Rubiaceae () is a family of flowering plants, commonly known as the coffee, madder, or bedstraw family. It consists of terrestrial trees, shrubs, lianas, 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).
Description The Rubiaceae are morphologically easily recognizable as a coherent group by a combination of characters: opposite or whorled leaves that are simple and entire, interpetiolar stipules, tubular sympetalous actinomorphic corollas and an inferior ovary.
A wide variety of growth forms are present: shrubs are most common (e.g. Coffea, Psychotria), but members of the family can also be trees (e.g. Cinchona, Nauclea), lianas (e.g. Psychotria samoritourei), or herbs (e.g. Galium, Spermacoce). Some epiphytes are also present (e.g. Myrmecodia). The plants usually contain iridoids, various alkaloids, and raphide crystals 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 stipules 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 calyx 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 stamens are alternipetalous and epipetalous. Anthers 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); ovules 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 endospermous.
Distribution and habitat Rubiaceae have a cosmopolitan 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 tribe Rubieae, 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 in Colombia, Venezuela, and New Guinea. When adjusted for area, Venezuela is the most diverse, followed by Colombia and Cuba.
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).
Ecology
Flower biology Most members of the Rubiaceae are zoophilous, pollinated mainly by insects. Entomophilous species produce nectar from an epigynous disk at the base of the corolla tube to attract insects. Ornithophily is rare and is found in red-flowered species of Alberta, Bouvardia, and Burchellia. Anemophilous species are found in the tribes Anthospermeae and Theligoneae and are characterized by hermaphroditic or unisexual flowers that exhibit a set of specialized features, such as striking sexual dimorphism, increased receptive surface of the stigmas and pendulous anthers.
Although most Rubiaceae species are hermaphroditic, outbreeding is promoted through sequential hermaphroditism 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 hawk moths.
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 dioecious. The two flower morphs are however difficult to observe as they are rather morphologically similar; male flowers have a rudimentary pistil with the ovaries empty and female flowers sterile or rudimentary stamens with empty anthers. Flowers that are morphologically hermaphrodite, but functionally dioecious occur in Pyrostria.
Fruit biology The dispersal units in Rubiaceae can be entire fruits, syncarps, mericarps, pyrenes or seeds. Fleshy fruit taxa are probably all (endo)zoochorous (e.g. tribes Pavetteae, Psychotrieae), while the dispersal of dry fruits is often unspecialized (e.g. tribes Knoxieae, Spermacoceae). When seeds function as diaspores, 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 tree Guettarda speciosa). Other dispersal mechanisms are absent or at least very rare. Some Spermacoceae having seeds with elaiosomes 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).
Associations with other organisms The genera Anthorrhiza, Hydnophytum, Myrmecodia, Myrmephytum, and Squamellaria are succulent epiphytes that have evolved a mutualistic relationship with ants. Their hypocotyl grows out into an ant-inhabited tuber. Some shrubs or trees have ant holes in their stems (e.g. Globulostylis). Some Rubiaceae species have domatia that are inhabited by mites (viz. acarodomatia; e.g. Plectroniella armata).
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.
Etymology
The family Rubiaceae is named after Rubia, a name used by Pliny the Elder in his Naturalis Historia for madder (Rubia tinctorum). The roots of this plant have been used since ancient times to extract alizarin and purpurin, two red dyes used for coloring clothes. The name rubia is therefore derived from the Latin word ruber, meaning red. The well-known genus Rubus (blackberries and raspberries) is unrelated and belongs to Rosaceae, the rose family.
Systematics
Taxonomy The name Rubiaceae (nomen conservandum) was published in 1789 by Antoine Laurent de Jussieu, but the name was already mentioned in 1782.
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
Subfamilies and tribes