Dicotyledon

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Image:Young castor bean plant showing prominent cotyledons.jpg Dicotyledons or "dicots" is a name for a group of flowering plants whose seed typically contains two embryonic leaves or cotyledons. Flowering plants that are not dicotyledons are monocotyledons, often having one embryonic leaf only.

It is now accepted, thanks to the Angiosperm Phylogeny Group, that monocotyledons evolved from within the dicotyledons, and as such the latter form a paraphyletic group. This means that dicotyledons no longer are regarded as a "good" group, and the names "dicotyledons" and "dicots" are no longer to be used, at least in a taxonomic sense. The vast majority of the former dicots, however, 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.

Image:Magnolia Watsoni.JPG

Contents

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Morphology, compared to monocotyledons

The schoolbooks list the differences between monocotyledons and dicotyledons as follows. Obviously this is only a broad sketch, not to be taken literally. There are many exceptions. The differences indicated are more true for monocots versus eudicots, as per APG:

Flowers: In monocots, flowers are trimerous (number of flower parts in a whorl in threes) while in dicots the flowers are tetramerous or pentamerous (flower parts are in fours or fives).

Pollen: In monocots, pollen has one furrow or pore while in dicots they have three.

Seeds: In monocots, the embryo has one cotyledon while the embryo of the dicot has two.

Stems: In monocots, vascular bundles in the stem are scattered, in dicots arranged in a ring.

Roots: In monocots, roots are adventitious, while in dicots they develop from the radicle.

Leaves: In monocots, the major leaf veins are parallel, while in dicots they are reticulate.

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Taxonomy

The dicots were recognised as a separate group by John Ray, so they predate Carolus Linnaeus (see also the history of plant systematics. The traditional name for this group has long been Dicotyledones, a descriptive botanical name that may be used at any rank. Early users, such as A.P. de Candolle preferred the spelling Dicotyledoneae. The taxonomy of the plants belonging to this group vary with the system being used:

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Dicotyledons according to de Candolle

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Dicotyledons according to the Bentham & Hooker system

The Bentham & Hooker system was an influential nineteenth century system, which treated the dicots:

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Dicotyledons according to the Wettstein system

An early twenthieth century system, the Wettstein system treated the dicots as:

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Dicotyledons according to Cronquist

In the Cronquist system, it was treated as class Magnoliopsida, a name formed from the family name Magnoliaceae (Art 16 ICBN). Template:Wikispecies

The Cronquist system has been very popular and there have been many versions of the system published. In some of these Cronquist-based systems the dicotyledons are treated at the rank of subclass, named Magnoliidae (as in this example). In that case the name Magnoliopsida (at the rank of class) refers to the flowering plants (the angiosperms).

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Dicotyledons according to Dahlgren

In the Dahlgren system, it was treated as subclass Magnoliidae, a name formed from the family name Magnoliaceae (Art 16 ICBN).

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Dicotyledons according to Thorne (1992)

In the Thorne system (1992), it was treated as subclass Magnoliidae, a name formed from the family name Magnoliaceae (Art 16 ICBN). The internal taxonomy in the Thorne system (in the version of the system as depicted by Reveal):

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According to APG II

As noted above, the APG emphatically reject the the dicots as a natural group, and in effect the plants belonging to the "dicots" are a paraphyletic gathering of all angiosperms except the monocots. The Angiosperm Phylogeny Group does not use ranks above the level of order and uses informal names for clades. The main clades in the APG II system:

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Number of species

There is no close agreement on the number of dicot species. One estimate assumes 199,350 species within this group [1].bg:Двусемеделни cs:Nižší dvouděložné cy:Deugotyledon da:Tokimbladede de:Einfurchenpollen-Zweikeimblättrige eo:Dukotiledonaj plantoj fr:Dicotylédone ko:쌍떡잎식물 he:דו־פסיגיים lt:Dviskilčiai lv:Divdīgļlapji nl:Tweezaadlobbigen ja:双子葉植物 pl:Dwuliścienne pt:Dicotiledónea ru:Двудольные sr:Дикотиледон fi:Kaksisirkkaiset sv:Tvåhjärtbladiga tr:İkiçenekliler uk:Дводольні vi:Thực vật hai lá mầm zh:双子叶植物

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