Dicotyledon

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Dicotyledon
Magnolia flower
Scientific classification
Kingdom:Plantae
Division:Magnoliophyta
Class:Magnoliopsida*
Brongniart
Included groups
Excluded groups
 
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Dicotyledon
Magnolia flower
Scientific classification
Kingdom:Plantae
Division:Magnoliophyta
Class:Magnoliopsida*
Brongniart
Included groups
Excluded groups
Young castor oil plant showing its prominent two embryonic leaves (cotyledons), that differ from the adult leaves.

The dicotyledons, also known as dicots, was a grouping formerly used for the flowering plants whose seed typically has two embryonic leaves or cotyledons. There are around 199,350 species within this group.[1] Flowering plants that were not dicotyledons were called monocotyledons, typically having one embryonic leaf.

Dicotyledons are not a monophyletic group, and therefore the names "dicotyledons" and "dicots" are paraphyletic terms. However, the vast majority of "dicots" do form a monophyletic group called the eudicots or tricolpates. These may be 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.

Traditionally the dicots have been called the Dicotyledones (or Dicotyledoneae), at any rank. If treated as a class, as in the Cronquist system, they may be called the Magnoliopsida after the type genus Magnolia. In some schemes, the eudicots are treated as a separate class, the Rosopsida (type genus Rosa), or as several separate classes. The remaining dicots (palaeodicots) may be kept in a single paraphyletic class, called Magnoliopsida, or further divided.

Compared to Monocotyledons[edit]

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.[2] In addition, some monocots have dicot characteristics such as reticulated leaf veins.[2]

FeatureIn monocotsIn dicots
Number of parts of each flowerIn threes (flowers are trimerous)In fours or fives (tetramerous or pentamerous)
Number of furrows or pores in pollenOneThree
Number of cotyledons (leaves in the seed)OneTwo
Arrangement of vascular bundles in the stemScatteredIn concentric circles
RootsAre adventitiousDevelop from the radicle
Arrangement of major leaf veinsParallelReticulate
Secondary growthAbsentOften present

Classification[edit]

APG (Angiosperm Phylogeny Group) vs. Cronquist Classification[edit]

The following lists are of the orders formerly placed in the dicots, giving their new placement in the APG-system and that under the older Cronquist system, which is still in wide use.

APG II
(polyphyletic)
Cronquist system
(classis Magnoliopsida)
Magnoliids
eudicots
Magnoliidae (mostly basal dicots)
Hamamelidae
Caryophyllidae
Dilleniidae
Rosidae
Asteridae
Note: "+ ..." = optionally separate family, that may be split off from the preceding family.


Dahlgren and Thorne systems[edit]

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:

Comparison of the Dahlgren and Thorne systems
Dahlgren systemThorne system
Magnolianae

Ranunculanae

Magnolianae

Rafflesianae

NymphaeanaeNymphaeanae
CaryophyllanaeCaryophyllanae
Theanae

Plumbaginanae
Polygonanae
Primulanae
Ericanae

Theanae
MalvanaeMalvanae
ViolanaeViolanae
RosanaeRosanae
ProteanaeProteanae
MyrtanaeMyrtanae
RutanaeRutanae

Celastranae
Geranianae

SantalanaeSantalanae
BalanophoranaeSantalanae
AsteranaeAsteranae
SolananaeSolananae
Cornanae

Vitanae

Cornanae

Aralianae

LoasanaeLoasanae
DicotyledonsGentiananae

Lamianae

Gentiananae


See also[edit]

References[edit]

  1. ^ Hamilton, Alan; Hamilton, Patrick (2006), Plant conservation : an ecosystem approach, London: Earthscan, p. 2, ISBN 978-1-84407-083-1 
  2. ^ a b "Monocots versus Dicots". University of California Museum of Paleontology. Retrieved 25 January 2012.