It consists of terrestrial trees, shrubs, lianas, or herbs that are recognizable by simple, opposite leaves with interpetiolar stipules and sympetalous actinomorphic flowers.
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.
In three genera (Pavetta, Psychotria, Sericanthe), bacterial leaf nodules can be observed as dark spots or lines on the leaves.
Their inside surface often bears glands called "colleters", which produce mucilaginous compounds protecting the young shoot.
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).
Placentation is axial, rarely parietal (e.g. Gardenia); ovules are anatropous to hemitropous, unitegmic, with a funicular obturator, one to many per carpel.
Entomophilous species produce nectar from an epigynous disk at the base of the corolla tube to attract insects.
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.
After elongation of the style, animals transport the pollen to flowers in the female or receptive stage with exposed stigmatic surfaces.
Epizoochorous taxa are limited to herbaceous Rubiaceae (e.g. Galium aparine fruits are densely covered with hooked bristly hairs).
The genera Anthorrhiza, Hydnophytum, Myrmecodia, Myrmephytum, and Squamellaria are succulent epiphytes that have evolved a mutualistic relationship with ants.
[14] The presence of endophytic bacteria is visible by eye because of the formation of dark spots or nodules in the leaf blades.
A second type of bacterial leaf symbiosis is found in the genera Fadogia, Fadogiella, Globulostylis, Rytigynia, and Vangueria (all belonging to the tribe Vanguerieae),[15][16][17] and in some species of Empogona and Tricalysia (both belonging to the tribe Coffeeae),[18] 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.
[20] The roots of this plant have been used since ancient times to extract alizarin and purpurin, two red dyes used for coloring clothes.
[22] Several historically accepted families are included in Rubiaceae: Aparinaceae, Asperulaceae, Catesbaeaceae, Cephalanthaceae, Cinchonaceae, Coffeaceae, Coutariaceae, Dialypetalanthaceae,[23] Galiaceae, Gardeniaceae, Guettardaceae, Hameliaceae, Hedyotidaceae, Henriqueziaceae,[24] Houstoniaceae, Hydrophylacaceae, Lippayaceae, Lygodisodeaceae, Naucleaceae, Nonateliaceae, Operculariaceae, Pagamaeaceae, Psychotriaceae, Randiaceae, Sabiceaceae, Spermacoceaceae, Theligonaceae.
[5] 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.
[5] The introduction of molecular phylogenetics in Rubiaceae research has corroborated or rejected several of the conclusions made in the pre-molecular era.
[28] 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.
[7] Molecular studies have demonstrated the phylogenetic placement of Rubiaceae within the order Gentianales and the monophyly of the family is confirmed.
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 the Late Cretaceous or Paleocene.
[41] 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.
[7] The bark of trees in the genus Cinchona is the source of a variety of alkaloids, the most familiar of which is quinine, one of the first agents effective in treating malaria.
Woodruff (Galium odoratum) is a small herbaceous perennial that contains coumarin, a natural precursor of warfarin, and the South American plant Carapichea ipecacuanha is the source of the emetic ipecac.
[43] 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 and stimulant qualities, behaving as a μ-opioid receptor agonist, and often being used in traditional Thai medicine in a similar way to and often as a replacement for opioid painkillers like morphine.
Mussaenda cultivars with enlarged, colored calyx lobes are shrubs with the aspect of Hydrangea; they are mainly cultivated in tropical Asia.