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Plants mayreproducesexually orasexually. Sexual reproduction produces offspring by the fusion ofgametes, resulting in offspring genetically different from either parent.Vegetative reproduction produces new individuals without the fusion of gametes, resulting in clonal plants that are genetically identical to the parent plant and each other, unlessmutations occur. In asexual reproduction, only one parent is involved.
Asexual reproduction does not involve the production and fusion of male and female gametes. Asexual reproduction may occur throughbudding,fragmentation,spore formation, regeneration andvegetative propagation.
Asexual reproduction is a type of reproduction where the offspring comes from one parent only, thus inheriting the characteristics of the parent. Asexual reproduction in plants occurs in two fundamental forms,vegetative reproduction andagamospermy.[1] Vegetative reproduction involves a vegetative piece of the original plant producing new individuals by budding,tillering, etc. and is distinguished fromapomixis, which is a replacement of sexual reproduction, and in some cases involves seeds. Apomixis occurs in many plant species such as dandelions (Taraxacum species) and also in some non-plant organisms. For apomixis and similar processes in non-plant organisms, seeparthenogenesis.
Natural vegetative reproduction is aprocess mostly found inperennial plants, and typically involves structural modifications of thestem orroots and in a few speciesleaves. Most plant species that employ vegetative reproduction do so as a means to perennialize the plants, allowing them to survive from one season to the next and often facilitating their expansion in size. A plant that persists in a location through vegetative reproduction of individuals gives rise to aclonal colony. A singleramet, or apparent individual, of a clonal colony is genetically identical to all others in the same colony. The distance that a plant can move during vegetative reproduction is limited, though some plants can produce ramets from branching rhizomes or stolons that cover a wide area, often in only a few growing seasons. In a sense, this process is not one of reproduction but one of survival and expansion of biomass of the individual. When an individualorganism increases in size via cell multiplication and remains intact, the process is called vegetative growth. However, in vegetative reproduction, the new plants that result are new individuals in almost every respect except genetic. A major disadvantage of vegetative reproduction is the transmission of pathogens from parent to offspring. It is uncommon for pathogens to be transmitted from the plant to its seeds (in sexual reproduction or in apomixis), though there are occasions when it occurs.[2][page needed]
Seeds generated by apomixis are a means of asexual reproduction, involving the formation and dispersal of seeds that do not originate from the fertilization of theembryos.Hawkweeds (Hieracium),dandelions (Taraxacum), some species ofCitrus andKentucky blue grass (Poa pratensis) all use this form of asexual reproduction.Pseudogamy occurs in some plants that have apomictic seeds, where pollination is often needed to initiate embryo growth, though the pollen contributes no genetic material to the developing offspring.[3] Other forms of apomixis occur in plants also, including the generation of aplantlet in replacement of a seed or the generation ofbulbils instead of flowers, where new cloned individuals are produced.
Arhizome is a modified underground stem serving as an organ of vegetative reproduction; the growing tips of the rhizome can separate as new plants, e.g.,polypody,iris,couch grass andnettles.
Prostrate aerial stems, called runners orstolons, are important vegetative reproduction organs in some species, such as thestrawberry, numerousgrasses, and someferns.
Adventitious buds form on roots near the ground surface, on damaged stems (as on the stumps of cut trees), or on old roots. These develop into above-ground stems and leaves. A form of budding calledsuckering is the reproduction orregeneration of a plant by shoots that arise from an existingroot system. Species that characteristically produce suckers includeelm (Ulmus)[4]: 299 and many members of therose family such asRosa,[4]: 285–296 Kerria[4]: 206 andRubus.[4]: 258
Bulbous plants such asonion (Allium cepa),hyacinths,narcissi andtulips reproduce vegetatively by dividing their undergroundbulbs into more bulbs. Other plants likepotatoes (Solanum tuberosum) anddahlias reproduce vegetatively from undergroundtubers.Gladioli andcrocuses reproduce vegetatively in a similar way withcorms.
