Dicotyledon

The dicotyledons, also known as dicots (or more rarely dicotyls[2]), are one of the two groups into which all the flowering plants or angiosperms were formerly divided. The name refers to one of the typical characteristics of the group, namely that the seed has two embryonic leaves or cotyledons. There are around 200,000 species within this group.[3] The other group of flowering plants were called monocotyledons or monocots, typically having one cotyledon. Historically, these two groups formed the two divisions of the flowering plants.

Dicotyledon
Lamium album (white dead nettle)
Scientific classification
Kingdom:	Plantae
Clade:	Tracheophytes
Clade:	Spermatophytes
Clade:	Angiosperms
Groups included
Eudicots Magnoliids Amborella Nymphaeales Austrobaileyales Chloranthales Ceratophyllum
Cladistically included but traditionally excluded taxa
Monocots
Synonyms
Dicotyledoneae Magnoliatae Takht.[1]

dicotyledon plant-let

Young castor oil plant showing its prominent two embryonic leaves (cotyledons), that differ from the adult leaves.

Largely from the 1990s onwards, molecular phylogenetic research confirmed what had already been suspected, namely that dicotyledons are not a group made up of all the descendants of a common ancestor (i.e. they are not a monophyletic group). Rather, a number of lineages, such as the magnoliids and groups now collectively known as the basal angiosperms, diverged earlier than the monocots did; in other words monocots evolved from within the dicots as traditionally defined. The traditional dicots are thus a paraphyletic group.[4] The eudicots are the largest clade within the dicotyledons. They are distinguished from all other flowering plants by the structure of their pollen. Other dicotyledons and monocotyledons have monosulcate pollen, or forms derived from it, whereas eudicots have tricolpate pollen, or derived forms, the pollen having three or more pores set in furrows called colpi.

Comparison with monocotyledons

Aside from cotyledon number, other broad differences have been noted between monocots and dicots, although these have proven to be differences primarily between monocots and eudicots. Many early-diverging dicot groups have "monocot" characteristics such as scattered vascular bundles, trimerous flowers, and non-tricolpate pollen.[5] In addition, some monocots have dicot characteristics such as reticulated leaf veins.[5]

Feature	In monocots	In dicots
Number of parts of each flower	In threes (flowers are trimerous)	In fours or fives (tetramerous or pentamerous)
Number of furrows or pores in pollen	One	Three
Number of cotyledons (leaves in the seed)	One	Two
Arrangement of vascular bundles in the stem	Scattered	In concentric circles
Roots	Are adventitious	Develop from the radicle
Arrangement of major leaf veins	Parallel	Reticulate
Secondary growth	Absent	Often present

Comparison of monocots and dicots

Classification

Phylogeny

The consensus phylogenetic tree used in the APG IV system shows that the group traditionally treated as the dicots is paraphyletic to the monocots:[6][7]

angiosperms	Amborellales       Nymphaeales       Austrobaileyales   core angiosperms   Chloranthales     magnoliids         Ceratophyllales      eudicots       monocots	traditional dicots

Historical

Traditionally the dicots have been called the Dicotyledones (or Dicotyledoneae), at any rank. If treated as a class, as in the Cronquist system, they could be called the Magnoliopsida after the type genus Magnolia. In some schemes, the eudicots were treated as a separate class, the Rosopsida (type genus Rosa), or as several separate classes. The remaining dicots (palaeodicots or basal angiosperms) may be kept in a single paraphyletic class, called Magnoliopsida, or further divided. Some botanists prefer to retain the dicotyledons as a valid class, arguing its practicality and that it makes evolutionary sense.[8]

APG vs. Cronquist

The following lists show the orders in the Angiosperm Phylogeny Group APG IV system traditionally called dicots,[7] together with the older Cronquist system.

