There are about 380,000 knownspecies of plants, of which the majority, some 260,000,produce seeds. They range in size from single cells to the tallesttrees. Green plants provide a substantial proportion of the world's molecular oxygen; the sugars they create supply the energy for most of Earth'secosystems, and otherorganisms, including animals, eithereat plants directly or rely on organisms which do so.
All living things were traditionally placed into one of two groups, plants andanimals. This classification dates fromAristotle (384–322 BC), who distinguished different levels of beings inhis biology,[5] based on whether living things had a "sensitive soul" or like plants only a "vegetative soul".[6]Theophrastus, Aristotle's student, continued his work in plant taxonomy and classification.[7] Much later,Linnaeus (1707–1778) created the basis of the modern system ofscientific classification, but retained the animal and plantkingdoms, naming the plant kingdom the Vegetabilia.[7]
Alternative concepts
When the name Plantae or plant is applied to a specific group of organisms ortaxa, it usually refers to one of four concepts. From least to most inclusive, these four groupings are:
Plants in a strict sense include thegreen algae, and land plants that emerged within them, includingstoneworts. The relationships between plant groups are still being worked out, and the names given to them vary considerably. Theclade Viridiplantae encompasses a group of organisms that havecellulose in theircell walls, possesschlorophyllsa andb and haveplastids bound by only two membranes that are capable of photosynthesis and of storing starch. This clade is the main subject of this article (e.g., PlantaeCopeland, 1956[10]).
Plants in a broad sense comprise the green plants listed above plus the red algae (Rhodophyta) and the glaucophyte algae (Glaucophyta) that storeFloridean starch outside theplastids, in the cytoplasm. This clade includes all of the organisms that eons ago acquired theirprimary chloroplasts directly by engulfingcyanobacteria (e.g., PlantaeCavalier-Smith, 1981[1]).
Plants in the widest sense included the unrelated groups ofalgae,fungi andbacteria on older, obsolete classifications (e.g. Plantae or VegetabiliaLinnaeus 1751,[11] PlantaeHaeckel 1866,[12] MetaphytaHaeckel, 1894,[13] PlantaeWhittaker, 1969[8]).
There are about 382,000 acceptedspecies of plants,[14] of which the great majority, some 283,000,produce seeds.[15] The table below shows some species count estimates of different green plant (Viridiplantae)divisions. About 85–90% of all plants are flowering plants.[clarification needed] Several projects are currently attempting to collect records on all plant taxa in online databases, e.g. theWorld Flora Online.[14][16]
The ancestors of land plants evolved in water. An algal scum formed on the land1,200 million years ago, but it was not until theOrdovician, around450 million years ago, that the first land plants appeared, with a level of organisation like that of bryophytes.[33][34] However, fossils of organisms with a flattenedthallus inPrecambrian rocks suggest that multicellular freshwater eukaryotes existed over 1000 mya.[35]
Primitive land plants began to diversify in the lateSilurian, around420 million years ago. Bryophytes, club mosses, and ferns then appear in the fossil record.[36] Early plant anatomy is preserved in cellular detail in an earlyDevonian fossil assemblage from theRhynie chert. These early plants were preserved by being petrified inchert formed in silica-rich volcanic hot springs.[37]
In 2019, aphylogeny based ongenomes andtranscriptomes from 1,153 plant species was proposed.[47] The placing of algal groups is supported by phylogenies based on genomes from theMesostigmatophyceae andChlorokybophyceae that have since been sequenced. Both the "chlorophyte algae" and the "streptophyte algae" are treated asparaphyletic (vertical bars beside phylogenetic tree diagram) in this analysis, as the land plants arose from within those groups.[48][49] The classification of Bryophyta is supported both by Putticket al. 2018,[50] and by phylogenies involving the hornwort genomes that have also since been sequenced.[51][52]
Plant cells have distinctive features that other eukaryotic cells (such as those of animals) lack. These include the large water-filled centralvacuole,chloroplasts, and the strong flexiblecell wall, which is outside thecell membrane. Chloroplasts arederived from what was once a symbiosis of a non-photosynthetic cell and photosyntheticcyanobacteria. The cell wall, made mostly ofcellulose, allows plant cells toswell up with water without bursting. The vacuole allows the cell to change in size while the amount ofcytoplasm stays the same.[53]
