Peridiscaceae
Peridiscaceae is a family of flowering plants in the order Saxifragales. Four genera comprise this family: "Medusandra, Soyauxia, Peridiscus", and "Whittonia"., with a total of 12 known species. It has a disjunct distribution, with "Peridiscus" occurring in Venezuela and northern Brazil, "Whittonia" in Guyana, "Medusandra" in Cameroon, and "Soyauxia" in tropical West Africa. "Whittonia" is possibly extinct, being known from only one specimen collected below Kaieteur Falls in Guyana. In 2006, archeologists attempted to rediscover it, however, it proved unsuccessful. The largest genus is "Soyauxia", with about seven species. "Medusandra" has two species. "Peridiscus" and "Whittonia" each contain one  species. The Peridiscaceae are small trees or erect shrubs of wet tropical forests. It was not until 2009 that all four of the genera were united into a single family. "Peridiscus" and "Whittonia" are clearly close relatives. This pair, and the other two genera have long been considered anomalous, being variously classified by different authors. Description. The following description was created by combining descriptions of "Medusandra" and "Peridiscus" by John Hutchinson with descriptions of "Soyauxia", "Peridiscus", and "Whittonia" by Clemens Bayer. Peridiscaceae are small trees or erect shrubs. The leaves are stipulate, alternate, and simple, with margins that are entire  or remotely crenulate ("Medusandra"). The petiole is pulvinate, at its apex, sometimes obscurely so. The stipules are in the axils of the leaves, sometimes enclosing an axillary bud. The inflorescence is a cluster of axillary racemes or spikes, the clusters often being reduced to a pair of racemes or to a single raceme. The flowers are bisexual and actinomorphic. The sepals are 4 to 7 in number, and free, that is, separate from each other. "Medusandra" and "Soyauxia" have five petals. "Peridiscus" and "Whittonia" have none. "Medusandra" lacks a nectary disk and has five stamens, inserted opposite the petals, and  alternating with five long, hairy staminodes. In the others, the stamens are numerous and arranged in a ring around the nectary disk. The anthers are tetrathecal in "Medusandra" and "Soyauxia"; bithecal in "Peridiscus" and "Whittonia". The perianth parts are attached below the ovary. The ovary is therefore superior, but appears half-inferior in "Peridiscus" because the ovary is embedded in the large, fleshy disk. The gynoecium consists of three or four carpels, united to form a unilocular ovary. The placentation is apical, with two ovules at the apex of each carpel. The ovary has a central column in "Medusandra" and  Soyauxia. Each carpel bears a stylulus and these are well separated at the apex of the ovary. The fruit is one-seeded; a capsule in "Medusandra" and "Soyauxia"; a drupe in "Peridiscus" and "Whittonia". History. George Bentham established the genus "Peridiscus" in 1862, naming its only species "Peridiscus lucidus". He placed it in a group which he called "Tribus Flacourtieae" and which later would be known as the family Flacourtiaceae. Bentham wrote no etymology for this name, but it is generally believed that the name refers to the fact that the stamens are attached along the outer edge of the  nectary disk. Daniel Oliver established the genus "Soyauxia" in 1880 for "Soyauxia gabonensis", placing it in the family Passifloraceae. He named it for the German botanist and plant collector Hermann Soyaux, saying "Mons. Soyaux, now settled in the Gaboon, well deserves that his name should be associated with one of his interesting discoveries in that region". The family Flacourtiaceae was, as Hermann Sleumer said, a fiction, and "Peridiscus" was, from the outset, one of its most doubtful members. Recognizing its distinctiveness, João Kuhlmann segregated it into its own family in 1947. In 1952, John Brenan named and described "Medusandra", erecting  a new family, Medusandraceae to accommodate it. In 1953, Brenan transferred "Soyauxia" from Passifloraceae to Medusandraceae, but few others agreed with his classification. In 1954, John Hutchinson and John McEwen Dalziel followed Brenan's treatment in the second edition of their "Flora of West Tropical Africa". Hutchinson, however, soon recanted, explaining in some detail why he thought that "Medusandra" and "Soyauxia" were not related. In 1962, Noel Y. Sandwith named and described "Whittonia". In an accompanying article, Charles Russell Metcalfe discussed its close relationship to "Peridiscus". For four decades thereafter, Peridiscaceae was viewed as a family of uncertain taxonomic position , containing two genera. In the year 2000, a DNA sequence for the "rbcL" gene of "Whittonia" was produced and used in a molecular phylogenetic study of the eudicots. This study placed Peridiscaceae in a clade with Elatinaceae and Malpighiaceae, a very surprising and unexpected result. On the basis of this phylogeny, the Angiosperm Phylogeny Group placed Peridiscaceae in Malpighiales when they published the APG II system of plant classification in 2003. It was soon found that the "rbcL" sequence for "Whittonia" was a chimera, formed by DNA from unidentified plants that had contaminated the sample. No subsequent attempt to extract  DNA from "Whittonia" has been made. In 2004, using DNA from "Peridiscus", it was shown that Elatinaceae and Malpighiaceae are indeed sister families and that Peridiscaceae belong to Saxifragales. "Medusandra" and "Soyauxia", meanwhile, were listed in APG II in an appendix entitled "TAXA OF UNCERTAIN POSITION". DNA from "Soyauxia" was eventually obtained, and in 2007, it was shown that "Soyauxia" is most closely related to "Peridiscus" and, presumably, "Whittonia". Since this result has a good morphological basis, "Soyauxia" was duly transferred to Peridiscaceae. This study also found strong statistical support for the inclusion of Peridiscaceae in Saxifragales, but no strong  support for any particular position within that order. In 2008, in a study employing a large amount of chloroplast DNA data, as well as some mitochondrial and nuclear DNA, it was shown that Peridiscaceae is sister to the rest of Saxifragales. It had been suspected that "Medusandra" might belong somewhere in Malpighiales, but a phylogeny of that order, generated in 2009, placed "Medusandra" in Saxifragales. The authors had included "Medusandra" and a few other members of Saxifragales in their outgroup, finding strong support for a clade of ["Medusandra" + ("Soyauxia" + "Peridiscus")]. When the APG III system was published in  October 2009, Peridiscaceae was expanded to include "Medusandra" and "Soyauxia". John Brenan, 57 years before, had been prescient in his perception of a relationship between "Medusandra" and "Soyauxia". Phylogeny. The phylogeny is diagrammed as a phylogenetic tree below. The relationships shown are from Wurdack and Davis (2009) except for the position of "Whittonia", for which no DNA sequences are known. "Peridiscus" and "Whittonia" are undoubtedly sister taxa due to their many shared morphological characters.