Young castor bean plant showing prominent cotyledons

The dicotyledons, also known as dicots (or, more rarely, dicotyls), are one of the two groups into which all the flowering plants (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. The other group of flowering plants were called monocotyledons (or monocots), typically each having one cotyledon. Historically, these two groups formed the two divisions of the flowering plants. Largely from the 1990s onwards,

molecular phylogenetic Molecular phylogenetics () is the branch of phylogeny that analyzes genetic, hereditary molecular differences, predominantly in DNA sequences, to gain information on an organism's evolutionary relationships. From these analyses, it is possible to ...

research confirmed what had already been suspected: that dicotyledons are not a group made up of all the descendants of a common ancestor (i.e., they are not a

monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gro ...

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 In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...

group. The eudicots are the largest monophyletic group within the dicotyledons. They are distinguished from all other flowering plants by the structure of their

pollen Pollen is a powdery substance produced by seed plants. It consists of pollen grains (highly reduced microgametophytes), which produce male gametes (sperm cells). Pollen grains have a hard coat made of sporopollenin that protects the gametophyt ...

. Other dicotyledons and the monocotyledons have monosulcate pollen (or derived forms): grains with a single sulcus. Contrastingly, eudicots have tricolpate pollen (or derived forms): grains with 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

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

Classification

Phylogeny

The consensus

phylogenetic tree A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree showing the evolutionary relationships among various biological spec ...

used in the APG IV system shows that the group traditionally treated as the dicots is

to the monocots:

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 The basal angiosperms are the flowering plants which diverged from the lineage leading to most flowering plants. In particular, the most basal (phylogenetics), basal angiosperms were called the ANITA grade, which is made up of ''Amborella'' (a sin ...

or basal angiosperms) may be kept in a single

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.

APG vs. Cronquist

The following lists show the orders in the Angiosperm Phylogeny Group APG IV system traditionally called dicots, 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 In geometry, the circumscribed circle or circumcircle of a polygon is a circle that passes through all the vertices of the polygon. The center of this circle is called the circumcenter and its radius is called the circumradius. Not every po ...

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.''

References

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
{{Authority control Historically recognized angiosperm taxa