APG IV (paraphyletic)

Cronquist system (classis Magnoliopsida)

Amborellales Austrobaileyales Nymphaeales magnoliids Canellales Laurales Magnoliales Piperales unplaced independent lineage Chloranthales probable sister of eudicots Ceratophyllales eudicots Buxales Proteales Ranunculales Trochodendrales core eudicots Dilleniales Gunnerales superrosids Brassicales Celastrales Crossosomatales Cucurbitales Fabales Fagales Geraniales Huerteales Malpighiales Malvales Myrtales Oxalidales Picramniales Rosales Sapindales Saxifragales Vitales Zygophyllales superasterids Apiales Aquifoliales Asterales Berberidopsidales Boraginales Bruniales Caryophyllales Cornales Dipsacales Ericales Escalloniales Garryales Gentianales Icacinales Lamiales Metteniusales Paracryphiales Santalales Solanales Vahliales

Magnoliidae (mostly basal dicots) Magnoliales Laurales Piperales Aristolochiales Illiciales Nymphaeales Ranunculales Papaverales Hamamelidae Trochodendrales Hamamelidales Daphniphyllales Didymelales Eucommiales Urticales Leitneriales Juglandales Myricales Fagales Casuarinales Caryophyllidae Caryophyllales Polygonales Plumbaginales Dilleniidae Dilleniales Theales Malvales Lecythidales Nepenthales Violales Salicales Capparales Batales Ericales Diapensiales Ebenales Primulales Rosidae Rosales Fabales Proteales Podostemales Haloragales Myrtales Rhizophorales Cornales Santalales Rafflesiales Celastrales Euphorbiales Rhamnales Linales Polygalales Sapindales Geraniales Apiales Asteridae Gentianales Solanales Lamiales Callitrichales Plantaginales Scrophulariales Campanulales Rubiales Dipsacales Calycerales Asterales

Dahlgren and Thorne systems

In the Dahlgren and the Thorne systems, the subclass name Magnoliidae was used for the dicotyledons. This is also the case in some of the systems derived from the Cronquist system. For each system, only the superorders are listed. The sequence of each system has been altered to pair corresponding taxa, although circumscription of superorders with the same name is not always the same.

The Thorne system (1992) as depicted by Reveal is:

Dahlgren system	Thorne system
Magnolianae Ranunculanae	Magnolianae Rafflesianae
Nymphaeanae	Nymphaeanae
Caryophyllanae	Caryophyllanae
Theanae Plumbaginanae Polygonanae Primulanae Ericanae	Theanae
Malvanae	Malvanae
Violanae	Violanae
Rosanae	Rosanae
Proteanae	Proteanae
Myrtanae	Myrtanae
Rutanae	Rutanae Celastranae Geranianae
Santalanae	Santalanae
Balanophoranae	Santalanae
Asteranae	Asteranae
Solananae	Solananae
Cornanae Vitanae	Cornanae Aralianae
Loasanae	Loasanae
Gentiananae Lamianae	Gentiananae

See also

• Calyciflorae

References

1. Takhtajan, A. (June 1964), "The Taxa of the Higher Plants above the Rank of Order", Taxon, 13 (5): 160–164, doi:10.2307/1216134, JSTOR 1216134CS1 maint: ref=harv (link)
2. "Dicotyl", The Free Dictionary, retrieved 2 January 2016
3. Hamilton, Alan; Hamilton, Patrick (2006), Plant conservation: An ecosystem approach, London: Earthscan, p. 2, ISBN 978-1-84407-083-1
4. Simpson, Michael G. (2011), "Chapter 7: Diversity and Classification of Flowering Plants", Plant Systematics, Elsevier, p. 139, ISBN 978-0-0805-1404-8
5. Monocots versus Dicots, University of California Museum of Paleontology, retrieved 25 January 2012
6. Cole, Theodor C.H.; Hilger, Hartmut H. & Stevens, Peter F. (2017), Angiosperm Phylogeny Poster - Flowering Plant Systematics (PDF), retrieved 2017-07-13
7. Angiosperm Phylogeny Group (2016), "An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG IV", Botanical Journal of the Linnean Society, 181 (1): 1–20, doi:10.1111/boj.12385
8. Stuessy, Tod F. (2010), "Paraphyly and the origin and classification of angiosperms." (PDF), Taxon, 59 (3): 689–693, doi:10.1002/tax.593001

External links

• World list of dicot species (aka Magnoliopsida) from the Catalogue of Life, 253,406 species.
• Tree browser for dicot orders, families and genera with species counts and estimates via the Catalogue of Life