Fossil range: Early Cretaceous - Recent
|Hemerocallis flower, with three flower parts in each whorl|
About 10; see text
Monocotyledons or monocots are one of two major groups of flowering plants (angiosperms) that are traditionally recognized, the other being dicotyledons or dicots. Monocot seedlings typically have one cotyledon (seed-leaf), in contrast to the two cotyledons typical of dicots. Monocots have been recognized at various taxonomic ranks, and under various names (see below). The APG II system recognises a clade called "monocots" but does not assign it to a taxonomic rank.
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 20,000 species. In agriculture the majority of the biomass produced comes from monocots. The economically most important family in this group (and in all plant families) are the true grasses, family Poaceae (Gramineae). These include all the true grains (rice, wheat, maize, etc.), the pasture grasses, sugar cane, and the bamboos. True grasses have evolved to become highly specialised 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 onion family Alliaceae, which includes such ubiquitously used vegetables as onions and garlic.
The name monocotyledons is derived from the traditional botanical name Monocotyledones, which derives from the fact that most members of this group have one cotyledon, or embryonic leaf, in their seeds. By contrast, the traditional dicotyledons 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. 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. That is to say, a monocotyledon's flower typically has three, six, or nine petals. Many monocots also have leaves with parallel veins.
Monocots evolved from a single ancestor, and are younger than dicots, from which they probably branched off, as recent genetic research has shown. They evolved 100-120 million years ago, shortly after the flowering plant explosion.
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.
|Feature||In monocots||In dicots|
|Number of parts of each flower||in threes (flowers are trimerous)||in fours or fives (tetramerous or pentamerous)|
|Number of furrows or pores in pollen||one||three|
|Number of cotyledons (leaves in the seed)||one||two|
|Arrangement of vascular bundles in the stem||scattered||in concentric circles|
|Roots||are adventitious||develop from the radicle|
|Arrangement of major leaf veins||parallel||reticulate|
A number of these differences are not unique to the monocots. For example, trimerous flowers and monosulcate pollen are also found in magnoliids. Exclusively adventitious roots are found also in Nymphaeaceae and some of the Piperaceae. Similarly, at least one of these traits, parallel leaf veins, is far from universal among the monocots. Monocots with reticula te leaf veins are found in a wide variety of monocot families: for example, Trillium, Smilax (greenbriar), and Pogonia (an orchid), and the Dioscoreales. Nevertheless, this list of traits is a generally valid set of contrasts, especially when contrasting monocots with eudicots rather than non-monocot flowering plants in general.
Some monocots, such as grasses, have hypogeal emergence, where the mesocotyl elongates and pushes the coleoptile (which encloses and protects the shoot tip) toward the soil surface. Since elongation occurs above the cotyledon, it is left in place in the soil where it was planted. Many dicots have epigeal emergence, the hypocotyl elongates and becomes arched in the soil. As the hypocotyl continues to elongate, it pulls the cotyledons upward, above the soil surface.
Monocots have a distinctive arrangement of vascular tissue known as an atactostele in which the vascular tissue is scattered rather than arranged in concentric rings. Many monocots are herbaceous and do not have the ability to increase the width of a stem (secondary growth) via the same kind of vascular cambium found in non-monocot woody plants. However, some monocots do have secondary growth, and because it does not arise from a single vascular cambium producing xylem inwards and phloem outwards, it is termed "anomalous secondary growth". Examples of large monocots which either exhibit secondary growth, or can reach large sizes without it, are palms (Arecaceae), screwpines (Pandanaceae), bananas (Musaceae), Yucca, Aloe, Dracaena, and Cordyline.
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:
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 (Petrosaviaceae and Dasypogonaceae) not yet assigned to any order. More recently, the Petrosaviaceae has been included in the Petrosaviales, and placed near the lilioid orders.
|clade monocots :||
|The current phylogeny and composition of the monocots.|
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.
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Classification System: APG II (down to family level)
Cladus: Familia: Petrosaviaceae; Ordines: Acorales - Alismatales - Asparagales - Dioscoreales - Liliales - Pandanales; Cladus: Commelinids
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