Celastrales is an order of flowering plants. They are found throughout the tropics and subtropics, with only a few species extending far into the temperate regions. There are about 1200  to 1350  species in about 100 genera. All but 7 of these genera are in the large family Celastraceae. Until recently, the composition of the order and its division into families varied greatly from one author to another.
Celastrales is a diverse order that has no conspicuous distinguishing characteristic and is consequently hard to recognize. The flowers are usually small with a conspicuous nectary disk. The stipules are small or rarely absent. The micropyle has 2 openings and is therefore called a bistomal micropyle. Flowers with well-developed male and female parts are often functionally unisexual. The seed often has an aril. In bud, the sepals have a quincuncial arrangement. This means that 2 sepals are inside, 2 are outside, and the other has one side covered and one side exposed.
Perhaps the most conspicuous and unusual trait of Celastrales is the nectary disk, a feature that it shares with another rosid order, Sapindales. Since the 2 orders are not closely related, the disk must have been an independent development in each of these 2 lines.
Celastrales is a member of the COM clade  of Fabidae, with Fabidae being one of the 2 groups of Eurosids. The COM clade consists of 3 orders, Celastrales, Oxalidales (including Huaceae), and Malpighiales.
The name Celastrales was first used by Thomas Baskerville in 1839. In the time since Baskerville first defined the order, until the 21st century, there were great differences of opinion about what should be included in the order and in its largest family, Celastraceae. The family Celastraceae was the only group consistently placed in the order by all authors who accepted it. Because of the ambiguity and complexity of its definition, Celastraceae became a dumping ground for genera of dubious affinity. Several genera were assigned to this family with considerable doubt about whether they really belonged there. Also, some genera that properly belong in Celastraceae were placed elsewhere.
By the end of the 20th century, Goupia and Forsellesia had been excluded from Celastraceae and also from Celastrales. Goupia is now in Malpighiales. Forsellesia is now in Crossosomatales. It continues to be the subject of a dispute about whether its proper name is Forsellesia or Glossopetalon.
After being placed elsewhere, Canotia, Brexia, and Plagiopteron were found to belong in Celastraceae. The family Hippocrateaceae was found to be deeply nested within Celastraceae and is no longer recognized as a separate family.
In 2000, Vincent Savolainen et alii found that 3 families - Lepidobotryaceae, Parnassiaceae, and Celastraceae - were closely related. They stated that these three families should constitute the order Celastrales, and this idea was accepted by the Angiosperm Phylogeny Group, which later subsumed Parnassiaceae into Celastraceae. Savolainen and co-authors also excluded Lophopyxis from Celastrales. Lophopyxis now constitutes a monogeneric family in Malpighiales.
In 2001, in a molecular phylogenetic study of DNA sequences, Mark Simmons and others confirmed all of these results except for the placement of Lophopyxis and Lepidobotryaceae, which they did not sample.
In 2006, Li-Bing Zhang and Mark Simmons produced a phylogeny of Celastrales based on nuclear ribosomal and chloroplast DNA. Their results showed that Bhesa and Perrottetia were misplaced in Celastraceae. Bhesa is now in Centroplacaceae, a family in Malpighiales. and Perrottetia is in Huerteales. Zhang and Simmons found Pottingeria and Mortonia to be closely related to the families Parnassiaceae and Celastraceae, as they were then defined, but not in either of them. These two genera are therefore in Celastrales. They found that Siphonodon and Empleuridium are proper members of Celastraceae, removing considerable doubt about their placement there. They also showed that the small family Stackhousiaceae, consisting of three genera, is embedded in Celastraceae. Except for taxa that were not sampled, these results were confirmed by the second phylogeny of Celastrales, which was produced by Mark Simmons and several co-authors in 2008.
Nicobariodendron sleumeri, the only member of its genus, continues to be an enigma. It is a small tree from the Andaman and Nicobar Islands of India. Little is known of it and it has never been sampled for DNA. It is generally thought to belong in Celastrales, but this is not a certainty. It is one of the five taxa placed incertae sedis in the angiosperms in the APG III system of classification.
Celastrales has been divided into families in various ways. In their APG II classification in 2003, the Angiosperm Phylogeny Group recognized 3 families in Celastrales - Lepidobotryaceae, Parnassiaceae, and Celastraceae. When they revised their classification in 2009, they recognized only two families because Pottingeria and the two genera of Parnassiaceae were transferred to Celastraceae. Nicobariodendron became one of the five taxa placed incertae sedis in the angiosperms.
In the 2006 phylogeny, Nicobariodendron was not sampled, but those species that were sampled fell into 2 strongly supported clades. One was a small clade consisting only of the family Lepidobotryaceae. Its sister was a very large clade containing the rest of the order. The large clade consisted of 5 strongly supported groups. These are the family Parnassiaceae, the genus Pottingeria, the genus Mortonia (in Celastraceae), and a pair of genera from Celastraceae (Quetzalia and Zinowiewia), and the rest of Celastraceae. No relationships were resolved among these 5 groups.
In 2008, Simmons and others produced a phylogeny of Celastrales that achieved better resolution than the 2006 study by sampling more species and more DNA. They found the same pentatomy of five strongly supported groups that the previous study had found, but only weak to moderate support for any relationships between the five groups. In the APG III system, Celastraceae was expanded to consist of these five groups. No one has yet published an intrafamilial classification for the expanded Celastraceae.
The following phylogenetic tree was made by combining parts of three different trees. Bootstrap support is 100% except where shown. Branches with less than 50% bootstrap support are collapsed. The clade numbers are from Simmons et alii (2008).
1. ^ "Lepidobotryaceae", "Parnassiaceae", and "Celastraceae" In: Klaus Kubitzki (ed.). The Families and Genera of Vascular Plants vol. VI. Springer-Verlag: Berlin;Heidelberg, Germany. (2004). ISBN 978-3-540-06512-8 (vol. VI).
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