Monocotyledon

Monocot
Temporal range: Early Cretaceous – Recent
Wheat, an important monocot
Scientific classification
Kingdom: Plantae
(unranked): Angiosperms
(unranked): Monocots
Orders
Synonyms
  • Monocotyledoneae
  • Liliopsida

Monocotyledons (/ˈmɒnɵˈkɒtɪˈliːdən/[1]), also known as monocots, are one of the major groups into which flowering plants (or angiosperms) are divided. Traditionally, the rest of the flowering plants were classed as dicotyledons, or dicots. Monocot seedlings typically have one cotyledon (seed-leaf), in contrast to the two cotyledons typical of dicots. Modern research using molecular phylogenetic methods has shown that the monocots form a monophyletic group – a clade – since they comprise all the descendants of a common ancestor. Dicots, by contrast, do not form a monophyletic group.

Monocots have been recognized as a group at various taxonomic ranks, and under various names (see below). The APG III system of 2009 recognises a clade called "monocots" but does not assign it to a taxonomic rank.

Comparison of a monocot and dicot sprouting. Note that the visible part of the monocot plant (left) is actually the first true leaf produced from the meristem; the cotyledon itself remains within the seed.

According to the IUCN there are 59,300 species of monocots.[2] 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.[3] In agriculture the majority of the biomass produced comes from monocots.[4] 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 amaryllis family (Amaryllidaceae), which includes such ubiquitously used vegetables as onions and garlic.

Many plants cultivated for their blooms are also from the monocot group, notably lilies, daffodils, irises, amaryllis, orchids, cannas, bluebells and tulips.

Description

The monocots are one of the major divisions of the flowering plants or angiosperms. They have been recognized as a natural group since at least the work of the English botanist John Ray in the 17th century. 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 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. 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.

Additionally, 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—having three, six, or nine petals. Many monocots also have leaves with parallel veins.

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

Morphology, compared to the (broadly defined) 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.[5]

Slice of onion, showing parallel veins in cross section
Ceroxylon quindiuense (Quindio wax palm) is considered the tallest monocot in the world
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.[5] Exclusively adventitious roots are found also in Nymphaeaceae and some of the Piperaceae.[5] 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.[5] 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]

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.[6] 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.

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. 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".[7] 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]

Classification

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:

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 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.[8] 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.

clade monocots :


Chloranthaceae

magnoliids
monocots

Acorales

Alismatales

Petrosaviales


Dioscoreales

Pandanales

Liliales

Asparagales

commelinids

Ceratophyllum

eudicots




The current phylogeny and composition of the monocots.[9]


Grass sprouting on left (a monocot), showing hypogeal development (the cotyledon remains underground and is not visible). Compare to a dicot (right)

Evolution

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.[10] At least some putative monocot fossils have been found in strata as old as the eudicots.[11] 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.[12][Note 1] They have also found flower fossils of Triuridaceae (Pandanales) in Upper Cretaceous rocks in New Jersey,[14] 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 for the age of extant monocots

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.[15][16] Similarly, Wikström et al.,[17] using Sanderson's non-parametric rate smoothing approach ("nonparametric rate smoothing approach"),[18] obtained ages of 158 or 141 million years for the crown group of monocots.[Note 2] All these estimates have large error ranges (usually 15-20%), and Wikström et al. used only a single calibration point,[17] 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[20] or 160 ± 16 million years,[21] while studies using relaxed clocks have obtained 135-131 million years[22] or 133.8 to 124 million years.[23] Bremer's estimate of 134 million years[15] has been used as a secondary calibration point in other analyses.[24]

Core group

The age of the core group of so-called 'nuclear monocot' or 'core monocots' by the Angiosperm Phylogeny Website ("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 (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[15]), and stem groups of all other orders, including Commelinidae would have diverged about or shortly after 115 million years.[24] These and many clades within these orders may have originated in southern Gondwana, i.e. Antarctica, Australasia, and southern South America.[25]

Aquatic monocots

The aquatic monocots of Alismatales have commonly been regarded as "primitive".[26][27][28][29][30][31][32][33][34] They have also been considered to have the most primitive foliage, which were cross-linked as Dioscoreales[35] and Melanthiales.[36][37] Keep in mind that the "most primitive" monocot is not necessarily "the sister of everyone else".[38] 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 & Loconte (1995).

