Monocotyledon

Monocotyledon Temporal range: Early Cretaceous – Recent PreЄ Є O S D C P T J K Pg N
Wheat, an economically important monocot
Scientific classification
Kingdom:	Plantae
Clade:	Angiosperms
Clade:	Monocots
Orders
Acorales Alismatales Asparagales Dioscoreales Liliales Pandanales Petrosaviales Commelinids: Arecales Commelinales Poales Zingiberales
Synonyms
Monocotyledoneae Lilianae Takht. Liliatae Cronquist, Takht. & W.Zimm.[1][lower-alpha 1] Liliopsida Batsch

Monocotyledons (/ˌmɒnəˌkɒtəˈliːdən, -ˌkɒtˈliː-/^{[lower-alpha 2][3][4]}), commonly referred to as monocots, (Lilianae sensu Chase & Reveal) are flowering plants (angiosperms) whose seeds typically contain only one embryonic leaf, or cotyledon. They constitute one of the major groups into which the flowering plants have traditionally been divided, the rest of the flowering plants having two cotyledons and therefore classified as dicotyledons, or dicots. However, molecular phylogenetic research has shown that while the monocots form a monophyletic group or clade (comprising all the descendants of a common ancestor), the dicots do not. Monocots have almost always been recognized as a group, but with various taxonomic ranks and under several different names. The APG III system of 2009 recognises a clade called "monocots" but does not assign it to a taxonomic rank.

The monocots include about 60,000 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 more than 20,000 species. About half as many species belong to the true grasses (Poaceae), who are economically the most important family of monocots. In agriculture the majority of the biomass produced comes from monocots. These include not only major grains (rice, wheat, maize, etc.), but also forage grasses, sugar cane, and the bamboos. Other economically important monocot crops include various palms (Arecaceae), bananas (Musaceae), gingers and their relatives, turmeric and cardamom (Zingiberaceae), asparagus and the onions and garlic family (Amaryllidaceae). Many houseplants are monocot epiphytes. Additionally most of the horticultural bulbs, plants cultivated for their blooms, such as lilies, daffodils, irises, amaryllis, cannas, bluebells and tulips, are monocots.

Description

Allium crenulatum, an onion, with a typical monocot perianth and parallel leaf venation

The monocots or monocotyledons have a single cotyledon, or embryonic leaf, in their seeds. Historically, this feature was used to contrast the monocots with the dicotyledons or dicots which typically have two cotyledons; however modern research has shown that the dicots are not a natural group. From a diagnostic point of view the number of cotyledons is neither a particularly useful characteristic (as they are only present for a very short period in a plant's life), nor is it completely reliable.

Comparison with dicotyledons

Comparison of a monocot (grass) sprouting (left) with a dicot (right), showing hypogeal development in which the cotyledon remains invisible within the seed, underground. The visible part is the first true leaf produced from the meristem; the cotyledon itself remains within the seed.

Slice of onion, showing parallel veins in cross section

Ceroxylon quindiuense (Quindio wax palm) is considered the tallest monocot in the world

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.^[5][6][7]

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.^[6] For example, trimerous flowers and monosulcate pollen are also found in magnoliids,^[5] of which exclusively adventitious roots are found in some of the Piperaceae.^[5] Similarly, at least one of these traits, parallel leaf veins, is far from universal among the monocots. Monocots with broad leaves and reticulate leaf veins, typical of dicots, are found in a wide variety of monocot families: for example, Trillium, Smilax (greenbriar), and Pogonia (an orchid), and the Dioscoreales (yams).^[5] Potamogeton are one of several 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.^[6] 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.^[5]

Vascular system

Stems of two Roystonea regia palms showing anomalous secondary growth in monocots. Note the characteristic fibrous roots, typical of monocots.

