Difference between revisions of "Cycad"

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{{Automatic taxobox
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#redirect [[:Category:Cycads]]
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| name = Cycadophyta
 
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| fossil_range = {{fossil range|Early Permian|0|earliest=Late Carboniferous|[[Early Permian]]–[[Holocene]]}}
 
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| image = Cycas circinalis.jpg
 
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| image_caption = ''[[Cycas rumphii]]'' with old and new male cones.
 
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| parent_authority = [[Charles Edwin Bessey|Bessey]] 1907: 321.<ref name="Bessey 1907">{{cite journal | last=Bessey | first=C.E. | year=1907 | title=A synopsis of plant phyla | journal=Nebraska Univ. Stud. | volume=7 | pages=275–373 }}</ref>
 
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| taxon = Cycadopsida
 
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| authority = [[Adolphe-Théodore Brongniart|Brongn.]]<ref name="Brongniart 1843">{{cite book | last=Brongniart | first=A. | year=1843 | title=Énumération des genres de plantes cultivées au Musée d'histoire naturelle de Paris }}</ref>
 
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| subdivision_ranks = Orders
 
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| subdivision =
 
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*[[Cycadales]] <small>Pers. ex Bercht. & J. Presl</small>
 
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*†[[Medullosales]] <small>Corsin 1960</small>
 
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}}
 
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'''Cycads''' {{IPAc-en|'|s|aI|k|ae|d|z}} are [[seed plant]]s with a long fossil history that were formerly more abundant and more diverse than they are today. They typically have a stout and woody ([[ligneous]]) [[trunk (botany)|trunk]] with a [[crown (botany)|crown]] of large, hard and stiff, [[evergreen]] leaves. They usually have [[pinnate]] leaves. The individual plants are either all male or all female ([[dioecious]]). Cycads vary in size from having trunks only a few centimeters to several meters tall. They typically grow very slowly and live very long, with some specimens known to be as much as 1,000 years old. Because of their superficial resemblance, they are sometimes mistaken for [[Arecaceae|palms]] or [[fern]]s, but they are not closely related to either group.
 
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Cycads are [[gymnosperms]] (naked seeded), meaning their [[fertilization|unfertilized]] seeds are open to the air to be directly fertilized by [[pollination]], as contrasted with [[angiosperms]], which have enclosed seeds with more complex fertilization arrangements. Cycads have very specialized [[pollinator]]s, usually a specific species of [[beetle]]. They have been reported to [[nitrogen fixation|fix nitrogen]] in association with various [[cyanobacteria]] living in the roots (the "coralloid" roots).<ref>{{citation |authors=Rai, A.N.; Soderback, E.; Bergman, B. |year=2000 |title=Tansley Review No. 116. Cyanobacterium-Plant Symbioses |journal=The New Phytologist |volume=147 |issue=3 |pages=449–481 |jstor=2588831 |doi=10.1046/j.1469-8137.2000.00720.x}}</ref> These photosynthetic bacteria produce a [[neurotoxin]] called [[BMAA]] that is found in the [[seed]]s of 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 [[hypothesize]]d that this is a source of some [[neurological disease]]s in humans.<ref name=Holtcamp>{{cite journal | author = Holtcamp, W. | year = 2012 | title = The emerging science of BMAA: do cyanobacteria contribute to neurodegenerative disease? | journal = Environmental Health Perspectives | volume = 120 | issue = 3 | doi = 10.1289/ehp.120-a110 | pmid=22382274 | pmc=3295368 | pages=a110–a116}}</ref><ref name="Cox and Davis">{{cite journal | authors = Cox, PA, Davis, DA, Mash, DC, Metcalf, JS, Banack, SA. | year = 2015 | title = Dietary exposure to an environmental toxin triggers neurofibrillary tangles and amyloid deposits in the brain | journal = Proceedings of the Royal Society B | volume = 283 | issue = 1823 | doi = 10.1098/rspb.2015.2397 | pages=20152397 | pmid=26791617 | pmc=4795023}}</ref>
 
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==Description==
 
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[[File:Cycad leaves semicircle.jpg|thumb|Cycads have a rosette of pinnate leaves around cylindrical trunk]]
 
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Cycads have a [[cylindrical]] trunk which usually does not [[branch]].
 
