Dicotyledon
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.
Phyletic Groups
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 a monophyletic 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 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. 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 between monocotyledons and dicotyledons
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.
| Feature | In monocots | In dicots |
|---|---|---|
| Flowers | Parts in threes (trimerous) or multiples of three (e.g. 3, 6 or 9 petals) | Fours (tetramerous) or fives (pentamerous) |
| Number of furrows or pores in pollen | Monocolpate (single aperture or colpus) | Tricolpate (three) |
| Number of cotyledons (leaves in the seed) | One, endosperm frequently present in seed | Two, endosperm present or absent |
| Plant stem: Vascular bundles | Numerous scattered bundles in ground parenchyma, cambium rarely present, no differentiation between cortical and stelar regions | Ring of primary bundles with cambium, differentiated into cortex and stele (eustelic) |
| Growth form | Mostly herbaceous, occasionally arboraceous | Herbaceous or arboraceous |
| Leaves | Leaf shape oblong or linear, often sheathed at base, petiole seldom developed, stipules absent. | Broad, seldom sheathed, petiole common often with stipules. |
| Arrangement of major leaf veins | Parallel | Reticulate (pinnate or palmate) |
| Arrangement of vascular bundles in the stem | Scattered | In concentric circles |
| Roots | Primary root of short duration, replaced by adventitial roots forming fibrous or fleshy root systems | Develops from the radicle. Primary root often persists forming strong taproot and secondary roots |
| Secondary growth | Absent | Often present |
| Stomata | Present on both the upper and lower epidermis of leaves | More common on the lower epidermis of leaves |