Cycads/ˈsaɪkædz/ areseed plants that typically have a stout and woody (ligneous)trunk with 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 specific species ofbeetle. Both male and female cycads bear cones (strobili), somewhat similar toconifer cones.
Cycads have been reported tofix nitrogen in association with variouscyanobacteria living in the roots (the "coralloid" roots).[4] These photosynthetic bacteria produce aneurotoxin calledBMAA that is found in theseeds 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 hypothesized that this is a source of someneurological diseases in 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][9][10]
Cycads have a rosette of pinnate leaves around a cylindrical trunk
Cycads have acylindrical trunk 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.[11] Leaves grow directly from the trunk, and typically fall when older, leaving a crown of leaves at the top. The leaves grow in arosette, 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 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 (withleaflets emerging from the leaf stalk as "ribs"), or have edges (margins) so deeply cut (incised) so as to appear compound. The Australian genusBowenia and some Asian species of Cycas, likeCycas multipinnata,C. micholitzii andC. debaoensis, have leaves that arebipinnate, the leaflets each having their own subleaflets, growingin the same form on the leaflet as the leaflets do on the stalk.[12][13]
Due to superficial similarities in foliage and plant structure, cycads andpalms are often mistaken for each other. They also can occur in similar climates. However, they belong to differentphyla and 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 aregymnosperms and bear cones (strobili), while palms areangiosperms and so flower and bear fruit. The mature foliage looks very similar between both groups, but the young emerging leaves of a cycad resemble afiddlehead fern before 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 arehelically arranged 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.[14]
The two extantfamilies of cycads both belong to the orderCycadales, and are theCycadaceae andZamiaceae (includingStangeriaceae). These cycads have changed little since the Jurassic in comparison to some other plant divisions. Five additional families belonging to theMedullosales became extinct by the end of the Paleozoic Era.
Based on genetic studies, cycads are thought to be more closely related toGinkgo than to other living gymnosperms. Both are thought to have diverged from each other during the earlyCarboniferous.[15][16]
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.[26] Cycads were generally uncommon during the Permian.[27] The two living cycad families are thought to have split from each other sometime between the Jurassic[17] and Carboniferous.[28] Cycads are thought to have reached their apex of diversity during the Mesozoic.[29] Although the Mesozoic is sometimes called the "Age of Cycads," some other groups of extinct seed plants with similar foliage, such asBennettitales andNilssoniales, that are not closely related, may have been more abundant.[30] The oldest records of the modern genusCycas are from the Paleogene of East Asia.[31] Fossils assignable to Zamiaceae are known from the Cretaceous,[30] with fossils assignable to living genera of the family known from the Cenozoic.[18]
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 gradient towards the equator but towards theTropic of Cancer and theTropic of Capricorn. However, the peak near the northern tropic is largely due toCycas in Asia andZamia in the New World, whereas the peak near the southern tropic is due toCycas again, and also to the diverse genusEncephalartos in southern and central Africa, andMacrozamia 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 partlyxerophytic rather than simplytropical.[citation needed]
Nuts of theCycas orientis (nyathu) are coveted by theYolngu in Australia'sArnhem Land as a source of food. 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.[36] Roots ofZamia integrifolia were used by theSeminole and other native peoples to produceFlorida arrowroot by a similar process.
InVanuatu, the cycad is known asnamele and is an important symbol of traditional culture. It serves as a powerfultaboo sign,[37] and a pair ofnamele leaves appears on thenational flag and coat of arms. Together with thenanggaria plant, another symbol of Vanuatu culture, thenamele also gives its name toNagriamel, an indigenous political movement.
^Hermsen, Elizabeth J.; Taylor, Edith L.; Taylor, Thomas N. (January 2009). "Morphology and ecology of theAntarcticycas plant".Review of Palaeobotany and Palynology.153 (1–2):108–123.Bibcode:2009RPaPa.153..108H.doi:10.1016/j.revpalbo.2008.07.005.
^Spiekermann, Rafael; Jasper, André; Siegloch, Anelise Marta; Guerra-Sommer, Margot; Uhl, Dieter (June 2021). "Not a lycopsid but a cycad-like plant:Iratinia australis gen. nov. et sp. nov. from the Irati Formation, Kungurian of the Paraná Basin, Brazil".Review of Palaeobotany and Palynology.289: 104415.Bibcode:2021RPaPa.28904415S.doi:10.1016/j.revpalbo.2021.104415.S2CID233860955.
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