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. Collenchyma is absent in monocot stems, roots and leaves. 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.^[5] 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".^[8] 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.^[5]

Synapomorphies

By contrast Douglas E. Soltis and others^{[9][10][11][12]} identify thirteen synapomorphies (shared characteristics that unite monophyletic groups of taxa);

1. Calcium oxalate raphides
2. Absence of vessels in leaves
3. Monocotyledonous anther wall formation
4. Successive microsporogenesis
5. Syncarpous gynoecium
6. Parietal placentation
7. Monocotyledonous seedling
8. Persistent radicle
9. Haustorial cotyledon tip
10. Open cotyledon sheath
11. Steroidal sapanonins
12. fly pollination
13. diffuse vascular bundles and absence of secondary growth

Taxonomy

The monocots form one of five major lineages of mesangiosperms, which in themselves form 99.95% of all angiosperms. The monocots and the eudicots, are the largest and most diversified angiosperm radiations accounting for 20% and 75% of all angiosperm species respectively.

Monocot diversity includes perennial geophytes including ornamental flowers (orchids, tulips and lilies) (Asparagales, Liliales respectively), rosette and succulent epiphytes (Asparagales), mycoheterotrophs (Liliales, Dioscoreales, Pandanales), all in the lilioid monocots, major grains (maize, rice and wheat) in the grass family (Poales) as well as woody tree-like palm trees (Arecales) and bamboo (Poales) in the commelinid monocots, as well as both emergent (Poales, Acorales) and floating or submerged aquatic plants (Alismatales).^{[13][14][15][16]}

Early history

Illustrations of cotyledons by John Ray 1682, after Malpighi

The monocots are one of the major divisions of the flowering plants or angiosperms. They have been recognized as a natural group since John Ray's studies of seed structure in the 17th century. Ray was the first botanical systematist,^[17] and in his examination of seeds, first observed the dichotomy of cotyledon structure. He reported his findings in a paper read to the Royal Society on 17 December 1674, entitled "A Discourse on the Seeds of Plants".^[5]

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^[19] but progressively developed this over successive publications. And since these were in Latin, "seed leaves" became folia seminalia^[20] and then cotyledon, following Malpighi.^[21][22] Malpighi and Ray were familiar with each other's work,^[19] and Malpighi in describing the same structures had introduced the term cotyledon,^[23] 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.^[24] In his Methodus plantarum nova^[25] 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 to develop the phyletic system that superseded it in the late nineteenth century, based on an understanding of the acquisition of characteristics.^[26][27][28] 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.^[29][24] He illustrated this with by quoting from Malpighi and including reproductions of Malpighi's drawings of cotyledons (see figure).^[30] 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,^[31] coining the terms Monocotyledones and Dicotyledones in 1703,^[32] 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).^[33]

Although Linnaeus did not utilise Ray's discovery, basing his own classification solely on floral reproductive morphology, every taxonomist since then, starting with De Jussie and De Candolle, has used Ray's distinction as a major classification characteristic.^[34]

Modern era

Modern research based on DNA has confirmed the status of the monocots as a monophyletic group or clade, in contrast to the other historical divisions of the flowering plants, which have had to be substantially reorganized.^[5] The monocots form about a quarter of all of the Angiosperms (flowering plants).^[35] Of some 60,000 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).^[36][37]

Taxonomists had 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 Angiosperm Phylogeny Group (APG) systems: the APG system, the APG II system, the APG III system and the APG IV system^{[38][39][40] [41] [42][43]}

Until the rise of the phylogenetic APG systems, it was widely accepted that angiosperms were neatly split between monocots and dicots, a state reflected in virtually all the systems. It is now understood that various groups, notably the Magnoliids and ancient lineages known as the basal angiosperms fall outside of this dichotomy. 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).

