Thegymnosperms (/ˈdʒɪmnəˌspɜːrmz,-noʊ-/ⓘnə-spurmz, -noh-;lit.'revealed seeds') are a group of woody, perennialseed-producing plants, typically lacking the protective outer covering which surrounds the seeds in flowering plants, that includeconifers,cycads,Ginkgo, andgnetophytes, forming the cladeGymnospermae.[2] The termgymnosperm comes from the composite word inGreek:γυμνόσπερμος (γυμνός,gymnos, 'naked' andσπέρμα,sperma, 'seed'), and literally means 'naked seeds'. The name is based on the unenclosed condition of their seeds (calledovules in their unfertilized state). The non-encased condition of their seeds contrasts with the seeds and ovules of flowering plants (angiosperms), which are enclosed within anovary. Gymnosperm seeds develop either on the surface of scales orleaves, which are often modified to formcones, or on their own as inyew,Torreya, andGinkgo.
The life cycle of a gymnosperm involvesalternation of generations, with a dominantdiploidsporophyte phase, and a reducedhaploidgametophyte phase, which is dependent on the sporophytic phase.[3] The term "gymnosperm" is often used inpaleobotany to refer to (theparaphyletic group of) all non-angiosperm seed plants. In that case, to specify the modernmonophyletic group of gymnosperms, the termAcrogymnospermae is sometimes used.[4]
The gymnosperms andangiosperms together constitute thespermatophytes or seed plants. The spermatophytes are subdivided into fivedivisions, the angiosperms and four divisions of gymnosperms: theCycadophyta,Ginkgophyta,Gnetophyta, andPinophyta (also known as Coniferophyta). Newer classification place the gnetophytes among the conifers.[5] Numerous extinct seed plant groups are recognised including those consideredpteridosperms/seed ferns, as well other groups like the Bennettitales.[6]
By far the largest group of living gymnosperms are the conifers (pines, cypresses, and relatives), followed by cycads, gnetophytes (Gnetum,Ephedra andWelwitschia), andGinkgo biloba (a single living species). About 65% of gymnosperms aredioecious,[7] but conifers are almost allmonoecious.[8] Some genera haveectomycorrhiza fungal associations with roots (Pinus),[9] while in some others (Cycas) small specialised roots called coralloid roots are associated with nitrogen-fixingcyanobacteria.[10]
Over 1,000 living species of gymnosperm exist.[11] It was previously widely accepted that the gymnosperms originated in theLate Carboniferous period, replacing thelycopsid rainforests of the tropical region, but more recent phylogenetic evidence indicates that they diverged from the ancestors ofangiosperms during theEarly Carboniferous.[12][13] The radiation of gymnosperms during the late Carboniferous appears to have resulted from a wholegenome duplication event around319 million years ago.[14] Early characteristics of seed plants are evident in fossilprogymnosperms of the lateDevonian period around 383 million years ago. It has been suggested that during the mid-Mesozoic era, pollination of some extinct groups of gymnosperms was by extinct species ofscorpionflies that had specializedproboscis for feeding on pollination drops. The scorpionflies likely engaged in pollination mutualisms with gymnosperms, long before the similar and independent coevolution of nectar-feeding insects on angiosperms.[15][16] Evidence has also been found that mid-Mesozoic gymnosperms were pollinated byKalligrammatid lacewings, a now-extinct family with members which (in an example ofconvergent evolution) resembled the modern butterflies that arose far later.[17]
Conifers are by far the most abundant extant group of gymnosperms with six to eight families, with a total of 65–70 genera and 600–630 species (696 accepted names).[23] Most conifers areevergreens.[24] Theleaves of many conifers are long, thin and needle-like, while other species, including mostCupressaceae and somePodocarpaceae, have flat, triangular scale-like leaves.Agathis in Araucariaceae andNageia in Podocarpaceae have broad, flat strap-shaped leaves.[citation needed]
Cycads, small palm-like trees,[2] are the next most abundant group of gymnosperms, with two or three families, 11 genera, and approximately 338 species. A majority of cycads are native to tropical climates and are most abundantly found in regions near the equator. The other extant groups are the 95–100 species ofGnetophytes and one species ofGinkgo. The ginkgo or maidenhair trees are tall and have bilobed leaves, while gnetophytes are a diverse groups of plants and shrubs including the horizontally growingwelwitschia[6]
Today, gymnosperms are the most threatened of all plant groups.[25]
A formal classification of the living gymnosperms is the "Acrogymnospermae", which form amonophyletic group within thespermatophytes.[30][31] The wider "Gymnospermae" group includes extinct gymnosperms and is thought to beparaphyletic. The fossil record of gymnosperms includes many distinctivetaxa that do not belong to the four modern groups, including seed-bearing trees that have a somewhatfern-like vegetative morphology (the so-called "seed ferns" orpteridosperms).[32] When fossil gymnosperms such as these and theBennettitales,glossopterids, andCaytonia are considered, it is clear that angiosperms are nested within a larger gymnospermae clade, although which group of gymnosperms is their closest relative remains unclear.
