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Monocotyledons (), commonly referred to as monocots, ( Lilianae sensu Chase & Reveal) are whose contain only one embryonic leaf, or cotyledon. A monocot taxon has been in use for several decades, but with various ranks and under several different names. The APG IV system recognises its monophyly but does not assign it to a taxonomic rank, and instead uses the term "monocots" to refer to the group.
Monocotyledons are contrasted with the Dicotyledon, which have two cotyledons. Unlike the monocots however, the dicots are not Monophyly and the two cotyledons are instead the ancestral characteristic of all flowering plants. Botanists now classify dicots into the eudicots ("true dicots") and several basal lineages from which the monocots emerged.
The monocots are extremely important economically, culturally, and ecologically, and make up a majority of plant biomass used in agriculture. Common crops such as date palm, , garlic, rice, wheat, maize, and sugarcane are all monocots. The Poaceae alone cover over 40% of Earth's land area and contribute a significant portion of the human diet. Other monocots, like , , daffodils, and Lilium are common and have been the subjects of several celebrations, holidays, and artworks for thousands of years.
With over 70,000 species, monocots are extremely evolutionarily successful and occupy a diverse set of Ecological niche: Perennial geophytes including orchids (Asparagales); tulips and lilies (Liliales); rosette and succulent epiphytes (Asparagales); mycoheterotrophs (Liliales, Dioscoreales, Pandanales), all in the lilioid monocots; major cereal food grains (maize, rice, barley, rye, oats, millet, sorghum and wheat) in the grass family; and forage grasses (Poales) as well as woody plants tree-like palm trees (Arecales), bamboo, reeds and bromeliads (Poales), bananas and ginger (Zingiberales) in the commelinid monocots, as well as floating or submerged aquatic plants such as seagrass (Alismatales) are all monocots.
| Growth form | Mostly herbaceous, occasionally arboraceous | Herbaceous or arboraceous |
| Leaves | Leaf shape oblong or linear, often sheathed at base, petiole seldom developed, stipules absent. Major leaf veins usually parallel | Broad, seldom sheathed, petiole common often with stipules. Veins usually reticulate (pinnate or palmate) |
| Primary root of short duration, replaced by Adventious roots roots forming fibrous or fleshy root systems | Develops from the radicle. Primary root often persists forming strong taproot and secondary roots | |
| 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 () |
| Flowers | Parts in threes (merosity) or multiples of three ( e.g. 3, 6 or 9 petals) | Fours (tetramerous) or fives (pentamerous) |
| Pollen: Number of apertures (furrows or pores) | Monocolpate (single aperture or colpus) | Tricolpate (three) |
| Plant embryo: Number of (leaves in the seed) | One, endosperm frequently present in seed | Two, endosperm present or absent |
A number of these differences are not unique to the monocots, and, while still useful, no one single feature will infallibly identify a plant as a monocot. For example, trimerous flowers and monosulcate pollen are also found in , and exclusively adventitious roots are found in some of the Piperaceae. Similarly, at least one of these traits, parallel leaf veins, is far from universal among the monocots. Broad leaves and reticulate leaf veins, features typical of dicots, are found in a wide variety of monocot families: for example, Trillium, Smilax (greenbriar), Pogonia (an orchid), and the Dioscoreales (yams). Potamogeton and Paris quadrifolia (herb-paris) are examples of monocots with tetramerous flowers. Other plants exhibit a mixture of characteristics. Nymphaeaceae (water lilies) have reticulate veins, a single cotyledon, adventitious roots, and a monocot-like vascular bundle. These examples reflect their shared ancestry. Nevertheless, this list of traits is generally valid, especially when contrasting monocots with , rather than non-monocot flowering plants in general.
Formal description dates from John Ray's studies of seed structure in the 17th century. Ray, who is often considered the first botanical systematist, observed the dichotomy of cotyledon structure in his examination of seeds. He reported his findings in a paper read to the Royal Society on 17 December 1674, entitled "A Discourse on the Seeds of Plants".
