Peridiscaceae
Encyclopedia
Peridiscaceae is a family
of flowering plant
s in the order
Saxifragales
. It comprises four genera
: Medusandra, Soyauxia
, Peridiscus, and Whittonia. 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. An attempt to rediscover it in 2006 was not successful.
The largest genus is Soyauxia, with about seven species. Medusandra has two species. Peridiscus and Whittonia each contain one species. The Peridiscaceae are small tree
s or erect shrub
s of wet tropical forest
s.
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.
with descriptions of Soyauxia, Peridiscus, and Whittonia by Clemens Bayer.
Peridiscaceae are small tree
s or erect shrub
s. 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 raceme
s or spike
s, the clusters often being reduced to a pair of racemes or to a single raceme. The flower
s are bisexual and actinomorphic. The sepal
s are 4 to 7 in number, and free, that is, separate from each other. Medusandra and Soyauxia have five petal
s. Peridiscus and Whittonia have none.
Medusandra lacks a nectary disk and has five stamen
s, inserted opposite the petals, and alternating with five long, hairy staminode
s. In the others, the stamens are numerous and arranged in a ring around the nectary disk. The anthers are tetratheca
l 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 ovule
s 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-seed
ed; a capsule
in Medusandra and Soyauxia; a drupe
in Peridiscus and Whittonia.
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 of 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.
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.
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
of flowering plant
Flowering plant
The flowering plants , also known as Angiospermae or Magnoliophyta, are the most diverse group of land plants. Angiosperms are seed-producing plants like the gymnosperms and can be distinguished from the gymnosperms by a series of synapomorphies...
s in the order
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
Saxifragales
Saxifragales
Saxifragales is an order of flowering plants. Their closest relatives are a large eudicot group known as the rosids by the definition of rosids given in the APG II classification system. Some authors define the rosids more widely, including Saxifragales as their most basal group. Saxifragales is...
. It comprises four genera
Genera
Genera is a commercial operating system and development environment for Lisp machines developed by Symbolics. It is essentially a fork of an earlier operating system originating on the MIT AI Lab's Lisp machines which Symbolics had used in common with LMI and Texas Instruments...
: Medusandra, Soyauxia
Soyauxia
Soyauxia is a genus of flowering plants in the family Peridiscaceae. They are small trees or erect shrubs from wet forests of tropical West Africa. Eight specific names have been published in Soyauxia. Additional species have been discovered, but their names and descriptions will not be published...
, Peridiscus, and Whittonia. It has a disjunct distribution
Disjunct distribution
In biology, a taxon with a disjunct distribution is one that has two or more groups that are related but widely separated from each other geographically...
, with Peridiscus occurring in Venezuela
Venezuela
Venezuela , officially called the Bolivarian Republic of Venezuela , is a tropical country on the northern coast of South America. It borders Colombia to the west, Guyana to the east, and Brazil to the south...
and northern Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...
, Whittonia in Guyana
Guyana
Guyana , officially the Co-operative Republic of Guyana, previously the colony of British Guiana, is a sovereign state on the northern coast of South America that is culturally part of the Anglophone Caribbean. Guyana was a former colony of the Dutch and of the British...
, Medusandra in Cameroon
Cameroon
Cameroon, officially the Republic of Cameroon , is a country in west Central Africa. It is bordered by Nigeria to the west; Chad to the northeast; the Central African Republic to the east; and Equatorial Guinea, Gabon, and the Republic of the Congo to the south. Cameroon's coastline lies on the...
, and Soyauxia in tropical
Tropics
The tropics is a region of the Earth surrounding the Equator. It is limited in latitude by the Tropic of Cancer in the northern hemisphere at approximately N and the Tropic of Capricorn in the southern hemisphere at S; these latitudes correspond to the axial tilt of the Earth...
West Africa
West Africa
West Africa or Western Africa is the westernmost region of the African continent. Geopolitically, the UN definition of Western Africa includes the following 16 countries and an area of approximately 5 million square km:-Flags of West Africa:...
