Monocotyledons, also known as monocots, are one of two major groups of flowering plants (or 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.
According to the IUCN there are 59,300 species of monocots. The largest family in this group (and in the flowering plants as a whole) by number of species are the orchids (family Orchidaceae), with more than 20,000 species. In agriculture the majority of the biomass produced comes from monocots. The true grasses, family Poaceae (Gramineae), are the most economically important family in this group. 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.
Morphology, compared to the (broadly defined) dicotyledonsEdit
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|
The vast majority of Monocots lack a petiole in their leaves.
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 reticulate 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.
- Main article: Cotyledon#Epigeal versus hypogeal development
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, in which 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:
- 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), and
- clade monocots in the APG system and the APG II system.
Each of these systems uses its 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. 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.
For a very long time, fossils of palm trees were believed to be the oldest monocots, first appearing 90 million years ago, but this estimate may not be entirely true. At least some putative monocot fossils have been found in strata as old as the eudicots. The oldest fossils that are unequivocally monocots are pollen from the Late Barremian-Aptian - Early Cretaceous period, about 120-110 million years ago, and are assignable to clade-Pothoideae-Monstereae Araceae; being Araceae, sister to other Alismatales. They have also found flower fossils of Triuridaceae (Pandanales) in Upper Cretaceous rocks in New Jersey, becoming the oldest known sighting of saprophytic /mycotrophic habits in angiosperm plants and among the oldest known fossils of monocotyledons.
Topology of the angiosperm phylogenetic tree could infer that the monocots would be among the oldest lineages of angiosperms, which would support the theory that they are just as old as the eudicots. The pollen of the eudicots dates back 125 million years, so the lineage of monocots should be that old too.
The molecular age estimates also hold the view that monocots are as old as eudicots. Bremer, using the rbcL sequence data and the method of the middle way ("mean-path lengths method") to estimate divergence times, dates back the origin of the crown group of the monocots (the time Acorus genus divides the rest of the group) to some 134 million years, which would mean that estimates of the main group of monocots are even older. However Wikström et al., using Sanderson’s non-parametric approach (nonparametric rate smoothing approach "), produced ages for the crown group of monocots between 158 and 141 million years up until now (see Sanderson et al. 2004 ), ages markedly older than Bremer’s, so that the trunk group of the monocots would also be older than Bremer’s estimations. The discrepancy between these two estimates is probably due to the highly conservative calibration point used in the study of Wikström et al. 2001 (the split between Fagales and Cucurbitales were considered to be in the late Santonian period).
In fact the age of the monocot crown has been variously estimated, besides the two mentioned, it has been estimated around 200 ± 20 million years BC, 160 ± 16 million years BC, 135-131 million, 133.8 to 124 million, etc.
Assuming Triuridaceae is a member of Pandanales, their fossils would give support to a crown group age closer to Bremer’s estimations.
Bremer’s estimation was used in a more recent analysis that formed the basis for dating the age of the monocots in general.
The age of the core group of so-called ‘nuclear monocot’ or ‘core monocots’ by the APW ("core monocots" in English), which correspond to all orders except Acorales and Alismatales, is about 131 million years to present, and crown group age is about 126 million years to the present. The subsequent branching in this part of the tree, including the crown Petrosaviaceae group may be in the period around 125-120 million years BC, and stem groups of all other orders, including Commelinadae would have diverged about or shortly after 115 million years. These and many clades within these orders may have originated in southern Gondwana, i.e., Antarctica, Australasia, and southern South America.
The aquatic monocot Alismatales have commonly been regarded as "primitive". They have also been considered to have the most primitive foliage, which were cross-linked as Dioscoreales. Keep in mind that, as stressed by Soltis et al. 2005, the "most primitive" monocot is not necessarily "the sister of everyone else." This is because the ancestral or primitive characters are inferred by means of the reconstruction of characteristic states, with the help of the phylogenetic tree. So primitive characters of monocots may be present in some derived groups. On the other hand, the basal taxa may exhibit many morphological autapomorphies. So although Acoraceae is the sister group to the remaining monocotyledons, the result does not imply that Acoraceae is "the most primitive monocot" in terms of its characteristics. In fact, Acoraceae is highly derived in most morphological characteristics, which is precisely why so many Alismatales Acoraceae occupied relatively imitative positions in trees produced by Chase et al. 1995b and Stevenson and Loconte 1995.
Some authors support the idea of an aquatic phase as the origin of monocots. The phylogenetic position of Alismatales (many water), which occupy a relationship with the rest except the Acoraceae, do not rule out the idea, because it could be ‘the most primitive monocots’ but not ‘the most basal’. The Atactostele stem, the long and linear leaves, the absence of secondary growth, roots in groups instead of a single root branching, including sympodial use, are consistent with a water source. However, while monocots were sisters of the aquatic Ceratophyllales, or their origin is related to the adoption of some form of aquatic habit, it would not help much to the understanding of how it evolved to develop their distinctive anatomical features: the monocots seem so different from the rest of angiosperms and it’s difficult to relate their morphology, anatomy and development and those of broad-leaved angiosperms.
In the past, taxa which had petiolate leaves with reticulate venation were considered "primitive" within the monocots, because of its superficial resemblance to the leaves of dicotyledons. Recent work suggests that these taxa are sparse in the phylogenetic tree of monocots, such as fleshy fruited taxa (excluding taxa with aril seeds dispersed by ants), the two features would be adapted to conditions that evolved together regardless. Among the taxa involved were Smilax, Trillium (Liliales), Dioscorea (Dioscoreales), etc. A number of these plants are vines that tend to live in shaded habitats for at least part of their lives, and may also have a relationship with their shapeless stomata (see Cameron and Dickison 1998  for references on this last characteristic). Reticulate venation seems to have appeared at least 26 times in monocots, in fleshy fruits 21 times (sometimes lost later), and the two characteristics, though different, showed strong signs of a tendency to be good or bad in tandem, a phenomenon Givnish et al. described as “concerted convergence” (“coordinated convergence”).
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