Contents 1 Description 2 Taxonomy 3 Historical diversity 3.1 Origins 4 Distribution 5 See also 6 References 7 External links


Description[edit] Cycads have a rosette of pinnate leaves around cylindrical trunk Cycads have a cylindrical trunk which usually does not branch. 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 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 basal rosette. The leaves are generally large in proportion to the trunk size, and sometimes even larger than the trunk. The leaves are pinnate (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 has leaflets emerging from it as "ribs"), or have edges (margins) 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 (self-similar geometry). Bowenia spectabilis : plant with single frond in the Daintree rainforest, north-east Queensland


Taxonomy[edit] Leaves and cone of Encephalartos sclavoi The three extant 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. 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] Cycads Ginkgo Conifers Anthophytes Bennettitales Gnetales Angiosperms angiosperms (flowering plants) gymnosperms cycads Ginkgo conifers gnetophytes Traditional view Modern view Classification of the Cycadophyta to the rank of family. Class Cycadopsida Order Medullosales † Family Alethopteridaceae Family Cyclopteridaceae Family Neurodontopteridaceae Family Parispermaceae Order Cycadales Suborder Cycadineae Family Cycadaceae Suborder Zamiineae Family Stangeriaceae Family Zamiaceae


Historical diversity[edit] 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 mya, indicating that diversity may have been much greater before the Jurassic and late Triassic 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. 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 species, compared to the 1080 remaining gymnosperms[6]. 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. Origins[edit] Fossil images of Cycads found in America (1906) The cycad fossil record dates to the early Permian, 280 million years ago (mya).[citation needed] There is controversy over older cycad fossils that date to the late Carboniferous period, 300–325  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.[7] Recent studies have indicated the common perception of existing cycad species as living fossils 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.[8] petrified cycad fossil, New York Botanical Garden The family Stangeriaceae (named for Dr. William Stanger, 1811–1854), consisting of only three extant species, is thought to be of Gondwanan origin, as fossils have been found in Lower Cretaceous deposits in Argentina, dating to 70–135 mya. The family Zamiaceae is more diverse, with a fossil record extending from the middle Triassic to the Eocene (54–200 mya) in North and South America, Europe, Australia, and Antarctica, 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 (38–54 mya) in Japan, China, and North America,[9] indicating this family originated in Laurasia. Cycas is the only genus in the family and contains 99 species, the most of any cycad genus. 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.


Distribution[edit] See also: List of cycad species by country The living cycads are found across much of the subtropical and tropical parts of the world. The greatest diversity occurs in South and Central America.[citation needed] They are also found in Mexico, the Antilles, southeastern United States, Australia, Melanesia, Micronesia, Japan, China, Southeast Asia, India, Sri Lanka, Madagascar, and southern and tropical Africa, where at least 65 species occur. Some can survive in harsh desert or semi-desert climates (xerophytic),[10] others in wet rain forest conditions,[11] and some in both.[12] Some can grow in sand or even on rock, some in oxygen-poor, swampy, bog-like soils rich in organic material.[citation needed] Some are able to grow in full sun, some in full shade, and some in both.[citation needed] Some are salt tolerant (halophytes).[citation needed] Approximate world distribution of living Cycadales 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 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. Notes: The distribution area on the map should be expanded to include the range of Macrozamia macdonnelliana in the central region of Australia, Zamia boliviana in Bolivia and Mato Grosso, Brazil, Cycas thouarsii on Comoros and Seychelles, and Cycas micronesica on the islands of Guam, Palau, Rota, & Yap. Also, the depiction of cycad distribution in Africa, particularly the western boundary, should be improved to show the actual range limits, rather than national borders.


See also[edit] Fossil Cycad National Monument, formerly in the U.S. state of South Dakota


References[edit] ^ Brongniart, A. (1843). Énumération des genres de plantes cultivées au Musée d'histoire naturelle de Paris.  ^ Bessey, C.E. (1907). "A synopsis of plant phyla". Nebraska Univ. Stud. 7: 275–373.  ^ Rai, A.N.; 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, JSTOR 2588831 CS1 maint: Uses authors parameter (link) ^ Holtcamp, W. (2012). "The emerging science of BMAA: do cyanobacteria contribute to neurodegenerative disease?". Environmental Health Perspectives. 120 (3): a110–a116. doi:10.1289/ehp.120-a110. PMC 3295368 . PMID 22382274.  ^ 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. PMC 4795023 . PMID 26791617. CS1 maint: Uses authors parameter (link) ^ Christenhusz, M. J. M.; Byng, J. W. (2016). "The number of known plants species in the world and its annual increase". Phytotaxa. Magnolia Press. 261 (3): 201–217. doi:10.11646/phytotaxa.261.3.1. ^ (Hermsen et al. 2006). ^ Nagalingum, N. S.; et al. (2011). "Recent Synchronous Radiation of a Living Fossil". Science. 334 (6057): 796–799. doi:10.1126/science.1209926. PMID 22021670.  ^ Hopkins, DJ; KR Johnson (December 1997). "First Record of cycad leaves from the Eocene Republic flora". Washington Geology. 25 (4): 37.  ^ National Recovery Plan for the MacDonnell Ranges Cycad Macrozamia macdonnellii (PDF), Department of Natural Resources, Environment, The Arts and Sport, Northern Territory, retrieved 16 July 2015  ^ Bermingham, E.; Dick, C.W.; Moritz, C. (2005), Tropical Rainforests: Past, Present, and Future, University of Chicago Press, ISBN 9780226044682  ^ "Macrozamia communis", The IUCN Red List of Threatened Species 