Since this paper appeared a year before the publication of [[Malpighi]]'s ''Anatome Plantarum'' (1675–1679), Ray has the priority. At the time, Ray did not fully realise the importance of his discovery but progressively developed this over successive publications. And since these were in Latin, "seed leaves" became ''folia seminalia'' and then ''cotyledon'', following [[Malpighi]]. Malpighi and Ray were familiar with each other's work, and Malpighi in describing the same structures had introduced the term cotyledon, which Ray adopted in his subsequent writing.
In this experiment, Malpighi also showed that the cotyledons were critical to the development of the plant, proof that Ray required for his theory. In his ''Methodus plantarum nova'' Ray also developed and justified the "natural" or pre-evolutionary approach to classification, based on characteristics selected ''[[a posteriori]]'' in order to group together taxa that have the greatest number of shared characteristics. This approach, also referred to as polythetic would last till evolutionary theory enabled [[Eichler|Eichler system]] to develop the [[phyletic]] system that superseded it in the late nineteenth century, based on an understanding of the acquisition of characteristics. He also made the crucial observation ''Ex hac seminum divisione sumum potest generalis plantarum distinctio, eaque meo judicio omnium prima et longe optima, in eas sci. quae plantula seminali sunt bifolia aut διλόβω, et quae plantula sem. adulta analoga.'' (From this division of the seeds derives a general distinction amongst plants, that in my judgement is first and by far the best, into those seed plants which are bifoliate, or bilobed, and those that are analogous to the adult), that is between monocots and dicots. He illustrated this by quoting from Malpighi and including reproductions of Malpighi's drawings of cotyledons (see figure). Initially Ray did not develop a classification of flowering plants (florifera) based on a division by the number of cotyledons, but developed his ideas over successive publications, coining the terms ''Monocotyledones'' and ''Dicotyledones'' in 1703, in the revised version of his ''Methodus'' (''Methodus plantarum emendata''), as a primary method for dividing them, ''Herbae floriferae, dividi possunt, ut diximus, in Monocotyledones & Dicotyledones'' (Flowering plants, can be divided, as we have said, into Monocotyledons & Dicotyledons).
Monocotyledons remained in a similar position as a major division of the flowering plants throughout the nineteenth century, with minor variations. George Bentham and Hooker (1862–1883) used Monocotyledones, as would Wettstein, while August Eichler used Mononocotyleae and Adolf Engler, following de Candolle, Monocotyledoneae. In the twentieth century, some authors used alternative names such as Charles Bessey's (1915) Alternifoliae and Arthur Cronquist's (1966) Liliatae. Later (1981) Cronquist changed Liliatae to Liliopsida, usages also adopted by Takhtajan simultaneously. Thorne (1992) and Rolf (1985) also used Liliidae as a synonym.
Taxonomists had considerable latitude in naming this group, as the Monocotyledons were a group above the rank of family. Article 16 of the ICBN allows either a descriptive botanical name or a name formed from the name of an included family.
In summary they have been variously named, as follows:
This DNA based molecular phylogenetic research confirmed on the one hand that the monocots remained as a well defined monophyly group or clade, in contrast to the other historical divisions of the flowering plants, which had to be substantially reorganized. No longer could the angiosperms be simply divided into monocotyledons and dicotyledons; it was apparent that the monocotyledons were but one of a relatively large number of defined groups within the angiosperms. Correlation with morphological criteria showed that the defining feature was not cotyledon number but the separation of angiosperms into two major pollen types, uniaperturate (monosulcate and monosulcate-derived) and triaperturate (tricolpate and tricolpate-derived), with the monocots situated within the uniaperturate groups. The formal taxonomic ranking of Monoctyledons thus became replaced with monocots as an informal clade. This is the name that has been most commonly used since the publication of the Angiosperm Phylogeny Group APG system in 1998 and regularly updated since.
Within the angiosperms, there are two major grades, a small early branching basal grade, the basal angiosperms (ANA grade) with three lineages and a larger late branching grade, the core angiosperms (mesangiosperms) with five lineages, as shown in the cladogram.
