APG system

The APG system (Angiosperm Phylogeny Group system) of plant classification is the first version of a modern, mostly molecular-based, system of plant taxonomy. Published in 1998 by the Angiosperm Phylogeny Group, it was replaced by the improved APG II in 2003, APG III system in 2009 and APG IV system in 2016.

History

The original APG system is unusual in being based, not on total evidence, but on the cladistic analysis of the DNA sequences of three genes, two chloroplast genes and one gene coding for ribosomes. Although based on molecular evidence only, its constituent groups prove to be supported by other evidence as well, for example pollen morphology supports the split between the eudicots and the rest of the former dicotyledons.

The system is rather controversial in its decisions at the family level, splitting a number of long-established families and submerging some other families. It also is unusual in not using botanical names above the level of order, that is, an order is the highest rank that will have a formal botanical name in this system. Higher groups are defined only as clades, with names such as monocots, eudicots, rosids, asterids.

The APG system was superseded in 2003 by a revision, the APG II system, in 2009 by a next revision, the APG III system, and then in 2016 by a further revision, the APG IV system.

Groups

The main groups in the system (all unranked clades) are:

Representation in color

The APG system recognises 462 families and 40 orders: these are assigned as follows. In the beginning of each listing some families or orders that are not placed in a further clade:

Note: "+ ..." = optional seggregrate family, that may be split off from the preceding family.

See also

References

  • The Angiosperm Phylogeny Group (1998). "An ordinal classification for the families of flowering plants". Annals of the Missouri Botanical Garden. 85 (4): 531–553. doi:10.2307/2992015. JSTOR 2992015. (Available online: (PDF))

External links

Note: This is a selected list of the more influential systems. There are many other systems, for instance a review of earlier systems, published by Lindley in his 1853 edition, and Dahlgren (1982). Examples include the works of Scopoli, Batsch and Grisebach.

APG II system

The APG II system (Angiosperm Phylogeny Group 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 by a further revision, the APG III system.

Angiosperm Phylogeny Group

The Angiosperm Phylogeny Group, or APG, refers to an informal international group of systematic botanists who collaborate to establish a consensus on the taxonomy of flowering plants (angiosperms) that reflects new knowledge about plant relationships discovered through phylogenetic studies.

As of 2016, four incremental versions of a classification system have resulted from this collaboration, published in 1998, 2003, 2009 and 2016. An important motivation for the group was what they considered deficiencies in prior angiosperm classifications since they were not based on monophyletic groups (i.e., groups that include all the descendants of a common ancestor).

APG publications are increasingly influential, with a number of major herbaria changing the arrangement of their collections to match the latest APG system.

Class (biology)

In biological classification, class (Latin: classis) is a taxonomic rank, as well as a taxonomic unit, a taxon, in that rank. Other well-known ranks in descending order of size are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the prefix sub-: subclass (Latin: subclassis). For example, dogs are in the class Mammalia.

The composition of each class is determined by a taxonomist. Often there is no exact agreement, with different taxonomists taking different positions. There are no hard rules that a taxonomist needs to follow in describing a class, but for well-known animals there is likely to be consensus.

In botany, classes are now rarely discussed. Since the first publication of the APG system in 1998, which proposed a taxonomy of the flowering plants up to the level of orders, many sources have preferred to treat ranks higher than orders as informal clades. Where formal ranks have been assigned, the ranks have been reduced to a very much lower level, e.g. class Equisitopsida for the land plants, with the major divisions within the class assigned to subclasses and superorders.

Commelinids

In plant taxonomy, commelinids (originally commelinoids) (plural, not capitalised) is a name used by the APG IV system for a clade within the monocots, which in its turn is a clade within the angiosperms. The commelinids are the only clade that the APG has informally named within the monocots. The remaining monocots are a paraphyletic unit. Also known as the commelinid monocots it forms one of three groupings within the monocots, and the final branch, the other two groups being the alismatid monocots and the lilioid monocots.

