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Cycad

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For the insect, seeCicada.

Cycadales
Temporal range:Early Permian–Holocene
Cycas rumphiiwith old and new male strobili.
Scientific classificationEdit this classification
Kingdom: Plantae
Clade: Tracheophytes
Clade: Spermatophytes
Clade: Gymnospermae
Division: Cycadophyta
Bessey1907: 321.[2]
Class: Cycadopsida
Brongn.[1]
Order: Cycadales
Pers. ex Bercht. & J. Presl
Extant groupings
Synonyms
  • CycadofilicalesNěmejc 1950
  • DioalesDoweld 2001
  • StangerialesDoweld 2001
  • ZamialesBurnett 1835
Cycads in South Africa

Cycads/ˈsaɪkædz/areseed plantsthat typically have a stout and woody (ligneous)trunkwith acrownof large, hard, stiff,evergreenand (usually)pinnateleaves. The species aredioecious,that is, individual plants of a species are either male or female. Cycads vary in size from having trunks only a few centimeters to several meters tall. They typically grow very slowly[3]and live very long. Because of their superficial resemblance topalmsorferns,they are sometimes mistaken for them, but they are not closely related to either group. Cycads aregymnosperms(naked-seeded), meaning theirunfertilizedseeds are open to the air to be directly fertilized bypollination,as contrasted withangiosperms,which have enclosed seeds with more complex fertilization arrangements. Cycads have very specializedpollinators,usually a specific species ofbeetle.Both male and female cycads bear cones (strobili), somewhat similar toconifer cones.

Cycads have been reported tofix nitrogenin association with variouscyanobacterialiving in the roots (the "coralloid" roots).[4]These photosynthetic bacteria produce aneurotoxincalledBMAAthat is found in theseedsof cycads. This neurotoxin may enter a human food chain as the cycad seeds may be eaten directly as a source of flour by humans or by wild or feral animals such as bats, and humans may eat these animals. It is hypothesized that this is a source of someneurological diseasesin humans.[5][6]Another defence mechanism against herbivores is the accumulation of toxins in seeds and vegetative tissues; throughhorizontal gene transfer,cycads have acquired a family of genes (fitD) from a microbial organism, most likely a fungus, which gives them the ability to produce an insecticidal toxin.[7]

Cycads all over the world are in decline, with four species on the brink of extinction and seven species having fewer than 100 plants left in the wild.[8]

Description

[edit]
Cycads have a rosette of pinnate leaves around a cylindrical trunk

Cycads have acylindricaltrunk which usually does notbranch.However, some types of cycads, such asCycas zeylanica,can branch their trunks. The apex of the stem is protected by modified leaves calledcataphylls.[9]Leaves grow directly from the trunk, and typically fall when older, leaving a crown of leaves at the top. The leaves grow in arosetteform, with new foliage emerging from the top and center of the crown. The trunk may be buried, so the leaves appear to be emerging from the ground, so the plant appears to be abasal rosette.The leaves are generally large in proportion to the trunk size, and sometimes even larger than the trunk.

The leaves arepinnate(in the form of bird feathers,pinnae), with a central leaf stalk from which parallel "ribs" emerge from each side of the stalk, perpendicular to it. The leaves are typically either compound (the leaf stalk hasleafletsemerging from it as "ribs" ), or have edges (margins) so deeply cut (incised) so as to appear compound. The Australian genusBoweniaand some Asian species of Cycas, likeCycas multipinnata,Cycas micholitziiandCycas debaoensis,have leaves that arebipinnate,which means the leaflets each have their own subleaflets, growing in the same form on the leaflet as the leaflets grow on the stalk of the leaf (self-similar geometry).[10][11]

Confusion with palms

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Due to superficial similarities in foliage and plant structure, cycads andpalmsare often mistaken for each other. They also can occur in similar climates. However, they belong to differentphylaand as such are not closely related. The similar structure is the product ofconvergent evolution.

