Peridiscaceae

Peridiscaceae is a family of flowering plants in the order Saxifragales.Peter F. Stevens. 2001 onwards. "Peridiscaceae". At: Angiosperm Phylogeny Website At: Missouri Botanical Garden Website. (see ''External links'' below). Four genera comprise this family: ''Medusandra, Soyauxia, Peridiscus'', and ''Whittonia''.,Kenneth J. Wurdack and Charles C. Davis. 2009. "Malpighiales phylogenetics: Gaining ground on one of the most recalcitrant clades in the angiosperm tree of life." ''American Journal of Botany'' 96(8):1551-1570. with a total of 12 known species. It has a disjunct distribution, with ''Peridiscus'' occurring in Venezuela and northern Brazil, ''Whittonia'' in Guyana,Clemens Bayer. 2007. "Peridiscaceae" pages 297-300. In: Klaus Kubitski (editor). ''The Families and Genera of Vascular Plants'' volume IX. Springer-Verlag: Berlin;Heidelberg, Germany. ''Medusandra'' in Cameroon, and ''Soyauxia'' in tropical West Africa.Vernon H. Heywood, Richard K. Brummitt, Ole Seberg, and Alastair Culham. ''Flowering Plant Families of the World''. Firefly Books: Ontario, Canada. (2007). ''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 HutchinsonJohn Hutchinson. ''The Families of Flowering Plants, Third Edition (1973)''. Oxford University Press: London. 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 tetra thecal 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.George Bentham and Joseph Dalton Hooker. 1862. ''Genera Plantarum'' volume 1, part 1, page 127. A. Black, William Pamplin, Lovell Reeve & Co., Williams & Norgate: London, England. (see ''External links'' below). 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.Umberto Quattrocchi. 2000. ''CRC World Dictionary of Plant Names''. volume III, page 2010. CRC Press: Baton Rouge, New York, London, Washington DC. . (see ''External links'' below)

Daniel Oliver established the genus ''Soyauxia'' in 1880 for ''Soyauxia gabonensis'', placing it in the family Passifloraceae.Joseph Dalton Hooker. 1880. ''Hooker's Icones Plantarum'' volume XIV (volume IV of the third series):page 73 and plate 1393. (see ''External links'' below). He named it for the German botanist and plant collector Hermann Soyaux,Umberto Quattrocchi. 2000. ''CRC World Dictionary of Plant Names''. volume IV, page 2521. CRC Press: Baton Rouge, New York, London, Washington DC. . 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,Regis B. Miller (1975). "Systematic anatomy of the xylem and comments on the relationships of Flacourtiaceae". ''Journal of the Arnold Arboretum'' 56(1):79.Mark W. Chase, Sue Zmarzty, M. Dolores Lledó, Kenneth J. Wurdack, Susan M. Swensen, and Michael F. Fay. 2002. "When in doubt, put it in Flacourtiaceae: a molecular phylogenetic analysis based on plastid ''rbcL'' DNA sequences." ''Kew Bulletin'' 57(1):141-181. 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.João G. Kuhlmann. 1947. "Peridiscaceae (Kuhlmann)". ''Arquivos do Serviço Florestal'' 3(1):3-7.

In 1952, John Brenan named and described ''Medusandra'', erecting a new family, Medusandraceae to accommodate it.John P.M. Brenan. 1952. "Plants of the Cambridge Expedition, 1947-1948: II. A new order of flowering plants from the British Cameroons". ''Kew Bulletin'' 7:227-236. In 1953, Brenan transferred ''Soyauxia'' from Passifloraceae to Medusandraceae,John P.M. Brenan. 1953. "''Soyauxia'', a second genus of Medusandraceae". ''Kew Bulletin'' 8:507-511. 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''.Noel Y. Sandwith. 1962. "Contributions to the flora of tropical America: LXIX. A new genus of Peridiscaceae". ''Kew Bulletin'' 15:467-471. 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.Vincent Savolainen, Michael F. Fay, Dirk C. Albach, Anders Backlund, Michelle van der Bank, Kenneth M. Cameron, S.A. Johnson, M. Dolores Lledo, Jean-Christophe Pintaud, Martyn P. Powell, Mary Clare Sheahan, Douglas E. Soltis, Pamela S. Soltis, Peter Weston, W. Mark Whitten, Kenneth J. Wurdack and Mark W. Chase. 2000. "Phylogeny of the eudicots: a nearly complete familial analysis based on ''rbcL'' gene sequences". ''Kew Bulletin'' 55(2):257-309. 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.The Angiosperm Phylogeny Group. 2003. "An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG II". ''Botanical Journal of the Linnean Society'' 141(4):399-436. 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.Shuguang Jian, Pamela S. Soltis, Matthew A. Gitzendanner, Michael J. Moore, Ruiqi Li, Tory A. Hendry, Yin-Long Qiu, Amit Dhingra, Charles D. Bell, and Douglas E. Soltis. 2008. "Resolving an Ancient, Rapid Radiation in Saxifragales". ''Systematic Biology'' 57(1):38-57.

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.

References

Bibliography

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;Websites

Peridiscaceae
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At
Missoure Botanical Garden Website

CRC World Dictionary of Plant Names: M-Q
At
Hooker's Icones Plantarum


in L. Watson and M.J. Dallwitz (1992 onwards)
The families of flowering plants
descriptions, illustrations, identification, information retrieval.
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{{Taxonbar, from=Q132930

Saxifragales families