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Rubiaceae () is a family of , commonly known as the coffee, madder, or bedstraw family. It consists of terrestrial trees, shrubs, , or herbs that are recognizable by simple, opposite leaves with interpetiolar stipules and sympetalous actinomorphic flowers. The family contains about 14,200 species in about 615 genera, which makes it the fourth-largest angiosperm family by number of species. Rubiaceae has a cosmopolitan distribution; however, the largest species diversity is concentrated in the tropics and subtropics. Economically important genera include Coffea, the source of coffee; Cinchona, the source of the antimalarial alkaloid quinine; ornamental cultivars ( e.g., Gardenia, Ixora, Pentas); and historically some dye plants ( e.g., Rubia).
A wide variety of growth forms are present: are most common (e.g. Coffea, Psychotria), but members of the family can also be (e.g. Cinchona, Nauclea), (e.g. Psychotria samoritourei), or herbaceous plant (e.g. Galium, Spermacoce). Some epiphytes are also present (e.g. Myrmecodia). The plants usually contain , various , and raphide are common. The leaves are simple, undivided, and entire; there is only one case of pinnately compound leaves ( Pentagonia osapinnata). 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. The phyllotaxis 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 of 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 tribe Rubieae have classically been interpreted as true leaves plus interpetiolar leaf-like stipules. The inflorescence is a cyme, 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 in Richardia) flowers are usually bisexual and usually epigynous. The perianth is usually biseriate, although the calyx is absent in some taxa (e.g. Theligonum). The Sepal 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"). The corolla 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). The are alternipetalous and epipetalous. are longitudinal in dehiscence, but are poricidal in some genera (e.g. Rustia, Tresanthera). The gynoecium is syncarpous with an inferior ovary (rarely secondarily superior, e.g. Gaertnera, Pagamea). Placentation is axial, rarely parietal (e.g. Gardenia); are anatropous to hemitropous, unitegmic, with a funicular obturator, one to many per carpel. Nectaries are often present as a nectariferous disk atop the ovary. The fruit is a berry, capsule (e.g. Oldenlandia), drupe (e.g. Coffea, Psychotria), or schizocarp (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 the Psychotrieae 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 the Gardenieae. The seeds are .
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).
Although most Rubiaceae species are hermaphroditic, outbreeding is promoted through proterandry and spatial isolation of the reproductive organs. More complex reproductive strategies include secondary pollen presentation, heterostyly, and unisexual flowers.
Secondary pollen presentation (also known as stylar pollen presentation or ixoroid pollen mechanism) is especially known from the Gardenieae 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 genera Molopanthera and Posoqueria (tribe Posoquerieae) that projects a spherical pollen mass onto visiting Sphingidae.
Heterostyly is another mechanism to avoid inbreeding and is widely present in the family Rubiaceae. The tribes containing the largest number of heterostylous species are Spermacoceae and Psychotrieae. 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 are dioecy. The two flower morphs are however difficult to observe as they are rather morphologically similar; male flowers have a pistillode with the ovaries empty and female flowers staminode with empty anthers. Flowers that are morphologically hermaphrodite, but functionally dioecious occur in Pyrostria.
An intimate association between bacteria and plants is found in three rubiaceous genera (viz. Pavetta, Psychotria, and Sericanthe). 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 as Burkholderia bacteria. A second type of bacterial leaf symbiosis is found in the genera Fadogia, Fadogiella, Globulostylis, Rytigynia, and Vangueria (all belonging to the tribe Vanguerieae), and in some species of Empogona and Tricalysia (both belonging to the tribe Coffeeae), where Burkholderia 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.
Several historically accepted families are included in Rubiaceae: Aparinaceae, Asperulaceae, Catesbaeaceae, Cephalanthaceae, Cinchonaceae, Coffeaceae, Coutariaceae, Dialypetalanthaceae, Galiaceae, Gardeniaceae, Guettardaceae, Hameliaceae, Hedyotidaceae, Henriqueziaceae, Houstoniaceae, Hydrophylacaceae, Lippayaceae, Lygodisodeaceae, Naucleaceae, Nonateliaceae, Operculariaceae, Pagamaeaceae, Psychotriaceae, Randiaceae, Sabiceaceae, Spermacoceaceae, Theligonaceae.
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 be polyphyletic. For a long time, the classification with three subfamilies (Cinchonoideae, Ixoroideae, and Rubioideae) was followed. However, an alternative opinion existed with only two subfamilies: an expanded Cinchonoideae (that includes Ixoroideae, Coptosapelteae, and Luculieae) and Rubioideae. Finally, more and more evidence pointed towards a two-family classification. The adoption of the Melbourne Code for botanical 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. This means that the two subfamilies in Rubiaceae now are: Dialypetalanthoideae and Rubioideae. The monogeneric tribes Coptosapelteae, Acranthereae, and Luculieae are not placed within a subfamily and are sister to the rest of Rubiaceae. The following overview shows the latest classification of the family, with two subfamilies and 72 tribes. The approximate number of species and genera are indicated between brackets (species/genera).
The median number of species per genus is 4 but there are 30 genera with more than 100 species and 191 genera are monotypic, which account for 31% of all genera but only for 1.3% of all species. Psychotria, with around 1645 species, is the largest genus in the family and the third-largest genus in angiosperms, after the legume Astragalus and the orchid Bulbophyllum.
Generic names are between 3 and 20 letters long and names beginning with P are the most frequent. The names often have their origin in Greek and commonly refer to a plant feature or are named after a person.
The oldest confirmed fossils, which are fruits that strongly resemble those of the genus Emmenopterys, were found in the Washington and are 48–49 million years old. A fossil infructescence and fruit found in 44 million-year-old strata in Oregon was assigned to Emmenopterys dilcheri, an extinct species. The next-oldest fossils date to the Late Eocene and include Canthium from Australia, Faramea from Panama, Guettarda from New Caledonia, and Paleorubiaceophyllum, an extinct genus from the southeastern United States.
Fossil Rubiaceae are known from three regions in the Eocene (North America north of Mexico, Mexico-Central America-Caribbean, and Southeast Pacific-Asia). In the Oligocene, they are found in these three regions plus Africa. In the Miocene, they are found in these four regions plus South America and Europe.
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