Monocotyledons or monocots are one of two major groups of flowering plants (angiosperms) that are traditionally recognized, dicotyledons or dicots being the other. Monocots have been recognized at various taxonomic ranks, and under various names (see below). The APG II system recognizes a clade called "monocots" but does not assign it to a taxonomic rank.
Monocots comprise the majority of agricultural plants in terms of biomass produced. There are between 50,000 and 60,000 species within this group; according to IUCN there are 59,300 species . The largest family in this group (and in the flowering plants as a whole) by number of species are the orchids (family Orchidaceae), with about twenty thousand species. The economically most important family in this group (and in the flowering plants) are the grasses, family Poaceae (Gramineae). These include all the true grains (rice, wheat, maize, etc.), the pasture grasses and the bamboos. This family of the true grasses have evolved in another direction, becoming highly specialized for wind pollination. Grasses produce much smaller flowers, which are gathered in highly visible plumes (inflorescences). Other economically important monocot families are the palm family (Arecaceae), banana family (Musaceae), ginger family (Zingiberaceae) and the Alliaceae, which includes such ubiquitously used vegetables as onions and garlic.
The name Monocotyledoneae is derived from the fact that most members of this group have one cotyledon, or embryonic leaf, in their seeds. This as opposed to the traditional Dicotyledones, which typically have two cotyledons. From a diagnostic point of view the number of cotyledons is neither a particularly handy (as they are only present for a very short period in a plant's life) nor totally reliable character.
Nevertheless, monocots are a distinctive group (Chase, 2004). One of the most noticeable traits is that a monocot's flower is trimerous, with the flower parts in threes or in multiples of three. For example, a monocotyledon's flower typically has three, six, or nine petals. Many monocots also have leaves with parallel veins.
Morphology compared to the (former) dicotyledons
The traditionally listed differences between monocotyledons and dicotyledons are as follows. This is a broad sketch only, not invariably applicable, as there are a number of exceptions. The differences indicated are more true for monocots versus eudicots, as per the APG II system:
Seeds: In monocots, the embryo has one cotyledon while the embryo of the dicot has two.
Stems: In monocots, vascular bundles in the stem are scattered, in dicots arranged in a ring.
Leaves: In monocots, the major leaf veins are parallel, while in dicots they are reticulate.
However, these differences are not hard and fast: some monocots have characteristics more typical of dicots, and vice-versa. This is in part because "dicots" are a paraphyletic group with respect to monocots, and some dicots may be more closely related to monocots than to other dicots. In particular, several early-branching lineages of "dicots" share "monocot" characteristics, suggesting that these are not defining characters of monocots. When monocots are compared to eudicots, the differences are more concrete.
The monocots are considered to form a monophyletic group arising early in the history of the flowering plants. The earliest fossils presumed to be monocot remains date from the early Cretaceous period.
Taxonomists have considerable latitude in naming this group, as the monocots are a group above the rank of family. Article 16 of the ICBN allows either a descriptive name or a name formed from the name of an included family.
Historically, the monocotyledons were named:
- Monocotyledoneae in the de Candolle system and the Engler system.
- Monocotyledones in the Bentham & Hooker system and the Wettstein system
- class Liliopsida in the Takhtajan system and the Cronquist system.
- subclass Liliidae in the Dahlgren system and the Thorne system (1992).
- clade monocots in the APG system and the APG II system.
Each of the systems mentioned above use their own internal taxonomy for the group. The monocotyledons are famous as a group that is extremely stable in its outer borders (it is a well-defined, coherent group), while in its internal taxonomy is extremely unstable (historically no two authoritative systems have agreed with each other on how the monocotyledons are related to each other).
Recent molecular studies have both confirmed the monophyly of the monocots and helped elucidate relationships within this group. The APG II system does not assign the monocots to a taxonomic rank, instead recognizing a monocots clade. This system recognizes ten orders of monocots and two families of monocots not yet assigned to any order:
- clade monocots :
- family Petrosaviaceae
- clade commelinids:
The family Hydatellaceae, assigned to order Poales in the APG II system, has since been recognized as being misplaced in the monocots, and instead proves to be most closely related to the water lilies, family Nymphaeaceae (Saarela et al., 2007).
- Tree of Life Web Project: Monocotyledons
- Numbers of threatened species by major groups of organisms (1996–2004)
Chase, M. W. 2004. Monocot relationships: an overview. American Journal of Botany 91: 1645–1655.
Chase, M. W., D. E. Soltis, P. S. Soltis, P. J. Rudall, M. F. Fay, W. J. Hahn, S. Sullivan, J. Joseph, M. Molvray, P. J. Kores, T. J. Givnish, K. J. Sytsma & J. C. Pires. 2000. Higher-level systematics of the monocotyledons: An assessment of current knowledge and a new classification. In Monocots: Systematics and Evolution (K. L. Wilson & D. A. Morrison, eds.) pp. 3-16. CSIRO: Melbourne.
Davis, J. I., D. W. Stevenson, G. Petersen, O. Seberg, L. M. Campbell, J. V. Freudenstein, D. H. Goldman, C. R. Hardy, F. A. Michelangeli, M. P. Simmons, C. D. Specht, F. Vergara-Silva & M. Gandolfo. 2004. A phylogeny of the monocots, as inferred from rbcL and atpA sequence variation, and a comparison of methods for calculating jackknife and bootstrap values. Systematic Botany 29 (3): 467-510.
Saarela, J. M., H. S. Rai, J. A. Doyle, P. K. Endress, S. Mathews, A. D. Marchant, B. G. Briggs & S. W. Graham. 2007. Hydatellaceae identified as a new branch near the base of the angiosperm phylogenetic tree. Nature 446: 312-315.