Dasypogonaceae

Dasypogonaceae is a family of flowering plants. Such a family has not been commonly recognized by taxonomists: the plants involved were usually included in the family Xanthorrhoeaceae. Dasypogonaceae includes four genera with 16 species.The APG IV system, of 2016 places the family in order Arecales, after some studies revealed the family as sister to Arecaceae.The earlier APG III of 2009, APG II of 2003, and APG system of 1998, accepted the family and assigns the family to the clade commelinids, unplaced as to order. In turn, the commelinids belong to the monocots.

The family is endemic to Australia, and comprises 16 species in four genera. The best known representative is Kingia australis.

Dioscoreaceae

Dioscoreaceae () is a family of monocotyledonous flowering plants, with about 715 known species in nine genera. The best-known member of the family is the yam (some species of Dioscorea).

The APG system (1998) and APG II system (2003) both place it in the order Dioscoreales, in the clade monocots. However, the circumscription changed in the APG II system, with the 2003 system expanded to include the plants that in the 1998 system were treated in the families Taccaceae and Trichopodaceae.

Eudicots

The eudicots, Eudicotidae or eudicotyledons are a clade of flowering plants that had been called tricolpates or non-magnoliid dicots by previous authors. The botanical terms were introduced in 1991 by evolutionary botanist James A. Doyle and paleobotanist Carol L. Hotton to emphasize the later evolutionary divergence of tricolpate dicots from earlier, less specialized, dicots. The close relationships among flowering plants with tricolpate pollen grains was initially seen in morphological studies of shared derived characters. These plants have a distinct trait in their pollen grains of exhibiting three colpi or grooves paralleling the polar axis. Later molecular evidence confirmed the genetic basis for the evolutionary relationships among flowering plants with tricolpate pollen grains and dicotyledonous traits. The term means "true dicotyledons", as it contains the majority of plants that have been considered dicots and have characteristics of the dicots. The term "eudicots" has subsequently been widely adopted in botany to refer to one of the two largest clades of angiosperms (constituting over 70% of the angiosperm species), monocots being the other. The remaining angiosperms include magnoliids and what are sometimes referred to as basal angiosperms or paleodicots, but these terms have not been widely or consistently adopted, as they do not refer to a monophyletic group.

The other name for the eudicots is tricolpates, a name which refers to the grooved structure of the pollen. Members of the group have tricolpate pollen, or forms derived from it. These pollens have three or more pores set in furrows called colpi. In contrast, most of the other seed plants (that is the gymnosperms, the monocots and the paleodicots) produce monosulcate pollen, with a single pore set in a differently oriented groove called the sulcus. The name "tricolpates" is preferred by some botanists to avoid confusion with the dicots, a nonmonophyletic group.Numerous familiar plants are eudicots, including many common food plants, trees, and ornamentals. Some common and familiar eudicots include members of the sunflower family such as the common dandelion, the forget-me-not, cabbage and other members of its family, apple, buttercup, maple, and macadamia. Most leafy trees of midlatitudes also belong to eudicots, with notable exceptions being magnolias and tulip trees which belong to magnoliids, and Ginkgo biloba, which is not an angiosperm.

The name "eudicots" (plural) is used in the APG system, of 1998, and APG II system, of 2003, for classification of angiosperms. It is applied to a clade, a monophyletic group, which includes most of the (former) dicots.

"Tricolpate" is a synonym for the "Eudicot" monophyletic group, the "true dicotyledons" (which are distinguished from all other flowering plants by their tricolpate pollen structure). The number of pollen grain furrows or pores helps classify the flowering plants, with eudicots having three colpi (tricolpate), and other groups having one sulcus.Pollen apertures are any modification of the wall of the pollen grain. These modifications include thinning, ridges and pores, they serve as an exit for the pollen contents and allow shrinking and swelling of the grain caused by changes in moisture content. The elongated apertures/ furrows in the pollen grain are called colpi (singular colpus), which, along with pores, are a chief criterion for identifying the pollen classes.

