Dicotyledon plantletYoungcastor oil plant showing its prominent two embryonic leaves (cotyledons), which differ from the adult leaves
Thedicotyledons, also known asdicots (or, more rarely,dicotyls),[2] are one of the two groups into which all theflowering plants (angiosperms) were formerly divided. The name refers to one of the typical characteristics of the group: namely, that theseed has two embryonic leaves orcotyledons. There are around 200,000 species within this group.[3] The other group of flowering plants were calledmonocotyledons (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 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 amonophyletic group). Rather, a number of lineages, such as themagnoliids and groups now collectively known as thebasal angiosperms, diverged earlier than the monocots did; in other words, monocots evolved from within the dicots, as traditionally defined. The traditional dicots are thus aparaphyletic group.[4]
Theeudicots are the largest monophyletic group within the dicotyledons. They are distinguished from all other flowering plants by the structure of theirpollen. Other dicotyledons and the monocotyledons havemonosulcate pollen (or derived forms): grains with a single sulcus. Contrastingly, eudicots havetricolpate pollen (or derived forms): grains with three or more pores set in furrows called colpi.
Aside fromcotyledon number, other broad differences have been noted betweenmonocots and dicots, although these have proven to be differences primarily between monocots andeudicots. Many early-diverging dicot groups have monocot characteristics such as scatteredvascular bundles, trimerous flowers, and non-tricolpatepollen.[5] In addition, some monocots have dicot characteristics such asreticulated leaf veins.[5]
Traditionally, the dicots have been called the Dicotyledones (orDicotyledoneae), at any rank. If treated as a class, as they are within theCronquist system, they could be called the Magnoliopsida after thetype genusMagnolia. In some schemes, the eudicots were either treated as a separateclass, theRosopsida (type genusRosa), or as several separate classes. The remaining dicots (palaeodicots or basal angiosperms) may be kept in a singleparaphyletic class, calledMagnoliopsida, or further divided. Some botanists prefer to retain the dicotyledons as a valid class, arguing its practicality and that it makes evolutionary sense.[9]
The following lists show the orders in the Angiosperm Phylogeny GroupAPG IV system traditionally called dicots,[8] together with the olderCronquist system.
Under the Dahlgren and Thorne systems, thesubclass nameMagnoliidae was used for the dicotyledons. This is also the case in some of the systems derived from the Cronquist system.[citation needed] 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
There exist variances between the superorderscircumscribed 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'sTheanae corresponds to five distinct superorders under Dahlgren's system, only one of which is calledTheanae.
.JPG)