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Leaves grow directly from the trunk, and typically fall when older, leaving a crown of leaves at the top. The leaves grow in a [[rosette (botany)|rosette]] form, 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 a [[rosette (botany)|basal rosette]]. The leaves are generally large in proportion to the trunk size, and sometimes even larger than the trunk.
 
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The leaves are [[pinnate]] (in the form of bird feathers, [[pinnation|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 has [[leaflet (botany)|leaflets]] emerging from it as "ribs"), or have edges ([[Leaf margin|margin]]s) so deeply cut ([[incised]]) so as to appear compound. Some species have leaves that are [[bipinnate]], 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 ([[fractal|self-similar geometry]]).
 
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[[File:Bowenia spectabilis.JPG|thumb|''[[Bowenia spectabilis]]'' : plant with single frond in the Daintree rainforest, north-east Queensland]]
 
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==Taxonomy==
 
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[[File:Cycad cone.jpg|right|thumb|Leaves and cone of ''[[Encephalartos sclavoi]]'']]
 
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The three extant [[family (biology)|families]] of cycads all belong to the order [[Cycadales]], and are [[Cycadaceae]], [[Stangeriaceae]], and [[Zamiaceae]]. These cycads have changed little since the Jurassic, compared to some major evolutionary changes in other plant divisions. Five additional families belonging to the [[Medullosales]] became extinct by the end of the Paleozoic Era.
 
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Cycads are most closely related to the extinct [[Bennettitales]], and are also relatively close relatives to the [[Ginkgoales]], as shown in the following phylogeny:{{citation needed|date=July 2015}}
 
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{|  style="border:solid 1px gray; margin:auto;"
 
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|{{phylogeny|Cycads|
 
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  ''[[Ginkgo]]''|
 
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      {{clade|Conifers|
 
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        label2=Anthophytes|
 
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          {{clade|Bennettitales|[[Gnetales]]|Angiosperms}}}}}}
 
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|{{clade |
 
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  1=[[angiosperm]]s (flowering plants)|
 
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  label2=[[gymnosperm]]s  |
 
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  2={{clade |
 
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    1=cycads  |
 
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    2=''[[Ginkgo]]'' |
 
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    3={{clade |
 
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        1=conifers |
 
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        2=gnetophytes
 
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    }}
 
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  }}
 
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}}
 
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|-
 
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!Traditional view
 
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!Modern view
 
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|}
 
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Classification of the Cycadophyta to the rank of family.
 
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;Class Cycadopsida
 
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:;Order Medullosales †
 
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:::Family [[Alethopteridaceae]]
 
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:::Family [[Cyclopteridaceae]]
 
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:::Family [[Neurodontopteridaceae]]
 
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:::Family [[Parispermaceae]]
 
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:;Order Cycadales
 
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::;Suborder Cycadineae
 
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:::Family [[Cycadaceae]]
 
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::;Suborder Zamiineae
 
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:::Family [[Stangeriaceae]]
 
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:::Family [[Zamiaceae]]
 
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==Historical diversity==
 
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The probable former range of cycads can be inferred from their global distribution. For example, the family Stangeriaceae only contains three extant species in Africa and Australia. Diverse fossils of this family have been dated to 135&nbsp;mya, indicating that diversity may have been much greater before the Jurassic and late Triassic [[Extinction event|mass extinction events]]. However, the cycad fossil record is generally poor and little can be deduced about the effects of each mass extinction event on their diversity.
 
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Instead, correlations can be made between the number of extant [[gymnosperms]] and [[angiosperms]]. It is likely that cycad diversity was affected more by the great angiosperm radiation in the mid-Cretaceous than by extinctions. Very slow cambial growth was first used to define cycads, and because of this characteristic the group could not compete with the rapidly growing, relatively short-lived angiosperms, which now number over 250,000&nbsp;species, compared to the 947&nbsp;remaining gymnosperms. It is surprising that the cycads are still extant, having been faced with extreme competition and five major extinctions. The ability of cycads to survive in relatively dry environments where plant diversity is generally lower, may explain their long persistence and longevity.
 
