Dicotyledon

Young castor oil plant showing its prominent two embryonic leaves (cotyledons), that differ from the adult leaves.

Dicotyledons, also known as dicots, is a name for a group of flowering plants whose seed typically has two embryonic leaves or cotyledons. There are around 199,350 species within this group.^[1] Flowering plants that are not dicotyledons are monocotyledons, typically having one embryonic leaf.

Dicotyledons are not a monophyletic group, and therefore the names "dicotyledons" and "dicots" are, strictly speaking, deprecated. However, the vast majority of "dicots", do 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.

Traditionally the dicots have been called the Dicotyledones (or Dicotyledoneae), at any rank. If treated as a class, as in the Cronquist system, they may be called the Magnoliopsida after the type genus Magnolia. In some schemes, the eudicots are treated as a separate class, the Rosopsida (type genus Rosa), or as several separate classes. The remaining dicots (palaeodicots) may be kept in a single paraphyletic class, called Magnoliopsida, or further divided.

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

Seeds: The embryo of the monocot has one cotyledon or seed leaf, while the embryo of the dicot has two.

Flowers: The flower parts in monocots are multiples of three, while in dicots are multiples of four or five.

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

Secondary growth: In monocots, stems rarely show secondary growth; in dicots, stems frequently have secondary growth.

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

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

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

Classification

APG (Angiosperm Phylogeny Group) vs. Cronquist Classification

The following lists are of the orders formerly placed in the dicots, giving their new placement in the APG-system and that under the older Cronquist system, which is still in wide use.

Dahlgren and Thorne systems

In the Dahlgrenff and the Thorne systems the name Magnoliidae is used for the dicotyledons. This is also the case in some of the systems derived from the Cronquist system. For each system, only the superorders are listed. The sequence of each system has been altered to pair corresponding taxa, although circumscription of superorders with the same name is not always the same. The Thorne system (1992) is the version of the system as depicted by Reveal.

References