Some authors support the idea of an aquatic phase as the origin of monocots.[39] 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.[40][41]

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.[42][43][44][45] 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.[Note 3] 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").[44][45]

Notes

  1. ↑ For fossil monocots, see Gandolfo et al., 2000 and Friis et al., 2006b.[13]
  2. ↑ See Sanderson et al. (2004).[19]
  3. ↑ See Cameron & Dickison (1998)[46] for references on this last characteristic.

References

  1. ↑ An Anglo-Latin pronunciation. OED: "Monocotyledon".
  2. ↑ http://cmsdata.iucn.org/downloads/2008rl_stats_tables_all.xls
  3. ↑ Peter H. Raven, Ray Franklin Evert & Susan E. Eichhorn. (2005) Biology of Plants, 7th ed., page 459
  4. ↑ Reed, Barbara (2008). Plant cryopreservation a practical guide. New York: Springer. p. 241. ISBN 978-0-387-72276-4.
  5. ↑ 5.0 5.1 5.2 5.3 5.4 5.5 5.6 5.7 Chase, Mark W. (2004). "Monocot relationships: an overview". American Journal of Botany 91 (10): 1645–1655. doi:10.3732/ajb.91.10.1645. PMID 21652314.
  6. ↑ Radosevich, Steven R.; Holt, Jodie S.; Ghersa, Claudio (1997). Weed ecology: implications for management. New York: J. Wiley. ISBN 0-471-11606-8.
  7. ↑ Donoghue, Michael J. (2005). "Key innovations, convergence, and success: macroevolutionary lessons from plant phylogeny" (PDF). Paleobiology 31: 77–93. doi:10.1666/0094-8373(2005)031[0077:KICASM]2.0.CO;2.
  8. ↑ Cantino, Philip D.; James A. Doyle; Sean W. Graham; Walter S. Judd; Richard G. Olmstead; Douglas E. Soltis; Pamela S. Soltis; Michael J. Donoghue (2007). "Towards a phylogenetic nomenclature of Tracheophyta" (PDF). Taxon 56 (3): E1–E44. doi:10.2307/25065865.
  9. ↑ "An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG III". Botanical Journal of the Linnean Society 161 (2): 105–121. 2009. doi:10.1111/j.1095-8339.2009.00996.x.
  10. ↑ Herendeen, P. S.; Crane, P. R. (1995). "The fossil history of the monocotyledons". In Rudall, P., Cribb, P. J., Cutler, D. F. & C. J. Humphries. Monocotyledons: systematics and evolution. London: Royal Botanic Gardens Kew. pp. 1–21.
  11. ↑ Herendeen, P. S.; P. R. Crane & A. Drinnan (1995). Fagaceous flowers, fruits, and cupules from the Campanian (Late Cretaceous) of Central Georgia, USA. International Journal of Plant Sciences 156. pp. 93–116. JSTOR 2474901.
  12. ↑ Friis, E. M.; Pedersen, K. R.; Crane, P. R. (2004). "Araceae from the early Cretaceous of Portugal: Evidence on the emergence of monocotyledons". Proceedings of the National Academy of Sciences 101 (47): 16565–16570. doi:10.1073/pnas.0407174101. PMC 534535. PMID 15546982.
  13. ↑ Friis, E. M.; Pedersen, K. R. & Crane, P. R. (2006). "Cretaceous angiosperm flowers: innovation and evolution in plant reproduction". Palaeogeog. Palaeoclim. Palaeoecol. 232: 251–293. doi:10.1016/j.palaeo.2005.07.006.
  14. ↑ Gandolfo, M. A.; K. C. Nixon & W. L. Crepet (2002). "Triuridaceae fossil flowers from the Upper Cretaceous of New Jersey". American Journal of Botany 89: 1940–1957. doi:10.3732/ajb.89.12.1940.