The extant gymnosperms include 12 main families and 83 genera which contain more than 1000 known species.[11][31][33]
Gymnosperms, like allvascular plants, have a sporophyte-dominant life cycle, which means they spend most of their life cycle with diploid cells, while thegametophyte (gamete-bearing phase) is relatively short-lived. Like allseed plants, they areheterosporous, having two spore types,microspores (male) produced inmicrosporangium andmegaspores (female) produced inmegasporangium that are typically present in pollen cones or ovulate cones respectively. The microsporangium is carried bymicrosporophyll (modified leaf) and seeds are carried by ovuliferous scales in the male and female cones respectively.[2][34] The exception is the females in the cycad genusCycas, which form a loose structure called megasporophylls instead of cones.[35] As with all heterosporous plants, the gametophytes develop within the spore wall. Pollen grains (microgametophytes) mature from microspores, and ultimately produce sperm cells.[34] Megagametophytes develop from megaspores and are retained within the ovule. Gymnosperms produce multiplearchegonia, which produce the female gametes.[citation needed]
During pollination, pollen grains are physically transferred between plants from the pollen cone to the ovule. Pollen is usually moved by wind or insects. Whole grains enter each ovule through a microscopic gap in the ovule coat (integument) called the micropyle. The pollen grains mature further inside the ovule and produce sperm cells. Two main modes of fertilization are found in gymnosperms. Cycads andGinkgo haveflagellated motile sperm[36] that swim directly to the egg inside the ovule, whereas conifers andgnetophytes have sperm with no flagella that are moved along apollen tube to the egg. Aftersyngamy (joining of the sperm and egg cell), the zygote develops into an embryo (young sporophyte). More than one embryo is usually initiated in each gymnosperm seed. The mature seed comprises the embryo and the remains of the femalegametophyte, which serves as a food supply, and theseed coat.[37]
Gymnosperms ordinarily reproduce bysexual reproduction, and only rarely express parthenogenesis.[38] Sexual reproduction in gymnosperms appears to be required for maintaining long-termgenomic integrity.[38]Meiosis in sexual land plants provides a direct mechanism forrepairing DNA in reproductive tissues.[38] The likely primary benefit of cross-pollination in gymnosperms, as in other eukaryotes, is that it allows the avoidance of inbreeding depression caused by the presence of recessive deleterious mutations.[39]
Gymnosperms have major economic uses. Some, such as pine, fir, spruce, and cedar, are used forlumber, paper production, and resin. Some other common uses for gymnosperms aresoap,varnish,nail polish, food, gum, andperfumes.[41]
^Li, Hong-Tao; Yi, Ting-Shuang; Gao, Lian-Ming; Ma, Peng-Fei; Zhang, Ting; Yang, Jun-Bo; Gitzendanner, Matthew A.; Fritsch, Peter W.; Cai, Jie; Luo, Yang; Wang, Hong (May 2019). "Origin of angiosperms and the puzzle of the Jurassic gap".Nature Plants.5 (5):461–470.Bibcode:2019NatPl...5..461L.doi:10.1038/s41477-019-0421-0.PMID31061536.S2CID146118264.
^Bond, W. J. (March 1989). "The tortoise and the hare: ecology of angiosperm dominance and gymnosperm persistence".Biological Journal of the Linnean Society.36 (3):227–249.doi:10.1111/j.1095-8312.1989.tb00492.x.
Cantino, Philip D.; Doyle, James A.; Graham, Sean W.; Judd, Walter S.; Olmstead, Richard G.; Soltis, Douglas E.; Soltis, Pamela S.; Donoghue, Michael J. (August 2007). "Towards a phylogenetic nomenclature of Tracheophyta".Taxon.56 (3):822–846.doi:10.2307/25065864.JSTOR25065864.
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