Beyond those superficial resemblances, there are a number of differences between cycads and palms. For one, both male and female cycads aregymnospermsand bear cones (strobili), while palms areangiospermsand so flower and bear fruit. The mature foliage looks very similar between both groups, but the young emerging leaves of a cycad resemble afiddlehead fernbefore they unfold and take their place in the rosette, while the leaves of palms are just small versions of the mature frond. Another difference is in thestem.Both plants leave some scars on the stem below the rosette where there used to be leaves, but the scars of a cycad arehelicallyarranged and small, while the scars of palms are a circle that wraps around the whole stem. The stems of cycads are also in general rougher and shorter than those of palms.[12]

Taxonomy

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The two extantfamiliesof cycads all belong to the orderCycadales,and are theCycadaceaeandZamiaceae(includingStangeriaceae). These cycads have changed little since the Jurassic in comparison to some other plant divisions. Five additional families belonging to theMedullosalesbecame extinct by the end of the Paleozoic Era.

Based on genetic studies, cycads are thought to be more closely related toGinkgothan to other living gymnosperms. Both are thought to have diverged from each other during the earlyCarboniferous.[13][14]

External phylogeny[13][14] Internal phylogeny[15][16]
Gymnosperms
Angiosperms

(flowering plants)


Traditional view
Modern view
Cycads
Cycadineae
Zamiineae

Classification of the Cycadophyta to the rank of family.

Fossil genera

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The following extinct cycad genera are known:[17]

  • AmuriellaLate Jurassic, Russian Far East (leaf fragments)
  • AndrostrobusTriassic to Cretaceous, worldwide (leaf form genus)
  • AntarcticycasMiddle Triassic, Antarctica (known from the whole plant)[18]
  • ?AnthrophyopsisLate Triassic, worldwide (leaf form genus, possibly apteridospermatophyte)[19]
  • ApoldiaTriassic-Jurassic, Europe
  • ArchaeocycasEarly Permian, Texas (leaf with sporophylls)
  • AricycasLate Triassic, Arizona (leaf form genus)
  • Beania(=Sphaereda), Triassic to Jurassic, Europe & Central Asia (leaf form genus)
  • BehuniniaLate Jurassic, Colorado & Utah (fruiting structures)
  • BucklandiaMiddle Jurassic to Early Cretaceous, Europe and India (leaf form genus)
  • BurejaLate Jurassic, Russia
  • CavamonocolpitesEarly Cretaceous, Brazil (pollen)
  • CrossozamiaEarly to Late Permian, China (leaf form genus)
  • CtenisMesozoic-Paleogene, Worldwide (leaf form genus)
  • CtenozamitesTriassic-Cretaceous, worldwide (leaf form genus)
  • CycadeniaTriassic, Pennsylvania (trunks)
  • CycadinorachisLate Jurassic, India (rachis)
  • FascisvarioxylonLate Jurassic, India (petrified wood)
  • Gymnovulites,Latest Cretaceous/earliest Paleocene, India (seed)
  • Heilungia,Late Jurassic to early Cretaceous, Russia & Alaska (leaf form genus)
  • LeptocycasLate Triassic, North Carolina & China (known from the whole plant)[20]
  • Mesosingeria,Jurassic to Early Cretaceous, Antarctica & Argentina (leaf form genus)
  • Michelilloa,Late Triassic, Argentina (stem)
  • ?Nikania,Early Cretaceous, Russia (leaf fragments)
  • ?Nilssonia,Middle Permian to Late Cretaceous, worldwide (leaf form genus) (possibly not a cycad)[21]
  • ?Nilssoniocladus,Early to Late Cretaceous, United States & Russia (stems, likely associated withNilssonia,possiblydeciduous)[22]
  • Palaeozamia,Middle Jurassic, England
  • Paracycas,Middle Jurassic to Late Jurassic, Europe and Central Asia
  • ?Phasmatocycas,Late Carboniferous to Early Permian, Kansas, Texas & New Mexico (leaf with sporophylls)[23]
  • Pleiotrichium,Late Cretaceous, Germany (leaf)
  • Pseudoctenis,Late Permian to Late Cretaceous, worldwide (leaf form genus)
  • Sarmatiella,Late Triassic, Ukraine
  • Stangerites,Late Triassic to Early Jurassic, Virginia and Mexico (leaf form genus)
  • Sueria,Early Cretaceous, Argentina (leaf)
  • Taeniopteris,Carboniferous to Cretaceous, worldwide (polyphyletic leaf form genus, also includes bennettitales andmarattialeanferns)