Magnoliales

The Magnoliales comprise an order of flowering plants.

Magnoliids

Magnoliids (or Magnoliidae or Magnolianae) are a group of flowering plants. Until recently, the group included about 9,000 species, including magnolias, nutmeg, bay laurel, cinnamon, avocado, black pepper, tulip tree and many others. That group is characterized by trimerous flowers, pollen with one pore, and usually branching-veined leaves.

Malvoideae

Malvoideae is a botanical name at the rank of subfamily, which includes in the minimum the genus Malva. It was first used by Burnett in 1835, but was not much used until recently, where, within the framework of the APG System, which unites the families Malvaceae, Bombacaceae, Sterculiaceae and Tiliaceae of the Cronquist system, the aggregate family Malvaceae is divided into 9 subfamilies, including Malvoideae. The Malvoideae of Kubitzki and Bayer includes 4 tribes:-

Malveae (Abutilon, Alcea, Malva, Sidalcea etc.)

Gossypieae (Gossypium, the cottons etc.)

Hibisceae (Hibiscus etc.)

Kydieae

- and two unplaced genera:-

Jumelleanthus

HowittiaBaum et al. have a wider concept (cladistically, all those plants more closely related to Malva sylvestris than to Bombax ceiba) of Malvoideae, which includes additionally the tribe Matisieae (three genera of Neotropical trees) and the genera Lagunaria, Camptostemon, Pentaplaris and Uladendron.

Melanthiaceae

Melanthiaceae, also called the bunchflower family, is a family of flowering perennial herbs native to the Northern Hemisphere. Along with many other lilioid monocots, early authors considered members of this family to belong to the family Liliaceae, in part because both their sepals and petals closely resemble each other and are often large and showy like those of lilies, while some more recent taxonomists have placed them in a family Trilliaceae. The most authoritative modern treatment, however, the APG III system of 2009 (unchanged from the 2003 APG II system and the 1998 APG system), places the family in the order Liliales, in the clade monocots. Circumscribed in this way, the family includes up to 17 genera.

Familiar members of the family include Herb Paris (Paris quadrifolia) and the trilliums.

Nyctaginaceae

Nyctaginaceae, the four o'clock family, is a family of around 33 genera and 290 species of flowering plants, widely distributed in tropical and subtropical regions, with a few representatives in temperate regions. The family has a unique fruit type, called an "anthocarp", and many genera have extremely large (>100 µm) pollen grains.

The family has been almost universally recognized by plant taxonomists. The APG II system (2003; unchanged from the APG system of 1998), assigns it to the order Caryophyllales in the clade core eudicots.

A phylogenetic study by Levin has justified the combination of Selinocarpus and Ammocodon into the genus Acleisanthes. The genus Izabalea is now considered a synonym of Agonandra, a genus in Opiliaceae. A more recent study by Douglas and Manos clarified the relationships among almost all of the genera in the family and demonstrated that a substantial diversification of herbaceous genera has occurred in arid North America. Many genera of Nyctaginaceae possess unusual characters. Notable examples include sticky bands on the stems between the nodes, cleistogamous flowers (which self-pollinate without opening), or gypsophily, the ability to grow on soils with a high concentration of gypsum.

Piperales

Piperales is a botanical name for an order of flowering plants. It necessarily includes the family Piperaceae but otherwise has been treated variously over time. Well-known plants which may be included in this order include black pepper, kava, lizard's tail, birthwort, and wild ginger.

Plumbaginaceae

Plumbaginaceae is a family of flowering plants, with a cosmopolitan distribution. The family is sometimes referred to as the leadwort family or the plumbago family.