Most plants aremulticellular. Plant cellsdifferentiate into multiple cell types, forming tissues such as thevascular tissue with specializedxylem andphloem of leaf veins andstems, and organs with different physiological functions such asroots to absorb water and minerals, stems for support and to transport water and synthesized molecules,leaves for photosynthesis, andflowers for reproduction.[54]
Plantsphotosynthesize, manufacturing food molecules (sugars) using energy obtained fromlight. Plant cells containchlorophylls inside their chloroplasts, which are green pigments that are used to capture light energy. The end-to-end chemical equation for photosynthesis is:[55]
This causes plants to releaseoxygen into the atmosphere. Green plants provide a substantial proportion of the world's molecular oxygen, alongside the contributions from photosynthetic algae and cyanobacteria.[56][57][58]
Plants that have secondarily adopted a parasitic lifestyle may lose the genes involved in photosynthesis and the production of chlorophyll.[59]
Growth and repair
Growth is determined by the interaction of a plant'sgenome with its physical and biotic environment.[60] Factors of the physical or abiotic environment includetemperature,water, light,carbon dioxide, andnutrients in the soil.[61] Biotic factors that affect plant growth include crowding, grazing, beneficial symbiotic bacteria and fungi, and attacks by insects orplant diseases.[62]
Frost and dehydration can damage or kill plants. Some plants haveantifreeze proteins,heat-shock proteins and sugars in their cytoplasm that enable them totolerate these stresses.[63] Plants are continuously exposed to a range of physical and biotic stresses which causeDNA damage, but they can tolerate and repair much of this damage.[64]
Plants reproduce to generate offspring, whethersexually, involvinggametes, orasexually, involving ordinary growth. Many plants use both mechanisms.[65]
When reproducing sexually, plants have complex lifecycles involvingalternation of generations. One generation, thesporophyte, which isdiploid (with 2 sets ofchromosomes), gives rise to the next generation, thegametophyte, which ishaploid (with one set of chromosomes). Some plants also reproduce asexually viaspores. In some non-flowering plants such as mosses, the sexual gametophyte forms most of the visible plant.[66] In seed plants (gymnosperms and flowering plants), the sporophyte forms most of the visible plant, and the gametophyte is very small. Flowering plants reproduce sexually using flowers, which contain male and female parts: these may be within the same (hermaphrodite) flower, ondifferent flowers on the same plant, oron different plants. Thestamens createpollen, which produces male gametes that enter theovule to fertilize the egg cell of the female gametophyte. Fertilization takes place within thecarpels orovaries, which develop intofruits that containseeds. Fruits may be dispersed whole, or they may split open and theseeds dispersed individually.[67]
Plants reproduce asexually by growing any of a wide variety of structures capable of growing into new plants. At the simplest, plants such as mosses or liverworts may be broken into pieces, each of which may regrow into whole plants. The propagation of flowering plants bycuttings is a similar process. Structures such asrunners enable plants to grow to cover an area, forming aclone. Many plants grow food storage structures such astubers orbulbs which may each develop into a new plant.[68]
Some non-flowering plants, such as many liverworts, mosses and some clubmosses, along with a few flowering plants, grow small clumps of cells calledgemmae which can detach and grow.[69][70]
Plants use pattern-recognition receptors to recognizepathogens such as bacteria that cause plant diseases. This recognition triggers a protective response. The first such plant receptors were identified inrice[71] and inArabidopsis thaliana.[72]
Plants have some of the largest genomes of all organisms.[73] The largest plant genome (in terms of gene number) is that ofwheat (Triticum aestivum), predicted to encode ≈94,000 genes[74] and thus almost 5 times as many as thehuman genome. The first plant genome sequenced was that ofArabidopsis thaliana which encodes about 25,500 genes.[75] In terms of sheer DNA sequence, the smallest published genome is that of the carnivorousbladderwort (Utricularia gibba) at 82 Mb (although it still encodes 28,500 genes)[76] while the largest, from theNorway spruce (Picea abies), extends over 19.6 Gb (encoding about 28,300 genes).[77]
