| Cycadales | |
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| Cycas rumphii with old and new male strobili | |
Scientific classification | |
| Kingdom: | Plantae |
| Clade: | Tracheophytes |
| Clade: | Spermatophytes |
| Clade: | Gymnospermae |
| Division: | Cycadophyta Bessey 1907: 321.[2] |
| Class: | Cycadopsida Brongn.[1] |
| Order: | Cycadales Pers. ex Bercht. & J. Presl |
| Extant groupings | |
| Synonyms | |
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Cycads/ˈsaɪkædz/—constituting thedivisionCycadophyta—areseed plants with a stout, woody cylindricaltrunk with acrown of large, hard, stiff,evergreen and usuallypinnate (feather-shaped) leaves. 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 slowly and have long lifespans. They superficially resemblepalms orferns, but are not closely related to either group. Cycads aregymnosperms. Cycads have specializedpollinators, usually a specificbeetle, and more rarely athrips or amoth.
Both male and female cycads bear cones (strobili) containing their seeds, somewhat resemblingconifer cones. Cycadsfix nitrogen in association withcyanobacteria living in the plants' roots. Some species are used asnarcotics, while inVanuatu the plant symbolizes peace and appears on the national flag. Cycads all over the world are in decline, with four species on the brink of extinction and seven species each with fewer than 100 plants left in the wild.
Cycads areseed plants with a stout, woody, and usually unbranched cylindricaltrunk, and acrown of large, hard, stiff,evergreen and (usually)pinnate leaves. 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 slowly[3] and have long lifespans. Because of their superficial resemblance topalms orferns, they are sometimes mistaken for them, but they are not closely related to either group.Cycads aregymnosperms (naked-seeded), meaning theirunfertilized seeds 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 specificbeetle, and more rarely athrips or amoth.[4]
The leaves arepinnate (shaped like feathers), with a central leaf stalk from which parallel ribs emerge from each side of the stalk, perpendicular to it. The leaves are typically eithercompound, or havemargins so deeply cut as to appear compound. The Australian genusBowenia and some Asian species likeCycas multipinnata,C. micholitzii andC. debaoensis, havebipinnate leaves, the leaflets each having subleaflets.[5] The apex of the stem is protected by modified leaves calledcataphylls.[6]
Cycads superficially resemblepalms in foliage and plant structure, occur in similar climates, and are often mistaken for them. However, they are so distantly related that they are classified in differentphyla. Their similarities are caused byconvergent evolution. Differences between cycads and palms include the cones (strobili) of cycads: they aregymnosperms, whereas palms areflowering plants and bear fruit. Both groups' mature foliage look similar, but young emerging cycad leaves – before they unfold and shift into place in the rosette crown – resemble afiddlehead fern; in contrast, new leaves of palms are just miniature versions of a mature frond. Another difference is in thestem: Both phyla show scarring on their stems – below the rosette, where leaves used to attach – but the scars on a cycad's trunks arehelically arranged and small; the scars on palm trunks are a circle, that wraps around the whole stem. The stems of cycads are generally rougher and shorter than those of palms.[7]
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The oldest probable cycad foliage is known from thelatest Carboniferous /early Permian (around 300 million years ago) of South Korea and China, such asCrossozamia. Unambiguous fossils are known from the early /middle Permian onwards.[8] Cycads were uncommon during the Permian.[9] The two living cycad families diverged from each other at some time between theCarboniferous[10] and theJurassic.[11] Cycads are thought to have reached their apex of diversity during theMesozoic.[12] Although the Mesozoic is sometimes called"The Age of Cycads", some other groups of distantly related extinct seed plants with similar foliage, such asBennettitales andNilssoniales, were considerably more abundant than cycads during the Mesozoic: the "true" cycads were only minor components of Mesozoic vegetation.[13] The oldest records of the modern genusCycas are from thePaleogene of east Asia.[14] Fossils assignable toZamiaceae are known from theCretaceous,[13] with fossils assignable to living genera of the family from theCenozoic.[15]
The two extantfamilies of cycads both belong to the orderCycadales, and are theCycadaceae andZamiaceae (includingStangeriaceae). These cycads may have changed little since the Jurassic in comparison to some other plant divisions, but are by no means "living fossils" as they have continued to evolve.[11] Based on genetic studies, cycads are thought to be more closely related toGinkgo than to other living gymnosperms. They diverged from each other during the earlyCarboniferous.[16][17]
The Cycads have traditionally been thought to be basal within the seed plants. A more modern view is that they are Gymnosperms.[16][17]
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The Cycads are in two clades, the Cycadaceae and the Zamiaceae.[11][15]
| Cycads |
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Classification of extant Cycadophyta to genus:[18]
The living cycads are found across much of thesubtropical andtropical parts of the world, with a few in temperate regions such as in Australia.[20] The greatest diversity is in the Americas, but they also grow in China, South and Southeast Asia, Pacific islands, and southern and tropical Africa.[21][19] Some arexerophytes that can survive indesert or semi-desert climates,[22] others in wetrain forest conditions,[23] and some in both.[24]
Cycads accommodatenitrogen-fixingcyanobacteria in their coralloid roots.[25] The cyanobacteria produce aneurotoxin,BMAA, that accumulates in the plant'sseeds.[26][27] Anotherdefence 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 microbe, most likely a fungus, which gives them the ability to produce an insecticidal toxin.[28]
Nuts ofCycas orientis (nyathu) are eaten by theYolngu in Australia'sArnhem Land. They are harvested on theirdry season to leach its poison under water overnight before ground into a paste, wrapped under bark and cooked on open fire until done.[29] A pair ofnamele cycad leaves, representing peace, appears on theFlag of Vanuatu.[30][31] Cycads are used asnarcotics in Mexico, where they are among the substances called "peyote", while in South Africa,Encephalartos is used for the same purpose. In both regions, collecting for the drugs market is harming wild cycad populations.[32] 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.[33][34][35] TheFossil Cycad National Monument was a protected area inSouth Dakota from 1922, containing many fossils ofCycadeoidea.[36] After vandalism of the exposed fossils, the status was withdrawn in 1957.[37]
The leaves of cycads ('namele' in Bislama) appear as one of the most prominent objects in the symbolic vocabulary of Vanuatu archipelago since the time the first ethnographic accounts on the territory were written. They continue to occupy a privileged place in the country's culture today: namele leaves are portrayed on Vanuatu's flag, used in the practice of traditional courts, and chiefly authorities.
Cycads are ingested for narcotic purposes and are likely hallucinogens. The Nahuatl term 'peyote' ...
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