External links[edit] Wikimedia Commons has media related to Cycadophyta. 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. Hill KD (1998–2004) The Cycad Pages, Royal Botanic Gardens Sydney. http://plantnet.rbgsyd.nsw.gov.au/PlantNet/cycad/index.html Gymnosperm Database: Cycads Fairchild Tropical Botanic Garden- one of the largest collection of cycads in the world in Florida, U.S.A. Palm and Cycad Societies of Australia (PACSOA) The Cycad Society of South Africa Cycad nitrogen fixation Cycad toxicity Cycads - Foto The Cult of the Cycads, New York Times Magazine article on cycad collectorship and cycad smuggling Cycads An annotated link directory "Biodiversity loss,.. continues at an alarming rate. Corals, amphibians and cycads are in serious decline due to distinct and worsening threats." as per "The UN Sustainable Development Goals Report 2017" v t e Classification of Archaeplastida / Plantae sensu lato Domain Archaea Bacteria Eukaryota (Supergroup Plant Hacrobia Heterokont Alveolata Rhizaria Excavata Amoebozoa Opisthokonta Animal Fungi) Rhodophyta (red algae) Cyanidiophyceae Porphyridiophyceae Compsopogonophyceae Stylonematophyceae Rhodellophyceae Bangiophyceae Florideophyceae Glaucocystophyta (glaucophytes) Glaucocystophyceae Glaucocystis Cyanophora Gloeochaete Viridiplantae (green algae & land plants) Chlorophyta Palmophyllales Nephroselmidophyceae Prasinophyceae Pseudoscourfieldiales Pyramimonadophyceae Scourfieldiales Pedinophyceae Chlorodendrophyceae UTC clade Ulvophyceae Trebouxiophyceae Chlorophyceae Streptophyta (charophytes, & land plants) Mesostigmatophyceae Chlorokybophyceae Klebsormidiophyceae Phragmo- plastophyta Charophyceae Coleochaetophyceae Zygnematophyceae Embryophyta (land plants) Bryophytes (non-vascular) Marchantiophyta Anthocerotophyta Bryophyta "Moss" †Horneophytopsida Tracheophyta (vascular) Lycopodiophyta (microphylls) †Zosterophyllopsida †Sawdoniales Isoetopsida Lycopodiopsida Euphyllophyta (megaphylls) Moniliformopses (ferns) †Cladoxylopsida †Stauropteridales †Zygopteridales Equisetopsida Psilotopsida Marattiopsida Filicopsida Spermatophyta (seed plants) †Seed ferns Gymnosperms Gnetopsida Pinopsida Cycadopsida Ginkgoopsida Angiosperms or flowering plants Amborellales Nymphaeales Austrobaileyales Magnoliids Monocots Eudicots Other †Trimerophytopsida †Progymnosperm Other †Rhyniopsida † = extinct. See also the list of plant orders. v t e Nuts True, or botanical nuts Acorn Beech American beech European beech Breadnut Candlenut Chestnut Sweet chestnut Hazelnut American hazel Beaked hazel European hazel Filbert Asian hazel Johnstone River almond Kola nut Kurrajong Malabar chestnut Mongongo Palm nut Karuka Planted karuka Wild karuka Red bopple nut Yellow walnut Drupes Almond Australian cashew nut Betel nut Borneo tallow nut Breadfruit Cashew Chilean hazel Coconut Durian Gabon nut Hickory Mockernut hickory Pecan Shagbark hickory Shellbark hickory Irvingia gabonensis Jack nut Panda oleosa Pekea nut Pili nut Pistachio Walnut Black walnut Butternut English walnut Heartnut Gymnosperms Cycad Burrawang nut Ginkgo nut Araucaria spp. Bunya nut Monkey-puzzle nut Pine nut Chilgoza pine Colorado pinyon Korean pine Mexican pinyon Single-leaf pinyon Stone pine Angiosperms Brazil nut Macadamia Macadamia nut Queensland macadamia nut Paradise nut Peanut Peanut tree Soybean Retrieved from "https://en.wikipedia.org/w/index.php?title=Cycad&oldid=811832282" Categories: CycadsLiving fossilsDioecious plantsCisuralian first appearancesExtant Permian first appearancesHidden categories: CS1 maint: Uses authors parameterArticles with 'species' microformatsAll articles with unsourced statementsArticles with unsourced statements from July 2015Articles with unsourced statements from April 2016Articles with unsourced statements from April 2011Use dmy dates from April 2011


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