  15. ↑ 15.0 15.1 15.2 Bremer, K. (2000). "Early Cretaceous lineages of monocot flowering plants" (PDF). Proceedings of the National Academy of Sciences USA 97: 4707–4711. doi:10.1073/pnas.080421597.
  16. ↑ Bremer, K. (2002). "Gondwanan evolution of the grass alliance families (Poales)". Evolution 56: 1374–1387. doi:10.1111/j.0014-3820.2002.tb01451.x.
  17. ↑ 17.0 17.1 Wikström, N.; V. Savolainen & M. W. Chase (2001). "Evolution of the angiosperms: calibrating the family tree". Proceedings of the Royal Society of London B 268: 2211–2220. doi:10.1098/rspb.2001.1782. PMC 1088868. PMID 11674868.
  18. ↑ Sanderson, M. J. (1997). "A nonparametric approach to estimating divergence times in the absence of rate constancy" (PDF). Molecular Biology and Evolution 14: 1218–1231. doi:10.1093/oxfordjournals.molbev.a025731.
  19. ↑ Sanderson, M. J.; J. L. Thorne, N. Wikström & K. Bremer (2004). "Molecular evidence on plant divergence times". American Journal of Botany 91: 1656–1665. doi:10.3732/ajb.91.10.1656.
  20. ↑ Savard, L.; Strauss, S. H., Chase, M. W., Michaud, M. & Bosquet, J. (1994). "Chloroplast and nuclear gene sequences indicate late Pennsylvanian time for the last common ancestor of extant seed plants". Proc. National Acad. Sci. U.S.A. 91: 5163–5167. doi:10.1073/pnas.91.11.5163.
  21. ↑ Goremykin, V. V.; Hansman, S., Samigullin, T., Antonov, A. & Martin, W. (1997). "Evolutionary analysis of 58 proteins encoded in six completely sequenced chloroplast genomes: revised molecular estimates of two seed plant divergence times". Plant Syst. Evol. 206: 337–351. doi:10.1007/bf00987956.
  22. ↑ Leebens-Mack, J.; Raubeson, L. A., Cui, L., Kuehl, J. V., Fourcade, M. H., Chumley, T. W., Boore, J. L., Jansen, R. K. & dePamphilis, C. W. (2005). "Identifying the basal angiosperm node in chloroplast genome phylogenies: Sampling one's way out of the Felsenstein zone". Mol. Biol. Evol. 22: 1948–1963. doi:10.1093/molbev/msi191. PMID 15944438.
  23. ↑ Moore, J. P.; Lindsey, G. G., Farrant, J. M. & Brandt, W. F. (2007). "An overview of the dessication-tolerant resurrection plant Myrothamnus flabellifer". Ann. Bot. 99: 211–217.
  24. ↑ 24.0 24.1 Janssen, T.; Bremer, K. (2004). "The age of major monocot groups inferred from 800+ rbcL sequences". Bot. J. Linnean Soc. 146: 385–398. doi:10.1111/j.1095-8339.2004.00345.x.
  25. ↑ Bremer, K.; Janssen, T. (2006). "Gondwanan origin of major monocot groups inferred from dispersal-vicariance analysis". Aliso 22: 22–27.
  26. ↑ Hallier, H. (1905). "Provisional scheme for the natural (phylogenetic) system of the flowering plants". New Phytologist 4: 151–162. doi:10.1111/j.1469-8137.1905.tb05894.x.
  27. ↑ Arber, A. (1925). Monocotyledons: a morphological study. Cambridge: Cambridge University Press.
  28. ↑ Hutchinson, J. (1934). The Families of Flowering Plants. Oxford: Oxford University Press.
  29. ↑ Cronquist, A. (1968). The Evolution and Classification of Flowering Plants. Boston: Houghton Mifflin.
  30. ↑ Takhtajan, A. (1969). Flowering Plants: Origin and Dispersal. Washington: Smithsonian Institution Press.