Molecular studies have both confirmed the monophyly of the monocots and helped elucidate relationships within this group. The APG III system does not assign the monocots to a taxonomic rank, instead recognizing a monocots clade.^{[44][45][46][47]} However, there has remained some uncertainty regarding the exact relationships between the major lineages, with a number of competing models (including APG).^[16]

Subdivisions

Historically, 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 Lindley as largely unsuccessful.^[48] Like most subsequent classification systems it failed to distinguish between two major orders, Liliales and Asparagales, now recognised as quite separate.^[36] A major advance in this respect was the work of Rolf Dahlgren (1980),^[49] 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)^[50] 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.^[51]

The APG system establishes 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.^[52] 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. Family Dasypogonaceae is placed in order Arecales in the APG IV system.^[41]

Evolution

The monocots form a monophyletic group arising early in the history of the flowering plants, but the fossil record is meagre.^[53] The earliest fossils presumed to be monocot remains date from the early Cretaceous period. For a very long time, fossils of palm trees were believed to be the oldest monocots,^[54] first appearing 90 million years ago, but this estimate may not be entirely true.^[55] At least some putative monocot fossils have been found in strata as old as the eudicots.^[56] 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.^[57][58][59] They have also found flower fossils of Triuridaceae (Pandanales) in Upper Cretaceous rocks in New Jersey,^[57] 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.

Molecular clock estimates

Kåre Bremer, using rbcL sequences and the mean path length method ("mean-path lengths method"), 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.^[60][61] Similarly, Wikström et al.,^[62] using Sanderson's non-parametric rate smoothing approach ("nonparametric rate smoothing approach"),^[63] obtained ages of 158 or 141 million years for the crown group of monocots.^[64] All these estimates have large error ranges (usually 15-20%), and Wikström et al. used only a single calibration point,^[62] 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^[65] or 160 ± 16 million years,^[66] while studies using relaxed clocks have obtained 135-131 million years^[67] or 133.8 to 124 million years.^[68] Bremer's estimate of 134 million years^[60] has been used as a secondary calibration point in other analyses.^[69] Some estimates place the emergence of the monocots as far back as 150 mya in the Jurassic period.^[16]

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,^[70] 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^[60]), and stem groups of all other orders, including Commelinidae would have diverged about or shortly after 115 million years.^[69] These and many clades within these orders may have originated in southern Gondwana, i.e. Antarctica, Australasia, and southern South America.^[71]

Aquatic monocots

The aquatic monocots of Alismatales have commonly been regarded as "primitive".^{[72][73][74][75][76][77][78][79][80]} They have also been considered to have the most primitive foliage, which were cross-linked as Dioscoreales^[81] and Melanthiales.^[82][83] Keep in mind that the "most primitive" monocot is not necessarily "the sister of everyone else".^[35] 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 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 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.^[44] and others.^[11][84]

Some authors support the idea of an aquatic phase as the origin of monocots.^[85] 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.^[86][87]

Other taxa

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.^{[88][89][90][91]} 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.^[92] 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 described as "concerted convergence" ("coordinated convergence").^[90][91]

Etymology

The name monocotyledons is derived from the traditional botanical name "Monocotyledones", which refers to the fact that most members of this group have one cotyledon, or embryonic leaf, in their seeds.

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.^[93] 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.

Uses

Of the monocots, the grasses are of enormous economic importance as a source of animal and human food,^[36] and form the largest component of agricultural species in terms of biomass produced.^[37]

Notes

References

Bibliography

External links

Wikispecies has information related to: Monocots

• Tree of Life Web Project: Monocotyledons
• "Numbers of threatened species by major groups of organisms (1996–2004)". International Union for Conservation of Nature and Natural Resources. Archived from the original on 2006-09-27. Retrieved 2006-12-15. 
• Monocots Plant Life Forms
• World list of monocot species via the Catalogue of Life
• Tree browser for monocot orders, families and genera with species counts and estimates via the Catalogue of Life
• Trias-Blasi, A. et al. 2015, Baker, W. J., Haigh, A. L., Simpson, D. A., Weber, O., & Wilkin, P. 2015. A genus-level phylogenetic sequence of monocots. Taxon 64: 552-581. doi:10.12705/643.9

Orders of Monocotyledons
Alismatid monocots	Acorales Alismatales
Lilioid monocots	Asparagales Dioscoreales Liliales Pandanales Petrosaviales
Commelinids	Arecales Commelinales Poales Zingiberales Dasypogonales