. Whittonia is possibly extinct, being known from only one specimen
Specimen
A specimen is a portion/quantity of material for use in testing, examination, or study.BiologyA laboratory specimen is an individual animal, part of an animal, a plant, part of a plant, or a microorganism, used as a representative to study the properties of the whole population of that species or...
collected below Kaieteur Falls
Kaieteur Falls
Kaieteur Falls is a high-volume waterfall on the Potaro River in central Guyana, Potaro-Siparuni region. It is located inKaieteur National Park. It is 226 meters high when measured from its plunge over a sandstone and conglomerate cliff to the first break...
in Guyana. An attempt to rediscover it in 2006 was not successful.
The largest genus is Soyauxia, with about seven species. Medusandra has two species. Peridiscus and Whittonia each contain one species. The Peridiscaceae are small tree
Tree
A tree is a perennial woody plant. It is most often defined as a woody plant that has many secondary branches supported clear of the ground on a single main stem or trunk with clear apical dominance. A minimum height specification at maturity is cited by some authors, varying from 3 m to...
s or erect shrub
Shrub
A shrub or bush is distinguished from a tree by its multiple stems and shorter height, usually under 5–6 m tall. A large number of plants may become either shrubs or trees, depending on the growing conditions they experience...
s of wet tropical forest
Forest
A forest, also referred to as a wood or the woods, is an area with a high density of trees. As with cities, depending where you are in the world, what is considered a forest may vary significantly in size and have various classification according to how and what of the forest is composed...
s.
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
John Hutchinson may refer to:*John Hutchinson , leader in the 17th century Puritan revolt in Britain*John Hutchinson , English writer...
with descriptions of Soyauxia, Peridiscus, and Whittonia by Clemens Bayer.
Peridiscaceae are small tree
Tree
A tree is a perennial woody plant. It is most often defined as a woody plant that has many secondary branches supported clear of the ground on a single main stem or trunk with clear apical dominance. A minimum height specification at maturity is cited by some authors, varying from 3 m to...
s or erect shrub
Shrub
A shrub or bush is distinguished from a tree by its multiple stems and shorter height, usually under 5–6 m tall. A large number of plants may become either shrubs or trees, depending on the growing conditions they experience...
s. The leaves are stipulate
Stipule
In botany, stipule is a term coined by Linnaeus which refers to outgrowths borne on either side of the base of a leafstalk...
, alternate, and simple, with margins that are entire
Leaf shape
In botany, leaf shape is characterised with the following terms :* Acicular : Slender and pointed, needle-like* Acuminate : Tapering to a long point...
or remotely crenulate (Medusandra). The petiole
Petiole (botany)
In botany, the petiole is the stalk attaching the leaf blade to the stem. The petiole usually has the same internal structure as the stem. Outgrowths appearing on each side of the petiole are called stipules. Leaves lacking a petiole are called sessile, or clasping when they partly surround the...
is pulvinate
Pulvinus
A pulvinus is a joint-like thickening at the base of a plant leaf or leaflet that facilitates growth-independent movement. It consists of a core of vascular tissue within a flexible, bulky cylinder of thin-walled parenchyma cells...
, at its apex, sometimes obscurely so. The stipules are in the axils of the leaves, sometimes enclosing an axillary bud
Bud
In botany, a bud is an undeveloped or embryonic shoot and normally occurs in the axil of a leaf or at the tip of the stem. Once formed, a bud may remain for some time in a dormant condition, or it may form a shoot immediately. Buds may be specialized to develop flowers or short shoots, or may have...
.
The inflorescence
Inflorescence
An inflorescence is a group or cluster of flowers arranged on a stem that is composed of a main branch or a complicated arrangement of branches. Strictly, it is the part of the shoot of seed plants where flowers are formed and which is accordingly modified...
is a cluster of axillary raceme
Raceme
A raceme is a type of inflorescence that is unbranched and indeterminate and bears pedicellate flowers — flowers having short floral stalks called pedicels — along the axis. In botany, axis means a shoot, in this case one bearing the flowers. In a raceme, the oldest flowers are borne...
