Dicotyledons or dicots are flowering plants whose seed contains two embryonic leaves or cotyledons. Flowering plants or angiosperms that are not dicotyledonous are monocotyledonous: having one embryonic leaf.
It is believed that monocots evolved from dicots, and as such the latter form a paraphyletic group, i.e. they include some forms that are as closely related to monocots as they are to the other dicots. The vast majority, 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. Basal dicots and monocots have monosulcate pollen, or forms derived from it, whereas eudicots have tricolpate pollen, or derived forms.
Traditionally the dicots have been treated as a class, originally called the Dicotyledoneae, but more recently called class Magnoliopsida after the type genus Magnolia. In some schemes, the eudicots are treated as a separate class Rosopsida (type genus Rosa), or as several separate classes. The remaining dicots may be kept in a single paraphyletic class Magnoliopsida, or further divided. The classification of dicots has undergone considerable revision as our understanding of their relationships has changed, and is still not entirely settled, though a general consensus is emerging.
The following lists are of the orders typical of new classification systems and those under the older Cronquist system, which is still in wide use.