The dicotyledons, or dicots, are one of the two traditional groups of flowering plants, characterized by seeds with two cotyledons. Comprising around 200,000 species, they were historically contrasted with monocots, which have one cotyledon. However, molecular phylogenetic studies have shown that dicots are a paraphyletic group, as some lineages like magnoliids and basal angiosperms diverged earlier than monocots. Within dicots, the eudicots form the largest monophyletic subgroup, distinguished by their unique tricolpate pollen, differing from the monosulcate pollen found in other dicots and monocots.
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.4 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 (Many orchids are acotyledonous, with none). | Two (Some exceptions, including Psittacanthus schiedeanus with twelve.6) |
| 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 |
| Stomata | Present on both the upper and lower epidermis of leaves | More common on the lower epidermis of leaves |
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:78
| angiosperms |
| traditional dicots | |||||||||||||||||||||||||||||||||||||||
Historical
Traditionally, the dicots have been called the Dicotyledones (or Dicotyledoneae), at any rank. If treated as a class, as they are within the Cronquist system, they could be called the Magnoliopsida after the type genus Magnolia. In some schemes, the eudicots were either 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.9
APG vs. Cronquist
The following lists show the orders in the Angiosperm Phylogeny Group APG IV system traditionally called dicots,10 together with the older Cronquist system.
Dahlgren and Thorne systems
Under the Dahlgren and 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. These two systems are contrasted in the table below in terms of how each categorises by superorder; note that the sequence within each system has been altered in order to pair corresponding taxa
The Thorne system (1992) as depicted by Reveal is:
There exist variances between the superorders circumscribed from each system. Namely, although the systems share common names for many of the listed superorders, the specific list orders classified within each varies. For example, Thorne's Theanae corresponds to five distinct superorders under Dahlgren's system, only one of which is called Theanae.
See also
Wikispecies has information related to Magnoliopsida.External links
Wikispecies has information related to Magnoliopsida.- World list of dicot species (a.k.a. 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
References
"Dicotyl", The Free Dictionary, retrieved 2 January 2016 http://www.thefreedictionary.com/dicotyl ↩
Hamilton, Alan; Hamilton, Patrick (2006), Plant conservation: An ecosystem approach, London: Earthscan, p. 2, ISBN 978-1-84407-083-1 978-1-84407-083-1 ↩
Simpson, Michael G. (2011), "Chapter 7: Diversity and Classification of Flowering Plants", Plant Systematics, Elsevier, p. 139, ISBN 978-0-0805-1404-8 978-0-0805-1404-8 ↩
Monocots versus Dicots, University of California Museum of Paleontology, retrieved 25 January 2012 http://www.ucmp.berkeley.edu/glossary/gloss8/monocotdicot.html ↩
Monocots versus Dicots, University of California Museum of Paleontology, retrieved 25 January 2012 http://www.ucmp.berkeley.edu/glossary/gloss8/monocotdicot.html ↩
Kuijt, Job (1969). The Biology of Parasitic Plants. Berkeley, Calif.: University of California Press. p. 39. ↩
Cole, Theodor C.H.; Hilger, Hartmut H. & Stevens, Peter F. (2017), Angiosperm Phylogeny Poster - Flowering Plant Systematics (PDF), archived from the original (PDF) on 2017-05-17, retrieved 2017-07-13 https://web.archive.org/web/20170517030259/http://www2.biologie.fu-berlin.de/sysbot/poster/poster1.pdf ↩
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 /wiki/Botanical_Journal_of_the_Linnean_Society ↩
Stuessy, Tod F. (2010), "Paraphyly and the origin and classification of angiosperms." (PDF), Taxon, 59 (3): 689–693, doi:10.1002/tax.593001 http://www.ktriop.bio.ug.edu.pl/upload/preview/d866ba1ba9b0c1ed4b27a44373a5e40b.pdf ↩
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 /wiki/Botanical_Journal_of_the_Linnean_Society ↩