Historically, George Bentham (1877), considered the monocots to consist of four alliances, Epigynae, Coronariae, Nudiflorae and Glumales, based on floral characteristics. He describes the attempts to subdivide the group since the days of John Lindley as largely unsuccessful. Like most subsequent classification systems it failed to distinguish between two major orders, Liliales and Asparagales, now recognised as quite separate. A major advance in this respect was the work of Rolf Dahlgren (1980), which would form the basis of the Angiosperm Phylogeny Group's (APG) subsequent modern classification of monocot families. Dahlgren who used the alternate name Lilliidae considered the monocots as a subclass of angiosperms characterised by a single cotyledon and the presence of triangular protein bodies in the sieve tube plastids. He divided the monocots into seven superorders, Alismatiflorae, Ariflorae, Triuridiflorae, Liliiflorae, Zingiberiflorae, Commeliniflorae and Areciflorae. With respect to the specific issue regarding Liliales and Asparagales, Dahlgren followed Huber (1969) in adopting a splitter approach, in contrast to the longstanding tendency to view Liliaceae as a very broad sensu lato family. Following Dahlgren's untimely death in 1987, his work was continued by his widow, Gertrud Dahlgren, who published a revised version of the classification in 1989. In this scheme the suffix -florae was replaced with -anae ( e.g. Alismatanae) and the number of superorders expanded to ten with the addition of Bromelianae, Cyclanthanae and Pandananae.
Molecular studies have both confirmed the monophyly of the monocots and helped elucidate relationships within this group. The APG system does not assign the monocots to a taxonomic rank, instead recognizing a monocots clade. However, there has remained some uncertainty regarding the exact relationships between the major lineages, with a number of competing models (including APG).
The APG system establishes eleven orders of monocots. These form three grades, the alismatid monocots, lilioid monocots and the commelinid monocots by order of branching, from early to late. In the following cladogram numbers indicate crown group (most recent common ancestor of the sampled species of the clade of interest) divergence times in mya (million years ago).
Of some 70,000 species, by far the largest number (65%) are found in two families, the orchids and grasses. The orchids (Orchidaceae, Asparagales) contain about 25,000 species and the grasses (Poaceae, Poales) about 11,000. Other well known groups within the Poales order include the Cyperaceae (sedges) and Juncaceae (rushes), and the monocots also include familiar families such as the palms (Arecaceae, Arecales) and lilies (Liliaceae, Liliales).
The monocots form a monophyletic group arising early in the history of the , but the fossil record is meagre. The earliest fossils presumed to be monocot remains date from the Cretaceous period. For a very long time, of palm trees were believed to be the oldest monocots, first appearing 90 million years ago (mya), 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 imply that the monocots are 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 lineage that led to monocots (stem group) split from other plants about 136 million years ago or 165-170 million years ago.
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 (see the biomechanics of living in the water), roots in groups instead of a single root branching (related to the nature of the substrate), 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.
Other economically important monocotyledon crops include various palms (Arecaceae), bananas and plantains (Musaceae), and their relatives, turmeric and cardamom (Zingiberaceae), asparagus (Asparagaceae), pineapple (Bromeliaceae), sedges (Cyperaceae) and rushes (Juncaceae), vanilla (Orchidaceae), yam (Dioscoreaceae), taro (Araceae), and , onion and garlic (Amaryllidaceae). Many are monocotyledon . Most of the horticultural bulbs, plants cultivated for their blooms, such as Lilium, , irises, amaryllis, cannas, bluebells and , are monocotyledons.
Modern era
Over the 1980s, a more general review of the classification of angiosperms was undertaken. The 1990s saw considerable progress in plant phylogenetics and cladistic theory, initially based on rbcL gene sequencing and cladistic analysis, enabling a phylogenetic tree to be constructed for the flowering plants. The establishment of major new clades necessitated a departure from the older but widely used classifications such as Cronquist and Thorne, based largely on morphology rather than genetic data. These developments complicated discussions on plant evolution and necessitated a major taxonomic restructuring.
Cladogram I: Phylogenetic position of the monocots within the angiosperms in APG IV (2016)
Subdivision
While the monocotyledons have remained extremely stable in their outer borders as a well-defined and coherent monophylectic group, the deeper internal relationships have undergone considerable flux, with many competing classification systems over time.