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===Origins===
 
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[[File:American fossil cycads (1906) (18143988832).jpg|thumb|Fossil images of Cycads found in America (1906)]]
 
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The cycad fossil record dates to the early [[Permian]], 280 million years ago ([[mya (unit)|mya]]). {{Citation needed|date=April 2016}} There is controversy over older cycad fossils that date to the late [[Carboniferous]] period, 300&ndash;325 &nbsp;mya. This [[clade]] probably diversified extensively within its first few million years, although the extent to which it radiated is unknown because relatively few fossil specimens have been found. The regions to which cycads are restricted probably indicate their former distribution in the [[Pangea]] before the supercontinents [[Laurasia]] and [[Gondwana]] separated.<ref>(Hermsen et al. 2006).</ref> Recent studies have indicated the common perception of existing cycad species as [[living fossil]]s is largely misplaced, with only ''[[Bowenia]]'' dating to the [[Cretaceous]] or earlier.  Although the cycad lineage itself is ancient, most extant species have evolved in the last 12 million years.<ref>{{cite journal | last1 = Nagalingum | first1 = N. S. | display-authors = etal  | year = 2011 | url = | journal = Science | volume =334 | issue =6057 | pages =796–799 | doi = 10.1126/science.1209926 |title=Recent Synchronous Radiation of a Living Fossil | pmid=22021670}}</ref>
 
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The [[family (biology)|family]] [[Stangeriaceae]] (named for Dr. [[William Stanger (surveyor)|William Stanger]], 1811&ndash;1854), consisting of only three [[extant taxon|extant]] [[species]], is thought to be of Gondwanan origin, as fossils have been found in [[Lower Cretaceous]] deposits in [[Argentina]], dating to 70&ndash;135&nbsp;mya. The family [[Zamiaceae]] is more diverse, with a fossil record extending from the middle [[Triassic]] to the [[Eocene]] (54&ndash;200&nbsp;mya) in [[North America|North]] and South America, [[Europe]], Australia, and [[Antarctic]]a, implying the family was present before the break-up of Pangea. The family [[Cycadaceae]] is thought to be an early offshoot from other cycads, with fossils from [[Eocene]] deposits (38&ndash;54&nbsp;mya) in [[Japan]], [[China]], and [[North America]],<ref>{{cite journal|last=Hopkins|first=DJ|author2=KR Johnson |title=First Record of cycad leaves from the Eocene Republic flora|journal=Washington Geology|date=December 1997|volume=25|issue=4|page=37|url=http://www.nwpaleo.org/Resources/WA_Geology/WA_Geol_1997-CycadLeaves.html}}</ref> indicating this family originated in Laurasia. ''[[Cycas]]'' is the only [[genus]] in the family and contains 99&nbsp;species, the most of any cycad genus. [[Molecular biology|Molecular]] data have recently shown ''Cycas'' species in [[Australasia]] and the east coast of Africa are recent arrivals, suggesting [[adaptive radiation]] may have occurred. The current distribution of cycads may be due to radiations from a few ancestral types sequestered on Laurasia and Gondwana, or could be explained by [[genetic drift]] following the separation of already evolved genera. Both explanations account for the strict [[endemism]] across present continental lines.
 
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==Distribution==
 
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{{see also|List of cycad species by country}}
 
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The living cycads are found across much of the [[subtropical]] and [[tropical]] parts of the world. The greatest diversity occurs in [[South America|South]] and [[Central America]].{{citation needed|date=July 2015}} They are also found in [[Mexico]], the [[Antilles]], southeastern [[United States of America|United States]], [[Australia]], [[Melanesia]], [[Micronesia]], [[Japan]], [[China]], [[Southeast Asia]], [[India]], [[Sri Lanka]], [[Madagascar]], and [[Southern Africa|southern]] and tropical [[Africa]], where at least 65 [[species]] occur. Some can survive in harsh [[desert]] or semi-desert [[climate]]s ([[xerophytic]]),<ref>{{citation |title=National Recovery Plan for the MacDonnell Ranges Cycad ''Macrozamia macdonnellii'' |url=http://www.environment.gov.au/system/files/resources/c17de516-c635-4c67-b040-c9db4da380e4/files/macrozamia-macdonnellii.pdf |publisher=Department of Natural Resources, Environment, The Arts and Sport, Northern Territory |accessdate=16 July 2015}}</ref> others in wet [[rain forest]] conditions,<ref>{{citation |author1=Bermingham, E. |author2=Dick, C.W. |author3=Moritz, C. |year=2005  |title=Tropical Rainforests: Past, Present, and Future |publisher=University of Chicago Press  |isbn=9780226044682 |url=https://books.google.ca/books?id=Y3dXZyCCDpEC}}</ref> and some in both.<ref>{{citation |chapter-url=http://www.iucnredlist.org/details/42000/0 |title=The IUCN Red List of Threatened Species |chapter=''Macrozamia communis''}}</ref> Some can grow in [[sand]] or even on [[rock (geology)|rock]], some in oxygen-poor, swampy, [[bog]]-like soils rich in [[organic material]].{{Citation needed|date=April 2011}} Some are able to grow in full sun, some in full shade, and some in both.{{Citation needed|date=April 2011}} Some are [[Sodium chloride|salt]] tolerant ([[halophyte]]s).{{citation needed|date=July 2015}}
 