Gemmae are single cells or masses of cells that detach from plants to form new clonal individuals. These are common inLiverworts andmosses and in thegametophyte generation of somefilmy fern. They are also present in someClub mosses such asHuperzia lucidula.[5] They are also found in some higher plants such as species ofDrosera.
The most common form of plant reproduction used by people is seeds, but a number of asexual methods are used which are usually enhancements of natural processes, including: cutting, grafting, budding,layering, division, sectioning of rhizomes, roots, tubers, bulbs, stolons, tillers, etc., and artificial propagation by laboratorytissue cloning. Asexual methods are most often used to propagatecultivars with individual desirable characteristics that do not come true from seed.[6]Fruit tree propagation is frequently performed by budding or grafting desirable cultivars (clones), ontorootstocks that are also clones, propagated bystooling.
In horticulture, a cutting is a branch that has been cut off from a motherplant below aninternode and then rooted, often with the help of arooting liquid or powder containinghormones. When a full root has formed and leaves begin to sprout anew, the clone is a self-sufficient plant,[7] genetically identical.
Examples include cuttings from the stems ofblackberries (Rubus occidentalis),African violets (Saintpaulia),verbenas (Verbena) to produce new plants. A related use of cuttings isgrafting, where a stem or bud is joined onto a different stem.Nurseries offer for sale trees with grafted stems that can produce four or more varieties of related fruits, includingapples. The most common usage of grafting is the propagation of cultivars onto already rooted plants, sometimes the rootstock is used to dwarf the plants or protect them from root damagingpathogens.[8]
Since vegetatively propagated plants are clones, they are important tools in plant research. When a clone is grown in various conditions, differences in growth can be ascribed to environmental effects instead of genetic differences.[7]
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| Biological terms |
| Sexual reproduction |
| Sexuality |
| Sexual system |
Sexual reproduction involves two fundamental processes:meiosis, which rearranges thegenes and reduces the number ofchromosomes, andfertilization, which restores the chromosome to a completediploid number. In between these two processes, different types ofplants andalgae vary, but many of them, including allland plants, undergoalternation of generations, with two different multicellular structures (phases), agametophyte and asporophyte. The evolutionary origin and adaptive significance of sexual reproduction are discussed in the pagesEvolution of sexual reproduction andOrigin and function of meiosis.
The gametophyte is the multicellular structure (plant) that ishaploid, containing a single set of chromosomes in each cell. The gametophyte produces male or femalegametes (or both), by a process of cell division, calledmitosis. In vascular plants with separate gametophytes, female gametophytes are known as mega gametophytes (mega=large, they produce the large egg cells) and the male gametophytes are called micro gametophytes (micro=small, they produce the small sperm cells).
The fusion of male and female gametes (fertilization) produces a diploidzygote, which develops by mitotic cell divisions into a multicellular sporophyte.
The mature sporophyte producesspores by meiosis, sometimes referred to as reduction division because the chromosome pairs are separated once again to form single sets.
In mosses and liverworts, the gametophyte is relatively large, and the sporophyte is a much smaller structure that is never separated from the gametophyte. Inferns,gymnosperms, andflowering plants (angiosperms), the gametophytes are relatively small and the sporophyte is much larger. In gymnosperms and flowering plants the megagametophyte is contained within theovule (that may develop into a seed) and the microgametophyte is contained within apollen grain.
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Unlike animals, plants are immobile, and cannot seek out sexual partners for reproduction. In the evolution of early plants, abiotic means, including water and much later, wind, transportedsperm for reproduction. The first plants wereaquatic, as described in the pageEvolutionary history of plants, and released sperm freely into the water to be carried with the currents. Primitive land plants such as liverworts and mosses had motile sperm that swam in a thin film of water or were splashed in water droplets from the male reproduction organs onto the female organs.[9] As taller and more complex plants evolved, modifications in the alternation of generations evolved. In thePaleozoic eraprogymnosperms reproduced by using spores dispersed on the wind. The seed plants includingseed ferns,conifers andcordaites, which were allgymnosperms, evolved about 350 million years ago.[10] They had pollen grains that contained the malegametes for protection of the sperm during the process of transfer from the male to female parts.
