similar; male flowers have arudimentary pistil with the ovaries empty and female flowerssterile or rudimentary stamens with empty anthers.[6] Flowers that are morphologically hermaphrodite, but functionally dioecious occur inPyrostria.[11]
The dispersal units in Rubiaceae can be entire fruits, syncarps, mericarps, pyrenes or seeds. Fleshy fruit taxa are probably all (endo)zoochorous (e.g. tribesPavetteae,Psychotrieae), while the dispersal of dry fruits is often unspecialized (e.g. tribesKnoxieae,Spermacoceae). When seeds function asdiaspores, the dispersal is either anemochorous or hydrochorous. The three types of wind-dispersed diaspores in Rubiaceae are dust seeds (rare, e.g.Lerchea), plumed seeds (e.g.Hillia), and winged seeds (e.g.Coutarea). Long-distance dispersal by ocean currents is very rare (e.g. the seashore treeGuettarda speciosa). Other dispersal mechanisms are absent or at least very rare. SomeSpermacoceae having seeds withelaiosomes are probably myrmecochorous (e.g.Spermacoce hepperiana). Epizoochorous taxa are limited to herbaceous Rubiaceae (e.g.Galium aparine fruits are densely covered with hooked bristly hairs).[citation needed]
An intimate association between bacteria and plants is found in three rubiaceous genera (viz.Pavetta,Psychotria, andSericanthe).[15] The presence of endophytic bacteria is visible by eye because of the formation of dark spots or nodules in the leaf blades. The endophytes have been identified asBurkholderia bacteria. A second type of bacterial leaf symbiosis is found in the generaFadogia,Fadogiella,Globulostylis,Rytigynia, andVangueria (all belonging to the tribeVanguerieae),[16][17][18] and in some species ofEmpogona andTricalysia (both belonging to the tribeCoffeeae),[19] whereBurkholderia bacteria are found freely distributed among the mesophyll cells and no leaf nodules are formed. The hypothesis regarding the function of the symbiosis is that the endophytes provide chemical protection against herbivory by producing certain toxic secondary metabolites.[20]
The family Rubiaceae is named afterRubia, a name used byPliny the Elder in hisNaturalis Historia for madder (Rubia tinctorum).[21] The roots of this plant have been used since ancient times to extract alizarin and purpurin, two red dyes used for coloring clothes. The namerubia is therefore derived from the Latin wordruber, meaningred. The well-known genusRubus (blackberries and raspberries) is unrelated and belongs toRosaceae, the rose family.[citation needed]
The classical classification system of Rubiaceae distinguished only two subfamilies: Cinchonoideae, characterized by more than one ovule in eachlocule, and Coffeoideae, having one ovule in each locule.[30][31] This distinction, however, was criticized because of the distant position of two obviously related tribes, viz. Gardenieae with many ovules in Cinchonoideae and Ixoreae with one ovule in Coffeoideae, and because in species ofTarenna the number of ovules varies from one to several in each locule.[32][33] During the 20th century, other morphological characters were used to delineate subfamilies, e.g. stylar pollen presentation,raphides,endosperm,heterostyly, etc. On this basis, three[34] or eight[35] subfamilies were recognised. The last subfamilial classification solely based on morphological characters divided Rubiaceae into four subfamilies: Cinchonoideae, Ixoroideae, Antirheoideae, and Rubioideae.[6] In general, problems of subfamilies delimitation in Rubiaceae based on morphological characters are linked with the extreme naturalness of the family, hence a relatively low divergence of its members.[6]
The introduction of molecular phylogenetics in Rubiaceae research has corroborated or rejected several of the conclusions made in the pre-molecular era. There was support for the subfamilies Cinchonoideae, Ixoroideae, and Rubioideae, although differently circumscribed, and Antirheoideae was shown to bepolyphyletic.[36] For a long time, the classification with three subfamilies (Cinchonoideae, Ixoroideae, and Rubioideae) was followed.[37] However, an alternative opinion existed with only two subfamilies: an expanded Cinchonoideae (that includes Ixoroideae, Coptosapelteae, and Luculieae) and Rubioideae.[28] Finally, more and more evidence pointed towards a two-family classification.[29] The adoption of theMelbourne Code forbotanical nomenclature had an unexpected impact on many names that have been long in use and are well-established in literature. According to the Melbourne Code, the subfamilial name Ixoroideae had to be replaced by Dialypetalanthoideae.[38] This means that the two subfamilies in Rubiaceae now are: Dialypetalanthoideae and Rubioideae.[29] The monogeneric tribesCoptosapelteae,Acranthereae, andLuculieae are not placed within a subfamily and are sister to the rest of Rubiaceae.[39][29] The following overview shows the latest classification of the family, with two subfamilies and 72 tribes.[29][1] The approximate number of species and genera are indicated between brackets (species/genera).