Cladogram 2: The phylogenetic composition of the monocots
Evolution
In phyletic classification systems monocots were generally positioned between plants other than angiosperms and dicots, implying that monocots were more primitive. With the introduction of phyletic thinking in taxonomy (from the Eichler system 1875–1878 onwards) the predominant theory of monocot origins was the ranalean (ranalian) theory, particularly in the work of Charles Bessey (1915), which traced the origin of all flowering plants to a Ranalean type, and reversed the sequence making dicots the more primitive group.
Molecular clock estimates
Kåre Bremer, using rbcL sequences and the Network science for estimating divergence times, estimated the age of the monocot crown group (i.e. the time at which the ancestor of today's Acorus diverged from the rest of the group) as 134 million years. Similarly, Wikström et al., using Sanderson's non-parametric rate smoothing approach, obtained ages of 127–141 million years for the crown group of monocots. All these estimates have large error ranges (usually 15-20%), and Wikström et al. used only a single calibration point, namely the split between Fagales and Cucurbitales, which was set to 84 Ma, in the late Santonian period. Early molecular clock studies using strict clock models had estimated the monocot crown age to 200 ± 20 million years ago or 160 ± 16 million years, while studies using relaxed clocks have obtained 135-131 million years or 133.8 to 124 million years. Bremer's estimate of 134 million years has been used as a secondary calibration point in other analyses. Some estimates place the diversification of the monocots as far back as 150 mya in the Jurassic period.
Core group
The age of the core group of so-called 'nuclear monocots' or 'core monocots', 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 (i.e. Petrosaviaceae, Dioscoreales + Pandanales and Liliales clades appeared), including the crown Petrosaviaceae group may be in the period around 125–120 million years BC (about 111 million years so far), and stem groups of all other orders, including Commelinidae 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.
Aquatic monocots
The aquatic monocots of Alismatales have commonly been regarded as "primitive". They have also been considered to have the most primitive foliage, which were cross-linked as Dioscoreales and Melanthiales. Keep in mind that 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 character 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 autapomorphy. 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 character states. In fact, Acoraceae is highly derived in many morphological characters, and that is precisely why Acoraceae and Alismatales occupied relatively derived positions in the trees produced by Chase et al. and others.
Other taxa
In the past, taxa which had petiolate leaves with reticulate venation were considered "primitive" within the monocots, because of the superficial resemblance to the leaves of dicotyledons. Recent work suggests that while 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 that tend to live in shaded habitats for at least part of their lives, and this fact may also relate to their shapeless stomata. Reticulate venation seems to have appeared at least 26 times in monocots, and fleshy fruits have appeared 21 times (sometimes lost later); the two characteristics, though different, showed strong signs of a tendency to be good or bad in tandem, a phenomenon described as "concerted convergence" ("coordinated convergence").
Etymology
The name monocotyledons is derived from the traditional botanical name "Monocotyledones" or Monocotyledoneae in Latin, which refers to the fact that most members of this group have one cotyledon, or embryonic leaf, in their .
Ecology
Emergence
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.
Conservation
The IUCN Red List describes four species as extinct, four as extinct in the wild, 626 as possibly extinct, 423 as critically endangered, 632 endangered, 621 vulnerable, and 269 near threatened of 4,492 whose status is known.
Uses
Monocots are among the most important plants economically and culturally, and account for most of the staple foods of the world, such as cereal food grains and starchy root crops, and palms, orchids and lilies, building materials, and many medicines. Of the monocots, the grasses are of enormous economic importance as a source of animal and human food, and form the largest component of agricultural species in terms of biomass produced.
See also
Notes
Citations
Bibliography
Books
Historical
Modern
(1982). 9780122006807, Academic Press. ISBN 9780122006807
(1966). 9780050017159, Наука. ISBN 9780050017159
Symposia
Chapters
(2025). 9780387722757 ISBN 9780387722757, in
(2025). 9780415257909 ISBN 9780415257909, in
Articles
(1970). 9780120059034 ISBN 9780120059034
Phylogenetics
APG
Websites and databases
External links
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