  31. ↑ Takhtajan, A. (1991). Evolutionary Trends in Flowering Plants. New York: Columbia University Press.
  32. ↑ Stebbins, G. L. (1974). Flowering Plants: Evolution Above the Species Level. Cambridge: Belknap Press.
  33. ↑ Thorne, R. F. (1976). "A phylogenetic classification of the Angiospermae". Evolutionary Biology 9: 35–106. doi:10.1007/978-1-4615-6950-3_2.
  34. ↑ Cronquist 1981
  35. ↑ Dahlgren, R. M.; Clifford, H. T. & Yeo, P. F. (1985). The families of the monocotyledons. Berlin: Springer.
  36. ↑ Thorne, R. F. (1992a). "Classification and geography of the flowering plants". Botanical Review 58: 225–348. doi:10.1007/bf02858611.
  37. ↑ Thorne, R. F. (1992b). "An updated phylogenetic classification of the flowering plants". Aliso 13: 365–389.
  38. ↑ Soltis et al. 2005
  39. ↑ Henslow, G. (1893). "A theoretical origin of the endogens from the exogens through self-adaptation to an aquatic habitat". Journal of the Linnean Society, Botany 29: 485–528. doi:10.1111/j.1095-8339.1893.tb02273.x.
  40. ↑ Zimmermann, M. H.; Tomlinson, P. B. (1972). "The vascular system of monocotyledonous stems". Bot. Gaz. 133: 141–155. doi:10.1086/336628.
  41. ↑ Tomlinson, P. B. (1995). "Non-homology of vascular organisation in monocotyledons and dicotyledons". In Rudall, P., Cribb, P. J., Cutler, D. F. & C. J. Humphries. Monocotyledons: systematics and evolution. London: Royal Botanic Gardens Kew. pp. 589–622.
  42. ↑ Dahlgren, R. M. T.; Clifford, H. T. The monocotyledons: a comparative study. London: Academic Press.
  43. ↑ Patterson, T. B.; Givnish, T. J. (2002). "Phylogeny, concerted convergence, and phylogenetic niche conservatism in the core Liliales: Insights from rbcL and ndhF sequence data". Evolution 56: 233–252. doi:10.1111/j.0014-3820.2002.tb01334.x. PMID 11926492.
  44. ↑ 44.0 44.1 Thomas J. Givnish, J. Chris Pires, Sean W. Graham, Marc A. McPherson, Linda M. Prince, Thomas B. Patterson, Hardeep S. Rai, Eric H. Roalson, Timothy M. Evans, William J. Hahn, Kendra C. Millam, Alan W. Meerow, Mia Molvray, Paul J. Kores, Heath E. O'Brien, Jocelyn C. Hall, W. John Kress & Kenneth J. Sytsma (2005). "Repeated evolution of net venation and fleshy fruits among monocots in shaded habitats confirms a priori predictions: evidence from an ndhF phylogeny". Proceedings of the Royal Society B: Biological Sciences 272 (1571): 1481–1490. doi:10.1098/rspb.2005.3067. PMC 1559828. PMID 16011923.
  45. ↑ 45.0 45.1 Givnish, T. J.; Pires, J. C., Graham, S. W., McPherson, M. A., Prince, L. M., Patterson, T. B., Rai, H. S., Roalson, E. H., Evans, T. M., Hahn, W. J., Millam, K. C., Meerow, A. W., Molvray, M., Kores, P. J., O'Brien, H. E., Hall, J. C., Kress, W. J. & Sytsma, K. J. (2006). "Phylogeny of the monocots based on the highly informative plastid gene ndhF : Evidence for widespread concerted convergence". Aliso 22: 28–51.
  46. ↑ Cameron, K. M.; Dickison, W. C. (1998). "Foliar architecture of vanilloid orchids: Insights into the evolution of reticulate leaf venation in monocots". Bot. J. Linnean Soc. 128: 45–70. doi:10.1006/bojl.1998.0183.

Further reading

External links

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