s or spike
Raceme
A raceme is a type of inflorescence that is unbranched and indeterminate and bears pedicellate flowers — flowers having short floral stalks called pedicels — along the axis. In botany, axis means a shoot, in this case one bearing the flowers. In a raceme, the oldest flowers are borne...
s, the clusters often being reduced to a pair of racemes or to a single raceme. The flower
Flower
A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants . The biological function of a flower is to effect reproduction, usually by providing a mechanism for the union of sperm with eggs...
s are bisexual and actinomorphic. The sepal
Sepal
A sepal is a part of the flower of angiosperms . Collectively the sepals form the calyx, which is the outermost whorl of parts that form a flower. Usually green, sepals have the typical function of protecting the petals when the flower is in bud...
s are 4 to 7 in number, and free, that is, separate from each other. Medusandra and Soyauxia have five petal
Petal
Petals are modified leaves that surround the reproductive parts of flowers. They often are brightly colored or unusually shaped to attract pollinators. Together, all of the petals of a flower are called a corolla. Petals are usually accompanied by another set of special leaves called sepals lying...
s. Peridiscus and Whittonia have none.
Medusandra lacks a nectary disk and has five stamen
Stamen
The stamen is the pollen producing reproductive organ of a flower...
s, inserted opposite the petals, and alternating with five long, hairy staminode
Staminode
In botany, a staminode is an often rudimentary, sterile or abortive stamen. This means that it does not produce pollen. Staminodes are frequently inconspicuous and stamen-like, usually occurring at the inner whorl of the flower, but are also sometimes long enough to protrude from the...
s. In the others, the stamens are numerous and arranged in a ring around the nectary disk. The anthers are tetratheca
Theca
A theca refers to any case, covering, or sheath.In botany, the theca of an angiosperm consists a pair of microsporangia that are adjacent to each other and share a common area of dehiscence called the stomium. Any part of a microsporophyll that bears microsporangia is called an anther. Most...
l in Medusandra and Soyauxia; bithecal in Peridiscus and Whittonia.
The perianth
Perianth
The term perianth has two similar but separate meanings in botany:* In flowering plants, the perianth are the outer, sterile whorls of a flower...
parts are attached below the ovary
Ovary (plants)
In the flowering plants, an ovary is a part of the female reproductive organ of the flower or gynoecium. Specifically, it is the part of the pistil which holds the ovule and is located above or below or at the point of connection with the base of the petals and sepals...
. The ovary is therefore superior, but appears half-inferior in Peridiscus because the ovary is embedded in the large, fleshy disk. The gynoecium
Gynoecium
Gynoecium is most commonly used as a collective term for all carpels in a flower. A carpel is the ovule and seed producing reproductive organ in flowering plants. Carpels are derived from ovule-bearing leaves which evolved to form a closed structure containing the ovules...
consists of three or four carpels, united to form a unilocular
Locule
A locule is a small cavity or compartment within an organ or part of an organism ....
ovary. The placentation is apical, with two ovule
Ovule
Ovule means "small egg". In seed plants, the ovule is the structure that gives rise to and contains the female reproductive cells. It consists of three parts: The integument forming its outer layer, the nucellus , and the megaspore-derived female gametophyte in its center...
s 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
Fruit
In broad terms, a fruit is a structure of a plant that contains its seeds.The term has different meanings dependent on context. In non-technical usage, such as food preparation, fruit normally means the fleshy seed-associated structures of certain plants that are sweet and edible in the raw state,...
is one-seed
Seed
A seed is a small embryonic plant enclosed in a covering called the seed coat, usually with some stored food. It is the product of the ripened ovule of gymnosperm and angiosperm plants which occurs after fertilization and some growth within the mother plant...
ed; a capsule
Capsule (fruit)
In botany a capsule is a type of simple, dry fruit produced by many species of flowering plants. A capsule is a structure composed of two or more carpels that in most cases is dehiscent, i.e. at maturity, it splits apart to release the seeds within. A few capsules are indehiscent, for example...
in Medusandra and Soyauxia; a drupe
Drupe
In botany, a drupe is a fruit in which an outer fleshy part surrounds a shell of hardened endocarp with a seed inside. These fruits develop from a single carpel, and mostly from flowers with superior ovaries...
in Peridiscus and Whittonia.