Fossil record

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Bowenia spectabilis:plant with single frond in the Daintree rainforest, north-east Queensland
Leaves and strobilus ofEncephalartos sclavoi

The oldest probable cycad foliage is known from the latest Carboniferous-Early Permian of South Korea and China, such asCrossozamia.Unambiguous fossils of cycads are known from the Early-Middle Permian onwards.[24]Cycads were generally uncommon during the Permian.[25]The two living cycad families are thought to have split from each other sometime between the Jurassic[15]and Carboniferous.[26]Cycads are thought to have reached their apex of diversity during the Mesozoic.[27]Although the Mesozoic is sometimes called the "Age of Cycads," some other groups of extinct seed plants with similar foliage, such asBennettitalesandNilssoniales,that are not closely related, may have been more abundant.[28]The oldest records of the modern genusCycasare from the Paleogene of East Asia.[29]Fossils assignable to Zamiaceae are known from the Cretaceous,[28]with fossils assignable to living genera of the family known from the Cenozoic.[16]

Petrified cycad fossil, New York Botanical Garden

Distribution

[edit]
See also:List of cycad species by country

The living cycads are found across much of thesubtropicalandtropicalparts of the world, with a few in temperate regions such as in Australia.[30]The greatest diversity occurs inSouthandCentral America.[citation needed]They are also found inMexico,theAntilles,southeasternUnited States,Australia,Melanesia,Micronesia,Japan,China,Southeast Asia,Bangladesh,India,Sri Lanka,Madagascar,andsouthernand tropicalAfrica,where at least 65speciesoccur. Some can survive in harshdesertor semi-desertclimates(xerophytic),[31]others in wetrain forestconditions,[32]and some in both.[33]Some can grow insandor even onrock,some in oxygen-poor, swampy,bog-like soils rich inorganic material.[citation needed]Some are able to grow in full sun, some in full shade, and some in both.[citation needed]Some aresalttolerant (halophytes).[citation needed]

Species diversity of the extant cycads peaks at 17˚ 15 "N and 28˚ 12" S, with a minor peak at theequator.There is therefore not alatitudinal diversity gradienttowards the equator but towards theTropic of Cancerand theTropic of Capricorn.However, the peak near the northern tropic is largely due toCycasin Asia andZamiain the New World, whereas the peak near the southern tropic is due toCycasagain, and also to the diverse genusEncephalartosin southern and central Africa, andMacrozamiain Australia. Thus, the distribution pattern of cycad species with latitude appears to be an artifact of the geographical isolation of the remaining cycad genera and their species, and perhaps because they are partlyxerophyticrather than simplytropical.

Cultural significance

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Nuts of theCycas orientis(nyathu) are coveted by theYolnguin Australia'sArnhem Landas a source of food. They are harvested on theirdry seasonto leach its poison under water overnight before ground into a paste, wrapped under bark and cooked on open fire until done.[34]

InVanuatu,the cycad is known asnameleand is an important symbol of traditional culture. It serves as a powerfultaboosign,[35]and a pair ofnameleleaves appears on thenational flagand coat of arms. Together with thenanggariaplant, another symbol of Vanuatu culture, thenamelealso gives its name toNagriamel,an indigenous political movement.

