Dicotyledon

Flowering plants, or angiosperms, were once split into two main groups: the dicotyledons (dicots) and the monocotyledons (monocots). The name "dicot" comes from the fact that their seeds typically contain two embryonic leaves, called cotyledons. Monocots, by contrast, usually have just one. For a long time, these two categories were considered the primary divisions of all flowering plants.

However, starting around the 1990s, studies of plant DNA (molecular phylogenetics) confirmed what some botanists had already suspected: dicots are not a single, unified group descended from one common ancestor (a monophyletic group). Instead, several lineages—including the magnoliids and other so-called basal angiosperms—split off before the monocots did. In other words, monocots actually evolved from within the traditional dicot group. This makes the traditional dicots a paraphyletic group (one that includes a common ancestor but not all of its descendants).

The largest true (monophyletic) group within the dicots is the eudicots. What sets eudicots apart from other flowering plants is their pollen structure: while most other dicots and monocots have pollen with a single furrow (monosulcate), eudicots have pollen with three or more furrows (tricolpate).

When comparing monocots and dicots, many of the classic differences—like scattered vascular bundles, three-parted flowers, and non-tricolpate pollen—actually separate monocots from eudicots, not from all dicots. Many early-diverging dicot groups share these monocot-like traits, and some monocots have dicot-like features, such as netted leaf veins.

In older classification systems, dicots were called Dicotyledones or Dicotyledoneae. In the Cronquist system, they were treated as the class Magnoliopsida (named after the genus *Magnolia*). Some schemes split the eudicots into a separate class, Rosopsida, while leaving the remaining dicots (sometimes called palaeodicots or basal angiosperms) in Magnoliopsida. A few botanists still argue for keeping dicots as a valid class, citing practical and evolutionary reasons.

The Angiosperm Phylogeny Group (APG) system, now in its fourth version (APG IV), provides a modern phylogenetic tree showing that the traditional dicots are paraphyletic to the monocots. Other systems, like those of Dahlgren and Thorne, used the subclass name Magnoliidae for dicots. Although these systems share many superorder names, the specific orders grouped under each superorder often differ. For instance, Thorne's Theanae corresponds to five separate superorders in Dahlgren's system, only one of which is also called Theanae.

For further reading, see the related term Calyciflorae. External links include a world list of dicot species (Magnoliopsida) from the Catalogue of Life, with over 253,000 species, and a tree browser for dicot orders, families, and genera with species counts.