History
George BenthamGeorge Bentham
George Bentham CMG FRS was an English botanist, characterized by Duane Isely as "the premier systematic botanist of the nineteenth century".- Formative years :...
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
Flacourtiaceae
Flacourtiaceae is a defunct family of flowering plants whose former members have been scattered to various other families, mostly to Achariaceae, Samydaceae, and Salicaceae. It was so vaguely defined that hardly anything seemed out of place there and it became a dumping ground for odd and anomalous...
. Bentham wrote no etymology
Etymology
Etymology is the study of the history of words, their origins, and how their form and meaning have changed over time.For languages with a long written history, etymologists make use of texts in these languages and texts about the languages to gather knowledge about how words were used during...
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
Daniel Oliver
Daniel Oliver, FRS was a British botanist.He was Librarian of the Herbarium, Royal Botanic Gardens, Kew from 1860–1890 and Keeper there from 1864–1890, and Professor of Botany at University College, London from 1861–1888....
established the genus Soyauxia in 1880 for Soyauxia gabonensis, placing it in the family Passifloraceae
Passifloraceae
Passifloraceae is a family of flowering plants, containing about 530 species classified in around 18 genera. They include trees, shrubs, lianas and climbing plants, and are mostly found in tropical regions....
. He named it for the German
Germans
The Germans are a Germanic ethnic group native to Central Europe. The English term Germans has referred to the German-speaking population of the Holy Roman Empire since the Late Middle Ages....
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
Hermann Otto Sleumer
Hermann Otto Sleumer was a Dutch botanist of German birth. The plant genera Sleumerodendron Virot and Sleumeria Utteridge, Nagam. & Teo , are named for him....
said, a fiction, and Peridiscus was, from the outset, one of its most doubtful members. Recognizing its distinctiveness, João Kuhlmann
João Geraldo Kuhlmann
João Geraldo Kuhlmann was aBrazilian botanist.Kuhlmann was a specialist on Taxonomy of Angiosperms...
segregated
Segregate (taxonomy)
In taxonomy, a segregate, or a segregate taxon is created when a taxon is split off, from another taxon. This other taxon will be better known, usually bigger, and will continue to exist, even after the segregate taxon has been split off...
it into its own family in 1947.
In 1952, John Brenan
John Patrick Micklethwait Brenan
John Patrick Micklethwait Brenan was a British botanist. He was born June 19, 1917 in Chislehurst and died September 26, 1985 at Kew. He is buried at St. Anne's Church, Kew....
named and described Medusandra, erecting a new family, Medusandraceae
Medusandraceae
Medusandra is a genus of flowering plants in the family Peridiscaceae. It has two species, Medusandra richardsiana and Medusandra mpomiana. M. richardsiana is the most common and well known. Both species are native to Cameroon and adjacent countries. Medusandra was named by John Brenan in 1952...
to accommodate it. In 1953, Brenan transferred Soyauxia from Passifloraceae to Medusandraceae, but few others agreed with his classification. In 1954, John Hutchinson
John Hutchinson
John Hutchinson may refer to:*John Hutchinson , leader in the 17th century Puritan revolt in Britain*John Hutchinson , English writer...
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
DNA sequence
The sequence or primary structure of a nucleic acid is the composition of atoms that make up the nucleic acid and the chemical bonds that bond those atoms. Because nucleic acids, such as DNA and RNA, are unbranched polymers, this specification is equivalent to specifying the sequence of...
for the rbcL
RuBisCO
Ribulose-1,5-bisphosphate carboxylase oxygenase, commonly known by the shorter name RuBisCO, is an enzyme involved in the first major step of carbon fixation, a process by which atmospheric carbon dioxide is converted by plants to energy-rich molecules such as glucose. RuBisCo is an abbreviation...
gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
of Whittonia was produced and used in a molecular phylogenetic study of the eudicots
Eudicots
Eudicots and Eudicotyledons are botanical terms introduced by Doyle & Hotton to refer to a monophyletic group of flowering plants that had been called tricolpates or non-Magnoliid dicots by previous authors...