The family Rubiaceae contains about 14,200 species in 615 genera.[1] This makes it the fourth-largest family of flowering plants by number of species and the fifth-largest by number of genera. In total, 1386 generic names have been published, with almost half of the names appearing in the 19th century. The average increase of new generic names is 5.05 per year.[citation needed]
The median number of species per genus is 4 but there are 30 genera with more than 100 species and 191 genera aremonotypic, which account for 31% of all genera but only for 1.3% of all species.[1]Psychotria, with around 1645 species, is the largest genus in the family and the third-largest genus in angiosperms, after the legumeAstragalus and the orchidBulbophyllum.[citation needed]
Generic names are between 3 and 20 letters long and names beginning with P are the most frequent.[1] The names often have their origin in Greek and commonly refer to a plant feature or are named after a person.[citation needed]
Molecular studies have demonstrated the phylogenetic placement of Rubiaceae within the orderGentianales and themonophyly of the family is confirmed.[40][41] The relationships of the two subfamilies of Rubiaceae together with the tribes Acranthereae, Coptosapelteae, and Luculieae are shown in the phylogenetic tree below. The placement of these three tribes relative to the two subfamilies has not been fully resolved.[41][29]
The fossil history of the Rubiaceae goes back at least as far as theEocene. The geographic distribution of these fossils, coupled with the fact that they represent all three subfamilies, is indicative of an earlier origin for the family, probably in theLate Cretaceous orPaleocene. Although fossils dating back to theCretaceous andPalaeocene have been referred to the family by various authors, none of these fossils has been confirmed as belonging to the Rubiaceae.[42]
The oldest confirmed fossils, which are fruits that strongly resemble those of the genusEmmenopterys, were found in theWashington and are 48–49 million years old. A fossil infructescence and fruit found in 44 million-year-old strata inOregon was assigned toEmmenopterys dilcheri, an extinct species. The next-oldest fossils date to theLate Eocene and includeCanthium fromAustralia,Faramea from Panama,Guettarda fromNew Caledonia, andPaleorubiaceophyllum, an extinct genus from the southeasternUnited States.[42]
Fossil Rubiaceae are known from three regions in the Eocene (North America north of Mexico, Mexico-Central America-Caribbean, and Southeast Pacific-Asia). In theOligocene, they are found in these three regions plus Africa. In theMiocene, they are found in these four regions plus South America and Europe.[42]
The most economically important member of the family is the genusCoffea used in the production ofcoffee.Coffea includes 124 species, but only three species are cultivated for coffee production:C. arabica,C. canephora, andC. liberica.[8]
The bark of trees in the genusCinchona is the source of a variety ofalkaloids, the most familiar of which isquinine, one of the first agents effective in treatingmalaria. Woodruff (Galium odoratum) is a small herbaceous perennial that containscoumarin, a natural precursor ofwarfarin, and the South American plantCarapichea ipecacuanha is the source of theemeticipecac.Psychotria viridis is frequently used as a source ofdimethyltryptamine in the preparation ofayahuasca, a psychoactive decoction.[43] The bark of the speciesBreonadia salicina have been used in traditional African medicine for many years.[44] The leaves of the Kratom plant (Mitragyna speciosa) contain a variety of alkaloids, including several psychoactive alkaloids and is traditionally prepared and consumed in Southeast Asia, where it has been known to exhibit both painkilling andstimulant qualities, behaving as aμ-opioid receptoragonist, and often being used in traditional Thai medicine in a similar way to and often as a replacement foropioid painkillers likemorphine.[citation needed]
Originally from China, the common gardenia (Gardenia jasminoides) is a widely grown garden plant and flower in frost-free climates worldwide. Several other species from the genus are also seen in horticulture. The genusIxora contains plants cultivated in warmer-climate gardens; the most commonly grown species,Ixora coccinea, is frequently used for pretty red-flowering hedges.Mussaenda cultivars with enlarged, colored calyx lobes are shrubs with the aspect ofHydrangea; they are mainly cultivated in tropical Asia. The New Zealand nativeCoprosma repens is a commonly used plant forhedges. The South AfricanRothmannia globosa is seen as a specimen tree in horticulture.Nertera granadensis is a well-known house plant cultivated for its conspicuous orange berries. Other ornamental plants includeMitchella,Morinda,Pentas, andRubia.[citation needed]
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