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[[File:Cycads world distribution.png|thumb|800px|center|<center>Approximate world distribution of living Cycadales</center>]]
 
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Species diversity of the extant cycads peaks at 17˚ 15"N and 28˚ 12"S, with a minor peak at the [[equator]]. There is therefore not a [[Latitudinal gradients in species diversity|latitudinal diversity gradient]] towards the equator but towards the [[Tropic of Cancer]] and the [[Tropic of Capricorn]]. However, the peak near the northern tropic is largely due to ''Cycas'' in Asia and ''Zamia'' in the New World, whereas the peak near the southern tropic is due to ''Cycas'' again, and also to the diverse genus ''Encephalartos'' in southern and central Africa, and ''Macrozamia'' in 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 partly [[xerophytic]] rather than simply [[tropical]].
 
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==See also==
 
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*[[Fossil Cycad National Monument]], formerly in the U.S. state of South Dakota
 
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==References==
 
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{{Reflist}}
 
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==External links==
 
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{{commons|Cycadophyta}}
 
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* [http://palm-trees.org/ Palm Trees, Small Palms, Cycads, Bromeliads and tropical plants] site with thousands of large, high quality photos of cycads and associated flora. Includes information on habitat and cultivation.
 
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*Hill KD (1998–2004) The Cycad Pages, Royal Botanic Gardens Sydney. http://plantnet.rbgsyd.nsw.gov.au/PlantNet/cycad/index.html
 
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* [https://web.archive.org/web/20100810215325/http://www.conifers.org/zz/cycadales.htm Gymnosperm Database: Cycads]
 
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* [http://www.fairchildgarden.org/horticulture/n_collections.html Fairchild Tropical Botanic Garden- one of the largest collection of cycads in the world in Florida, U.S.A.]
 
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* [http://www.pacsoa.org.au/ Palm and Cycad Societies of Australia (PACSOA)]
 
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* [http://cycadsociety.org/ The Cycad Society of South Africa]
 
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* [http://academic.reed.edu/biology/Nitrogen/Nfix1.html Cycad nitrogen fixation]
 
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* [http://plantnet.rbgsyd.nsw.gov.au/PlantNet/cycad/toxic.html Cycad toxicity]
 
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* [http://www.kpr.sk/flora/cycads/ Cycads - Foto]
 
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* [https://www.nytimes.com/2005/08/28/magazine/28CYCADS.html?8hpib=&pagewanted=print ''The Cult of the Cycads''], [[New York Times]] Magazine article on cycad collectorship and cycad smuggling
 
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* [http://www.equisetites.de/palbot/taxa/cycads.html Cycads] An annotated link directory
 
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* "Biodiversity loss,.. continues at an alarming rate. [[Corals]], [[amphibians]] and cycads are in serious decline due to distinct and worsening threats." as per [http://www.un.org/sustainabledevelopment/blog/2017/07/pace-of-progress-must-accelerate-to-achieve-the-sdgs-finds-latest-un-progress-report/ "The UN Sustainable Development Goals Report 2017"]
 
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{{Plant classification}}
 
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{{Nuts}}
 
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{{Use dmy dates|date=April 2011}}
 
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'''
 
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[[Category:Cycads| ]]
 
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[[Category:Living fossils]]
 
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[[Category:Dioecious plants]]
 
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[[Category:Cisuralian first appearances]]
 
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[[Category:Extant Permian first appearances]]
 

Latest revision as of 13:18, 9 September 2017

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