Monocotyledon

Monocotyledons
Temporal range: Early Cretaceous – Recent
Wheat close-up.JPG
Wheat

An economically important monocot

Scientific classification e
Kingdom:Plantae
Clade:Angiosperms
Clade:Monocots
Type genus
Lilium
L.[1]
Orders
Synonyms

Monocotyledons (n/),[d][13][14] 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), which 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 and plantains (Musaceae), gingers and their relatives, turmeric and cardamom (Zingiberaceae), asparagus (Asparagaceae), pineapple (Bromeliaceae), water chestnut (Cyperaceae), and leeks, onion and garlic (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 (Asparagales), an onion, with typical monocot perianth and parallel leaf venation
Onion slice: Parallel veins in cross section

General

The monocots or monocotyledons have, as the name implies, a single (mono-) 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, and the term can only be used to indicate all angiosperms that are not monocots and is used in that respect here. 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. The single cotyledon is only one of a number of modifications of the body plan of the ancestral monocotyledons, whose adaptive advantages are poorly understood, but may have been related to adaption to aquatic habitats, prior to radiation to terrestrial habitats. Nevertheless, monocots are sufficiently distinctive that there has rarely been disagreement as to membership of this group, despite considerable diversity in terms of external morphology.[15] However, morphological features that reliably characterise major clades are rare.[16]

Thus monocots are distinguishable from other angiosperms both in terms of their uniformity and diversity. On the one hand the organisation of the shoots, leaf structure and floral configuration are more uniform than in the remaining angiosperms, yet within these constraints a wealth of diversity exists, indicating a high degree of evolutionary success.[17] Monocot diversity includes perennial geophytes such as ornamental flowers including (orchids (Asparagales), tulips and lilies) (Liliales), rosette and succulent epiphytes (Asparagales), mycoheterotrophs (Liliales, Dioscoreales, Pandanales), all in the lilioid monocots, major cereal grains (maize, rice, barley, rye and wheat) in the grass family and forage grasses (Poales) as well as woody tree-like palm trees (Arecales), bamboo, reeds and bromeliads (Poales), bananas and ginger (Zingiberales) in the commelinid monocots, as well as both emergent (Poales, Acorales) and aroids, as well as floating or submerged aquatic plants such as seagrass (Alismatales).[18][19][20][21]

Vegetative

Organisation, growth and life forms

The most important distinction is their growth pattern, lacking a lateral meristem (cambium) that allows for continual growth in diameter with height (secondary growth), and therefore this characteristic is a basic limitation in shoot construction. Although largely herbaceous, some arboraceous monocots reach great height, length and mass. The latter include agaves, palms, pandans, and bamboos.[22][23] This creates challenges in water transport that monocots deal with in various ways. Some, such as species of Yucca, develop anomalous secondary growth, while palm trees utilise an anomalous primary growth form described as establishment growth (see Vascular system). The axis undergoes primary thickening, that progresses from internode to internode, resulting in a typical inverted conical shape of the basal primary axis (see Tillich, Figure 1). The limited conductivity also contributes to limited branching of the stems. Despite these limitations a wide variety of adaptive growth forms has resulted (Tillich, Figure 2) from epiphytic orchids (Asparagales) and bromeliads (Poales) to submarine Alismatales (including the reduced Lemnoideae) and mycotrophic Burmanniaceae (Dioscreales) and Triuridaceae (Pandanales). Other forms of adaptation include the climbing vines of Araceae (Alismatales) which use negative phototropism (skototropism) to locate host trees (i.e. the darkest area),[24] while some palms such as Calamus manan (Arecales) produce the longest shoots in the plant kingdom, up to 185 m long.[25] Other monocots, particularly Poales, have adopted a therophyte life form.[26][27][28][29][30]

Leaves

The cotyledon, the primordial Angiosperm leaf consists of a proximal leaf base or hypophyll and a distal hyperphyll. In monocots the hypophyll tends to be the dominant part in contrast to other angiosperms. From these, considerable diversity arises. Mature monocot leaves are generally narrow and linear, forming a sheathing around the stem at its base, although there are many exceptions. Leaf venation is of the striate type, mainly arcuate-striate or longitudinally striate (parallel), less often palmate-striate or pinnate-striate with the leaf veins emerging at the leaf base and then running together at the apices. There is usually only one leaf per node because the leaf base encompasses more than half the circumference.[31] The evolution of this monocot characteristic has been attributed to developmental differences in early zonal differentiation rather than meristem activity (leaf base theory).[15][16][32]

Roots and underground organs

The lack of cambium in the primary root limits its ability to grow sufficiently to maintain the plant. This necessitates early development of roots derived from the shoot (adventitious roots). In addition to roots, monocots develop runners and rhizomes, which are creeping shoots. Runners serve vegetative propagation, have elongated internodes, run on or just below the surface of the soil and in most case bear scale leaves. Rhizomes frequently have an additional storage function and rhizome producing plants are considered geophytes (Tillich, Figure 11). Other geophytes develop bulbs, a short axial body bearing leaves whose bases store food. Additional outer non-storage leaves may form a protective function (Tillich, Figure 12). Other storage organs may be tubers or corms, swollen axes. Tubers may form at the end of underground runners and persist. Corms are short lived vertical shoots with terminal inflorescences and shrivel once flowering has occurred. However, intermediate forms may occur such as in Crocosmia (Asparagales). Some monocots may also produce shoots that grow directly down into the soil, these are geophilous shoots (Tillich, Figure 11) that help overcome the limited trunk stability of large woody monocots.[33][32][34][15]