See also

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References

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  2. ^Bessey, C.E. (1907). "A synopsis of plant phyla".Nebraska Univ. Stud.7:275–373.
  3. ^Dehgan, Bijan (1983)."Propagation and Growth of Cycads—A Conservation Strategy".Proceedings of the Florida State Horticultural Society.96:137–139 – via Florida Online Journals.
  4. ^Rai AN, Soderback E, Bergman B (2000)."Tansley Review No. 116. Cyanobacterium-Plant Symbioses".The New Phytologist.147(3): 449–481.doi:10.1046/j.1469-8137.2000.00720.x.JSTOR2588831.PMID33862930.
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  6. ^Cox PA, Davis DA, Mash DC, Metcalf JS, Banack SA (2015)."Dietary exposure to an environmental toxin triggers neurofibrillary tangles and amyloid deposits in the brain".Proceedings of the Royal Society B.283(1823): 20152397.doi:10.1098/rspb.2015.2397.PMC4795023.PMID26791617.
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  14. ^abStull, Gregory W.; Qu, Xiao-Jian; Parins-Fukuchi, Caroline; Yang, Ying-Ying; Yang, Jun-Bo; Yang, Zhi-Yun; Hu, Yi; Ma, Hong; Soltis, Pamela S.; Soltis, Douglas E.; Li, De-Zhu (19 July 2021)."Gene duplications and phylogenomic conflict underlie major pulses of phenotypic evolution in gymnosperms".Nature Plants.7(8): 1015–1025.doi:10.1038/s41477-021-00964-4.ISSN2055-0278.PMID34282286.S2CID236141481.
  15. ^abNagalingum, N. S.; Marshall, C. R.; Quental, T. B.; Rai, H. S.; Little, D. P.; Mathews, S. (2011). "Recent synchronous radiation of a living fossil".Science.334(6057): 796–799.Bibcode:2011Sci...334..796N.doi:10.1126/science.1209926.PMID22021670.S2CID206535984.
  16. ^abCondamine, Fabien L.; Nagalingum, Nathalie S.; Marshall, Charles R.; Morlon, Hélène (17 April 2015)."Origin and diversification of living cycads: a cautionary tale on the impact of the branching process prior in Bayesian molecular dating".BMC Evolutionary Biology.15.65.Bibcode:2015BMCEE..15...65C.doi:10.1186/s12862-015-0347-8.PMC4449600.PMID25884423.S2CID14815027.
  17. ^"PBDB".paleobiodb.org.Retrieved16 March2024.
  18. ^Hermsen, Elizabeth J.; Taylor, Edith L.; Taylor, Thomas N. (January 2009). "Morphology and ecology of theAntarcticycasplant ".Review of Palaeobotany and Palynology.153(1–2): 108–123.Bibcode:2009RPaPa.153..108H.doi:10.1016/j.revpalbo.2008.07.005.
  19. ^Xu, Yuanyuan; Popa, Mihai Emilian; Zhang, Tingshan; Lu, Ning; Zeng, Jianli; Zhang, Xiaoqing; Li, Liqin; Wang, Yongdong (1 September 2021)."Re-appraisal of Anthrophyopsis (Gymnospermae): New material from China and global fossil records".Review of Palaeobotany and Palynology.292:104475.Bibcode:2021RPaPa.29204475X.doi:10.1016/j.revpalbo.2021.104475.ISSN0034-6667.
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  21. ^Vajda, Vivi; Pucetaite, Milda; McLoughlin, Stephen; Engdahl, Anders; Heimdal, Jimmy; Uvdal, Per (August 2017)."Molecular signatures of fossil leaves provide unexpected new evidence for extinct plant relationships".Nature Ecology & Evolution.1(8): 1093–1099.Bibcode:2017NatEE...1.1093V.doi:10.1038/s41559-017-0224-5.ISSN2397-334X.PMID29046567.S2CID3604369.
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  26. ^Coiro, Mario; Allio, Rémi; Mazet, Nathan; Seyfullah, Leyla J.; Condamine, Fabien L. (11 June 2023)."Reconciling fossils with phylogenies reveals the origin and macroevolutionary processes explaining the global cycad biodiversity".New Phytologist.240(4): 1616–1635.doi:10.1111/nph.19010.ISSN0028-646X.PMC10953041.PMID37302411.
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  28. ^abCoiro, Mario; Pott, Christian (December 2017)."Eobowenia gen. nov. from the Early Cretaceous of Patagonia: indication for an early divergence of Bowenia?".BMC Evolutionary Biology.17(1): 97.Bibcode:2017BMCEE..17...97C.doi:10.1186/s12862-017-0943-x.ISSN1471-2148.PMC5383990.PMID28388891.
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  34. ^Midawarr | Harvest: The Art of Mulkun Wirrpanda and John Wolseley.National Museum of Australia Press. 2017. p. 48.ISBN978-1-921953-31-6.
  35. ^Dan McGarry (9 April 2018)."A Princely Title".Vanuatu Daily Post.
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