Most species in this family are perennial herbaceous plants, but a few grow as lianas or shrubs. The plants have perfect flowers and are pollinated by insects. They are found in many different climatic regions, from arctic to tropical conditions, but are particularly associated with salt-rich steppes, marshes, and sea coasts.

The family has been recognized by most taxonomists. The APG II system (2003; unchanged from the APG system of 1998), recognizes this family and assigns it to the order Caryophyllales in the clade core eudicots. It includes ca 30 genera and about 725 species.The 1981 Cronquist system placed the family in a separate order Plumbaginales, which included no other families. The Dahlgren system had segregated some of these plants as family Limoniaceae.

Portulacaceae

The Portulacaceae are a family of flowering plants, comprising 115 species in a single genus Portulaca. Formerly some 20 genera with about 500 species, were placed there, but it is now restricted to encompass only one genus, the other genera being placed elsewhere. The family has been recognised by most taxonomists, and is also known as the purslane family; it has a cosmopolitan distribution, with the highest diversity in semiarid regions of the Southern Hemisphere in Africa, Australia, and South America, but with a few species also extending north into Arctic regions. The family is very similar to the Caryophyllaceae, differing in the calyx, which has only two sepals.

The APG II system (2003; unchanged from the APG system of 1998) assigns it to the order Caryophyllales in the clade core eudicots. In the APG III system, several genera were moved to the Montiaceae, Didiereaceae, Anacampserotaceae and Talinaceae, thus making the family monotypic and only containing the genus Portulaca.

Posidonia

Posidonia is a genus of flowering plants. It contains nine species of marine plants ("seagrass"), found in the seas of the Mediterranean and around the south coast of Australia.

The APG system (1998) and APG II system (2003) accept this genus as constituting the sole genus in the family Posidoniaceae, which it places in the order Alismatales, in the clade monocots. The AP-Website concludes that the three families Cymodoceaceae, Posidoniaceae and Ruppiaceae form a monophyletic group. Earlier systems classified this genus in the family Potamogetonaceae or in the family Posidoniaceae but belonging to order Zosterales.

Rapateaceae

The Rapateaceae are a family of flowering plants. The botanical name has been recognized by most taxonomists.

The APG II system of 2003 also recognizes this family, and assigns it to the order Poales in the clade commelinids, in the monocots. This represents a slight change from the APG system, 1998, which left the family unplaced as to order, but placed it in the same clade (although it used the spelling "commelinoids"). The family is divided into 16 genera with a total of about 94 known species, found in tropical South America and tropical west Africa.

The Cronquist system of 1981 also recognized this family and placed it in the order Commelinales in the subclass Commelinidae in class Liliopsida in division Magnoliophyta.

Rubiales

Rubiales was an order of flowering plants in the Cronquist system, including the families Rubiaceae and Theligonaceae. The latest APG system (2009) does not recognize this order and places the families within Gentianales.

Xyridaceae

The Xyridaceae are a family of flowering plants. The botanical name has been recognized by many taxonomists and is known as the yellow-eyed grass family.

The APG II system, of 2003 (unchanged from the APG system of 1998), also recognizes this family, and assigns it to the order Poales in the clade commelinids, in the monocots. This treatment in APG II represents a slight change from the APG system of 1998, which had recognized the family Abolbodaceae for some of the plants included here; that family was unplaced as to order, but was assigned to this same clade (although APG used the spelling "commelinoids").

The family contains almost 400 species in five genera, but most of the species are found in the genus Xyris (see also Abolboda). The species are mostly tropical and subtropical.

The Cronquist system of 1981 also recognized such a family and placed it in the order Commelinales in the subclass Commelinidae in class Liliopsida in division Magnoliophyta.

The Wettstein system, last updated in 1935, placed the family in order Enantioblastae.

Xyris torta, twisted yellow-eyed grass, is on Minnesota's endangered species list.

Pre-Linnaean
Pre-Darwinian
Post-Darwinian (Phyletic)
Angiosperm Phylogeny Group
System (1998–2009)
See also

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