| Peridiscaceae | |
|---|---|
.png) | |
| Botanical illustration ofPeridiscus lucidus | |
Scientific classification | |
| Kingdom: | Plantae |
| Clade: | Tracheophytes |
| Clade: | Angiosperms |
| Clade: | Eudicots |
| Order: | Saxifragales |
| Family: | Peridiscaceae Kuhlm.[1] |
| Type genus | |
| Peridiscus | |
| Genera | |
Peridiscaceae is afamily offlowering plants in theorderSaxifragales.[2] Fourgenera comprise this family:Medusandra,Soyauxia,Peridiscus, andWhittonia.,[3] with a total of 12 known species.[4] It has adisjunct distribution, withPeridiscus occurring inVenezuela and northernBrazil,Whittonia inGuyana,[5]Medusandra inCameroon, andSoyauxia intropicalWest Africa.[6]Whittonia is possiblyextinct, being known from only onespecimen collected belowKaieteur Falls in Guyana. In 2006, archeologists attempted to rediscover it, however, it proved unsuccessful.[3]
The largest genus isSoyauxia, with about seven species.Medusandra has two species.Peridiscus andWhittonia each contain one species. The Peridiscaceae are smalltrees or erectshrubs of wet tropicalforests.
It was not until 2009 that all four of the genera were united into a single family.[3]Peridiscus andWhittonia are clearly close relatives. This pair, and the other two genera have long been considered anomalous, being variously classified by different authors.
The following description was created by combining descriptions ofMedusandra andPeridiscus byJohn Hutchinson[7] with descriptions ofSoyauxia,Peridiscus, andWhittonia by Clemens Bayer.[5]
Peridiscaceae are smalltrees or erectshrubs. The leaves arestipulate,alternate, andsimple, with margins that areentire or remotelycrenulate (Medusandra). Thepetiole ispulvinate, at itsapex, sometimes obscurely so. The stipules are in theaxils of the leaves, sometimes enclosing an axillarybud.
Theinflorescence is a cluster of axillaryracemes orspikes, the clusters often being reduced to a pair of racemes or to a single raceme. Theflowers are bisexual andactinomorphic. Thesepals are 4 to 7 in number, andfree, that is, separate from each other.Medusandra andSoyauxia have fivepetals.Peridiscus andWhittonia have none.
Medusandra lacks anectary disk and has fivestamens, inserted opposite the petals, and alternating with five long, hairystaminodes. In the others, the stamens arenumerous and arranged in a ring around the nectary disk. Theanthers are tetrathecal inMedusandra andSoyauxia; bithecal inPeridiscus andWhittonia.
Theperianth parts are attached below theovary. The ovary is thereforesuperior, but appears half-inferior inPeridiscus because the ovary is embedded in the large, fleshy disk. Thegynoecium consists of three or fourcarpels, united to form aunilocular ovary. Theplacentation is apical, with twoovules at the apex of each carpel. The ovary has a central column inMedusandra andSoyauxia. Each carpel bears astylulus and these are well separated at the apex of the ovary.
Thefruit is one-seeded; acapsule inMedusandra andSoyauxia; adrupe inPeridiscus andWhittonia.
George Bentham established the genusPeridiscus in 1862, naming its only speciesPeridiscus lucidus. He placed it in a group which he called "Tribus Flacourtieae" and which later would be known as the familyFlacourtiaceae.[8] Bentham wrote noetymology 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.[9]
Daniel Oliver established the genusSoyauxia in 1880 forSoyauxia gabonensis, placing it in the familyPassifloraceae.[10] He named it for theGerman botanist and plant collectorHermann Soyaux,[11] 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".[10]
The family Flacourtiaceae was, asHermann Sleumer said, a fiction,[12][13] andPeridiscus was, from the outset, one of its most doubtful members.[5][7] Recognizing its distinctiveness,João Kuhlmannsegregated it into its own family in 1947.[14]
In 1952,John Brenan named and describedMedusandra, erecting a new family,Medusandraceae to accommodate it.[15] In 1953, Brenan transferredSoyauxia from Passifloraceae to Medusandraceae,[16] but few others agreed with his classification. In 1954,John Hutchinson andJohn McEwen Dalziel followed Brenan's treatment in the second edition of theirFlora of West Tropical Africa. Hutchinson, however, soon recanted, explaining in some detail why he thought thatMedusandra andSoyauxia were not related.[7]
In 1962, Noel Y. Sandwith named and describedWhittonia.[17] In an accompanying article, Charles Russell Metcalfe discussed its close relationship toPeridiscus. For four decades thereafter, Peridiscaceae was viewed as a family of uncertain taxonomic position, containing two genera.
In the year 2000, aDNA sequence for therbcLgene ofWhittonia was produced and used in amolecular phylogenetic study of theeudicots.[18] This study placed Peridiscaceae in aclade withElatinaceae andMalpighiaceae, a very surprising and unexpected result. On the basis of thisphylogeny, theAngiosperm Phylogeny Group placed Peridiscaceae inMalpighiales when they published theAPG II system ofplant classification in 2003.[19] It was soon found that therbcL sequence forWhittonia was achimera, formed byDNA from unidentified plants that had contaminated thesample.[20] No subsequent attempt to extract DNA fromWhittonia has been made.
In 2004, using DNA fromPeridiscus, it was shown that Elatinaceae and Malpighiaceae are indeedsister families and that Peridiscaceae belong to Saxifragales.[20]Medusandra andSoyauxia, meanwhile, were listed in APG II in an appendix entitled "TAXA OF UNCERTAIN POSITION".[19]
DNA fromSoyauxia was eventually obtained, and in 2007, it was shown thatSoyauxia is most closely related toPeridiscus and, presumably,Whittonia.[21] Since this result has a goodmorphological 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.[21]
In 2008, in a study employing a large amount ofchloroplast DNA data, as well as somemitochondrial andnuclear DNA, it was shown that Peridiscaceae is sister to the rest of Saxifragales.[22]
It had been suspected thatMedusandra might belong somewhere in Malpighiales, but a phylogeny of that order, generated in 2009, placedMedusandra in Saxifragales. The authors had includedMedusandra and a few other members of Saxifragales in theiroutgroup, finding strong support for a clade of [Medusandra + (Soyauxia +Peridiscus)].[3] When theAPG III system was published in October 2009, Peridiscaceae wasexpanded to includeMedusandra andSoyauxia.[1] John Brenan, 57 years before, had been prescient in his perception of a relationship betweenMedusandra andSoyauxia.
Thephylogeny is diagrammed as aphylogenetic tree below. The relationships shown are from Wurdack and Davis (2009)[3] except for the position ofWhittonia, for which no DNA sequences are known.Peridiscus andWhittonia are undoubtedlysistertaxa due to their many shared morphological characters.
| Peridiscaceae | |
| National | |
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| Other | |