. This study placed Peridiscaceae in a clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
with Elatinaceae
Elatinaceae
Elatinaceae is a family of flowering plants with 35-50 species in 2 genera: Elatine and Bergia. The Elatine are mostly aquatic herbs, and the Bergia are subshrubs to shrubs. Elatine species are widely distributed throughout the world from temperate to tropical zones, with its greatest diversity...
and Malpighiaceae
Malpighiaceae
Malpighiaceae is a family of flowering plants in the order Malpighiales. It comprises approximately 75 genera and 1300 species, all of which are native to the tropics and subtropics...
, a very surprising and unexpected result. On the basis of this phylogeny, the Angiosperm Phylogeny Group
Angiosperm Phylogeny Group
The Angiosperm Phylogeny Group, or APG, refers to an informal international group of systematic botanists who came together to try to establish a consensus on the taxonomy of flowering plants that would reflect new knowledge about plant relationships discovered through phylogenetic studies., three...
placed Peridiscaceae in Malpighiales
Malpighiales
Malpighiales is one of the largest orders of flowering plants, containing about 16000 species, approximately 7.8% of the eudicots. The order is very diverse and hard to recognize except with molecular phylogenetic evidence. It is not part of any of the classification systems that are based only on...
when they published the APG II system
APG II system
The APG II system of plant classification is the second, now obsolete, version of a modern, mostly molecular-based, system of plant taxonomy that was published in April 2003 by the Angiosperm Phylogeny Group. It was a revision of the first APG system, published in 1998, and was superseded in 2009...
of plant classification
History of plant systematics
The history of plant systematics—the biological classification of plants—stretches from the work of ancient Greek to modern evolutionary biologists. As a field of science, plant systematics came into being only slowly, early plant lore usually being treated as part of the study of...
in 2003. It was soon found that the rbcL sequence for Whittonia was a chimera
Fusion protein
Fusion proteins or chimeric proteins are proteins created through the joining of two or more genes which originally coded for separate proteins. Translation of this fusion gene results in a single polypeptide with functional properties derived from each of the original proteins...
, formed by DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
from unidentified plants that had contaminated the sample
Sample (material)
In general, a sample is a limited quantity of something which is intended to be similar to and represent a larger amount of that thing. The things could be countable objects such as individual items available as units for sale, or a material not countable as individual items. Samples of countable...
. 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
Plant morphology
Plant morphology or phytomorphology is the study of the physical form and external structure of plants. This is usually considered distinct from plant anatomy, which is the study of the internal structure of plants, especially at the microscopic level...
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
Chloroplast
Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...
DNA data, as well as some mitochondrial and nuclear
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
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
Outgroup
In cladistics or phylogenetics, an outgroup is a group of organisms that serves as a reference group for determination of the evolutionary relationship among three or more monophyletic groups of organisms....
, finding strong support for a clade of [Medusandra + (Soyauxia + Peridiscus)]. When the APG III system
APG III system
The APG III system of flowering plant classification is the third version of a modern, mostly molecular-based, system of plant taxonomy...
was published in October of 2009, Peridiscaceae was expanded
Circumscription (taxonomy)
In taxonomy, circumscription is the definition of the limits of a taxonomic group of organisms. One goal of taxonomy is to achieve a stable circumscription for every taxonomic group. Achieving stability can be simple or difficult....
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 treePhylogenetic tree
A phylogenetic tree or evolutionary tree is a branching diagram or "tree" showing the inferred evolutionary relationships among various biological species or other entities based upon similarities and differences in their physical and/or genetic characteristics...
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
Taxon
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...
due to their many shared morphological characters.
External links
- Peridiscaceae At: Angiosperm Phylogeny Website At: Missoure Botanical Garden Website
- CRC World Dictionary of Plant Names: M-Q At: Google Books
- Genera Plantarum by Bentham & Hooker
- Hooker's Icones Plantarum
- APG III
- Peridiscaceae in L. Watson and M.J. Dallwitz (1992 onwards). The families of flowering plants: descriptions, illustrations, identification, information retrieval.