Reproductive

Flowers

In nearly all cases the perigone consists of two alternating trimerous whorls of tepals, being homochlamydeous, without differentiation between calyx and corolla. In zoophilous (pollinated by animals) taxa, both whorls are corolline (petal-like). Anthesis (the period of flower opening) is usually fugacious (short lived). Some of the more persistent perigones demonstrate thermonastic opening and closing (responsive to changes in temperature). About two thirds of monocots are zoophilous, predominantly by insects. These plants need to advertise to pollinators and do so by way of phaneranthous (showy) flowers. Such optical signalling is usually a function of the tepal whorls but may also be provided by semaphylls (other structures such as filaments, staminodes or stylodia which have become modified to attract pollinators). However, some monocot plants may have aphananthous (inconspicuous) flowers and still be pollinated by animals. In these the plants rely either on chemical attraction or other structures such as coloured bracts fulfill the role of optical attraction. In some phaneranthous plants such structures may reinforce floral structures. The production of fragrances for olfactory signalling are common in monocots. The perigone also functions as a landing platform for pollinating insects. [17]

Fruit and seed

The embryo consists of a single cotyledon, usually with two vascular bundles.[32]

Comparison with "dicots"

Comparison of a monocot (grass: Poales) sprouting (left) with a dicot (right)[e]
Yucca brevifolia (Joshua Tree: Asparagales)

The traditionally listed differences between monocots and "dicots" 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.[34][35][36]

Feature In monocots In "dicots"
Growth form Mostly herbaceous, occasionally arboraceous Herbaceous or arboraceous
Leaves[16] 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)
Roots Primary root of short duration, replaced by adventitial 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 (eustelic)
Flowers Parts in threes (trimerous) 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)
Embryo: Number of cotyledons (leaves in the seed) One, endosperm frequently present in seed Two, endosperm present or absent
Comparison of monocots and "dicots"
Illustrations of differences between monocots and dicots

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.[35] For example, trimerous flowers and monosulcate pollen are also found in magnoliids,[34] of which exclusively adventitious roots are found in some of the Piperaceae.[34] 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).[34] 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.[35] 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.[34]

Apomorphies

Monocot apomorphies (characteristics that are derived during radiation rather than inherited from an ancestral form) include herbaceous habit, leaves with parallel venation and sheathed base, embryo with a single cotyledon, atactostele stele, numerous adventitious roots, sympodial growth, and trimerous (3 parts per whorl) flowers that are pentacyclic (5 whorled) with 3 sepals, 3 petals, 2 whorls of 3 stamens each and 3 carpels. In contrast monosculate pollen is considered an ancestral trait, probably plesiomorphic.[36]

Synapomorphies

The distinctive features of the monocots have contributed to the relative taxonomic stability of the group. Douglas E. Soltis and others[37][38][39][40] 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[41]
  10. Open cotyledon sheath
  11. Steroidal saponins*
  12. Fly pollination*
  13. Diffuse vascular bundles and absence of secondary growth[f]

Vascular system

Roystonea regia palm (Arecales) stems showing anomalous secondary growth in monocots, with characteristic fibrous roots

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.[34] 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".[42] 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.[34]

Other Languages
Alemannisch: Monokotyledonen
azərbaycanca: Birləpəlilər
башҡортса: Бер өлөшлөләр
беларуская: Аднадольныя
беларуская (тарашкевіца)‎: Аднадольныя
български: Едносемеделни
català: Liliòpsids
čeština: Jednoděložné
Cymraeg: Monocotyledon
euskara: Liliopsida
français: Monocotylédone
Gaeilge: Monocotylidae
한국어: 외떡잎식물
हिन्दी: एकबीजपत्री
hrvatski: Jednosupnice
italiano: Liliopsida
Kiswahili: Monokotiledoni
Kreyòl ayisyen: Monokotiledòn
kurdî: Liliopsida
latviešu: Viendīgļlapji
Lëtzebuergesch: Liliopsida
lietuvių: Lelijainiai
magyar: Egyszikűek
македонски: Монокотиледони
მარგალური: ართლებერამეფი
Bahasa Melayu: Monokotiledon
Nederlands: Eenzaadlobbigen
日本語: 単子葉植物
norsk nynorsk: Einfrøblada plantar
oʻzbekcha/ўзбекча: Bir pallali oʻsimliklar
پنجابی: اکدالے
português: Monocotiledónea
русский: Однодольные
sicilianu: Monocotyledon
Simple English: Monocotyledon
slovenščina: Enokaličnice
srpskohrvatski / српскохрватски: Monokotiledone biljke
Basa Sunda: Liliopsida
తెలుగు: ఏకదళబీజాలు
Türkçe: Tek çenekliler
українська: Однодольні
اردو: یک دالہ
žemaitėška: Vėinskiltē augalā