| Buxales | |
|---|---|
 | |
| Buxus sempervirens | |
Scientific classification | |
| Kingdom: | Plantae |
| Clade: | Tracheophytes |
| Clade: | Angiosperms |
| Clade: | Eudicots |
| Order: | Buxales Takht. exReveal[1] |
| Families | |
| |
TheBuxales are a small order ofeudicotflowering plants, recognized by theAPG IV system of 2016. The order includes the familyBuxaceae; the familiesDidymelaceae andHaptanthaceae may also be recognized or may be included in the Buxaceae. Many members of the order are evergreen shrubs or trees, although some are herbaceous perennials. They have separate "male" (staminate) and "female" (carpellate) flowers, mostly on the same plant (i.e. they are mostlymonoecious). Some species are of economic importance either for the wood they produce or asornamental plants.
The Buxales have relatively few obvious shared features distinguishing them from related groups of plants (i.e. few obvioussynapomorphies). One is the presence of a particular type ofalkaloid or pseudoalkaloid,pregnane steroids. They have unisexual flowers (i.e. separate staminate or "male" flowers and carpellate or "female" flowers), most beingmonoecious. The flowers are small, less than 7 mm across. Thetepals have a singlevascular bundle (trace). Other characters common to the order include leaves with entire (untoothed) margins, flowers arranged inracemes, smallstyles capable of receiving pollen along their entire length rather than having a separate stigma, one to twoovules percarpel, and seeds withtestae (coats) made up of several cell layers.[2]
Didymeles, the sole genus in the familyDidymelaceae, consists of two species of evergreen tree occurring only inMadagascar. It is dioecious, i.e. with staminate and carpellate flowers on separate plants.Buxaceae (includingHaptanthaceae) is more diverse, with five or six genera totalling about 115 species, and is found in most temperate and tropical areas of the world. Most species are evergreen shrubs or small trees, but some (such as those ofPachysandra) are herbaceous perennials.[2]
In theAPG III system of 2009, the order includes two families,Buxaceae andHaptanthaceae. Unlike previous classifications (including theAPG II system of 2003), APG III does not recognize the familyDidymelaceae, but includes the genusDidymeles in Buxaceae.[1] Subsequent research published in 2011 suggests that the Haptanthaceae are embedded in the Buxaceae, possibly as the sister toBuxus,[3] and as of September 2014[update] theAngiosperm Phylogeny Website no longer recognizes Haptanthaceae.[2] In the 2016APG IV system, Haptanthaceae is incorporated into Buxaceae, which leaves Buxaceae the only family underBuxales.[4]
The Buxales are placed within theeudicots but outside thecore eudicots, in a paraphyletic group of basal eudicots. Themonophyly of the order and its general position relative to other eudicots has been confirmed by many studies.[2] One possible phylogenetic tree is shown below, where the precise ordering of the basal eudicots is still uncertain.[2]
| basal eudicots |
One difference between the basal eudicots and the core eudicots is that the latter appear to have undergone a complete duplication of the nuclear genome which is absent in the former.[5] Molecular phylogenetic studies have produced slightly different internal arrangements of the families and genera within the order;[3] hence the variation in the number of families into which it is divided.
Didymelaceae[2] or Buxaceae[1]
Buxaceae
Buxaceae[2] or Haptanthaceae[1]
In theCronquist system of 1981, the Buxaceae were associated with theEuphorbiaceae s.l.,[2] and the Didymelaceae were given their own order. In theAPG II system of 2003, theBuxaceae were a family unplaced as to order in theeudicots, optionally including the genusDidymeles, which could alternatively be placed in its own family.[6]
According tomolecular clock calculations, the lineage that led to Buxales split from other plants about 129million years ago[7] or 120 million years ago.[8]
Some species of the Buxaceae are of economic importance.Buxus sempervirens (common box) andBuxus macowanii (Cape box) produce hard wood valued for carving and engraving. Species are also used as ornamental garden plants and forbonsai. Common box is used for hedging and border edging. Species ofPachysandra are used as ground cover. Species ofSarcococca produce small but strongly scented flowers in the winter.[9]