It is believed that insects fed on the pollen, and plants thus evolved to useinsects to actively carry pollen from one plant to the next.[11] Seed producing plants, which include the angiosperms and the gymnosperms, have a heteromorphic alternation of generations with large sporophytes containing much-reduced gametophytes. Angiosperms have distinctive reproductive organs called flowers, withcarpels, and the female gametophyte is greatly reduced to a female embryo sac, with as few as eight cells. Each pollen grains contains a greatly reduced male gametophyte consisting of three or four cells. The sperm of seed plants are non-motile, except for two older groups of plants, theCycadophyta and theGinkgophyta, which have flagella.
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Flowering plants, the dominant plant group,[12]: 168 reproduce both by sexual and asexual means. Their distinguishing feature is that their reproductive organs are contained inflowers. Sexual reproduction in flowering plants involves the production of separate male and female gametophytes that producegametes.
Theanther producespollen grains that contain malegametophytes. The pollen grains attach to the stigma on top of acarpel, in which the female gametophytes (inside ovules) are located. Plants may eitherself-pollinate orcross-pollinate. The transfer of pollen (the male gametophytes) to the femalestigmas occurs is calledpollination. After pollination occurs, the pollen grain germinates to form a pollen tube that grows through the carpel's style and transports male nuclei to the ovule to fertilize the egg cell and central cell within the female gametophyte in a process termeddouble fertilization. The resulting zygote develops into an embryo, while the triploid endosperm (one sperm cell plus a binucleate female cell) and female tissues of the ovule give rise to the surrounding tissues in the developing seed.[13] The fertilized ovules develop into seeds within afruit formed from the ovary. When the seeds are ripe they may bedispersed together with the fruit or freed from it by various means togerminate and grow into the next generation.
Plants that use insects or other animals to move pollen from one flower to the next have developed greatly modified flower parts to attract pollinators and to facilitate the movement of pollen from one flower to the insect and from the insect to the next flower. Flowers of wind-pollinated plants tend to lack petals and or sepals; typically large amounts of pollen are produced and pollination often occurs early in the growing season before leaves can interfere with the dispersal of the pollen. Many trees and all grasses and sedges are wind-pollinated.
Plants have a number of different means to attract pollinators including color, scent, heat, nectar glands, edible pollen and flower shape. Along with modifications involving the above structures two other conditions play a very important role in the sexual reproduction of flowering plants, the first is the timing of flowering and the other is the size or number of flowers produced. Often plant species have a few large, very showy flowers while others produce many small flowers, often flowers are collected together into large inflorescences to maximize their visual effect, becoming more noticeable to passing pollinators. Flowers are attraction strategies and sexual expressions are functional strategies used to produce the next generation of plants, with pollinators and plants having co-evolved, often to some extraordinary degrees, very often rendering mutual benefit.
The largest family of flowering plants is the orchids (Orchidaceae), estimated by some specialists to include up to 35,000 species,[14] which often have highly specialized flowers that attract particular insects for pollination. The stamens are modified to produce pollen in clusters calledpollinia, which become attached to insects that crawl into the flower. The flower shapes may force insects to pass by the pollen, which is "glued" to the insect. Some orchids are even more highly specialized, with flower shapes that mimic the shape of insects to attract them to attempt to 'mate' with the flowers, a few even have scents that mimic insectpheromones.
Another large group of flowering plants is theAsteraceae or sunflower family with close to 22,000 species,[15] which also have highly modified inflorescences composed of many individual flowers called florets. Heads with florets of one sex, when the flowers are pistillate or functionally staminate or made up of all bisexual florets, are called homogamous and can include discoid and liguliflorous type heads. Some radiate heads may be homogamous too. Plants with heads that have florets of two or more sexual forms are called heterogamous and include radiate and disciform head forms.
Ferns typically produce large diploids with stem, roots, and leaves. On fertile leavessporangia are produced, grouped together insori and often protected by anindusium. If the spores are deposited onto a suitable moist substrate they germinate to produce short, thin, free-living gametophytes calledprothalli that are typically heart-shaped, small and green in color. The gametophytes produce both motile sperm in theantheridia and egg cells in separatearchegonia. After rains or when dew deposits a film of water, the motile sperm are splashed away from the antheridia, which are normally produced on the top side of the thallus, and swim in the film of water to the antheridia where they fertilize the egg. To promote out crossing or cross-fertilization the sperm is released before the eggs are receptive of the sperm, making it more likely that the sperm will fertilize the eggs of the different thallus. Azygote is formed after fertilization, which grows into a new sporophytic plant. The condition of having separate sporophyte and gametophyte plants is calledalternation of generations. Other plants with similar reproductive strategies includePsilotum,Lycopodium,Selaginella andEquisetum.
Thebryophytes, which includeliverworts,hornworts andmosses, can reproduce both sexually andvegetatively. Thelife cycles of these plants start with haploid spores that grow into the dominant form, which is a multicellular haploid gametophyte, with thalloid or leaf-like structures thatphotosynthesize. The gametophyte is the most commonly known phase of the plant. Bryophytes are typically small plants that grow in moist locations and like ferns, have motile sperm which swim to the ovule usingflagella and therefore need water to facilitate sexual reproduction. Bryophytes show considerable variation in their reproductive structures, and a basic outline is as follows: Haploid gametes are produced in antheridia and archegonia by mitosis. The sperm released from the antheridia respond to chemicals released by ripe archegonia and swim to them in a film of water and fertilize the egg cells, thus producing zygotes that are diploid. The zygote divides repeatedly bymitotic division and grows into a diploid sporophyte. The resulting multicellular diploid sporophyte produces spore capsules calledsporangia. The spores are produced bymeiosis, and when ripe, the capsules burst open to release the spores. In some species each gametophyte isone sex while other species may bemonoicous, producing bothantheridia andarchegonia on the same gametophyte which is thushermaphrodite.[16]
Sexual reproduction in the multicellular facultatively sexualgreen algaVolvox carteri is induced byoxidative stress.[17] A two-fold increase in cellularreactive oxygen species (associated with oxidative stress) activates theV. carteri genes needed forsexual reproduction.[18] Exposure toantioxidants inhibits the induction of sex inV. carteri.[19] It was proposed on the basis of these observations that sexual reproduction emerged inV. carteri evolution as an adaptive response to oxidative stress and theDNA damage induced by reactive oxygen species.[18] Oxidative stress induced DNA damage may be repaired during themeiotic event associated with germination of thezygospore and the start of a new generation.[19]
One of the outcomes of plant reproduction is the generation of seeds, spores, and fruits[20] that allow plants to move to new locations or new habitats.[21]
Plants do not have nervous systems or any will for their actions. Even so, scientists are able to observe mechanisms that help their offspring thrive as they grow. All organisms have mechanisms to increase survival in offspring.[citation needed]
Offspring care is observed in theMammillaria hernandezii, a small cactus found in Mexico. A cactus is a type of succulent, meaning it retains water when it is available for future droughts.M. hernandezii also stores a portion of its seeds in its stem, and releases the rest to grow.[22] This can be advantageous for many reasons. By delaying the release of some of its seeds, the cactus can protect these from potential threats from insects, herbivores, or mold caused by micro-organisms. A study found that the presence of adequate water in the environment causesM. Hernandezii to release more seeds to allow for germination. The plant was able to perceive a water potential gradient in the surroundings, and act by giving its seeds a better chance in this preferable environment.[23] This evolutionary strategy gives a better potential outcome for seed germination.
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