Plants are distributed almost worldwide. While they inhabit manybiomes which can be divided into a multitude ofecoregions,[78] only the hardy plants of theAntarctic flora, consisting of algae, mosses, liverworts, lichens, and just two flowering plants, have adapted to the prevailing conditions on that southern continent.[79]
Plants are often the dominant physical and structural component of the habitats where they occur. Many of the Earth's biomes are named for the type of vegetation because plants are the dominant organisms in those biomes, such asgrassland,savanna, andtropical rainforest.[80]
The photosynthesis conducted by land plants and algae is the ultimate source of energy and organic material in nearly all ecosystems. Photosynthesis, at first by cyanobacteria and later by photosynthetic eukaryotes, radically changed the composition of the early Earth's anoxic atmosphere, which as a result is now 21%oxygen. Animals and most other organisms areaerobic, relying on oxygen; those that do not are confined to relatively rareanaerobic environments. Plants are theprimary producers in most terrestrial ecosystems and form the basis of thefood web in those ecosystems.[81] Plants form about 80% of the worldbiomass at about 450 gigatonnes (4.4×1011 long tons; 5.0×1011 short tons) of carbon.[82]
Many animalsdisperse seeds that are adapted for such dispersal. Various mechanisms of dispersal have evolved. Some fruits offer nutritious outer layers attractive to animals, while the seeds are adapted to survive the passage through the animal's gut; others have hooks that enable them to attach to a mammal's fur.[84]Myrmecophytes are plants that have coevolved withants. The plant provides a home, and sometimes food, for the ants. In exchange, the ants defend the plant fromherbivores and sometimes competing plants. Ant wastes serve as organicfertilizer.[85]
The majority of plant species have fungi associated with their root systems in amutualisticsymbiosis known asmycorrhiza. The fungi help the plants gain water and mineral nutrients from the soil, while the plant gives the fungi carbohydrates manufactured in photosynthesis.[86]Some plants serve as homes forendophytic fungi that protect the plant from herbivores by producing toxins. The fungal endophyteNeotyphodium coenophialum intall fescue grass has pest status in the American cattle industry.[87]
Manylegumes haveRhizobium nitrogen-fixing bacteria in nodules of their roots, which fix nitrogen from the air for the plant to use; in return, the plants supply sugars to the bacteria.[88] Nitrogen fixed in this way can become available to other plants, and is important in agriculture; for example, farmers may grow acrop rotation of a legume such as beans, followed by a cereal such as wheat, to providecash crops with a reduced input ofnitrogen fertilizer.[89]
Some 1% of plant species areparasitic. They range from the semi-parasiticmistletoe that merely takes some nutrients from its host, but still has photosynthetic leaves, to the fully-parasiticbroomrape andtoothwort that acquire all their nutrients through connections to the roots of other plants, and so have no chlorophyll. Full parasites can be extremely harmful to their plant hosts.[90]
Plants that grow on other plants, usually trees, without parasitizing them, are calledepiphytes. These may support diverse arboreal ecosystems. Some may indirectly harm their host plant, such as by intercepting light.Hemiepiphytes like thestrangler fig begin as epiphytes, but eventually set their own roots and overpower and kill their host. Manyorchids,bromeliads, ferns, and mosses grow as epiphytes.[91] Among the epiphytes, the bromeliads accumulate water in their leaf axils; thesewater-filled cavities can support complex aquatic food webs.[92]
Some 630 species of plants arecarnivorous, such as theVenus flytrap (Dionaea muscipula) andsundew (Drosera species). They trap small animals and digest them to obtain mineral nutrients, especiallynitrogen andphosphorus.[93]
Competition for shared resources reduces a plant's growth.[94][95] Shared resources include sunlight, water and nutrients. Light is a critical resource because it is necessary for photosynthesis.[94] Plants use their leaves to shade other plants from sunlight and grow quickly to maximize their own expose.[94] Water too is essential for photosynthesis; roots compete to maximize water uptake from soil.[96] Some plants have deep roots that are able to locate water stored deep underground, and others have shallower roots that are capable of extending longer distances to collect recent rainwater.[96]Minerals are important for plant growth and development.[97] Common nutrients competed for amongst plants include nitrogen, phosphorus, and potassium.[98]
Structural resources and fibres from plants are used to construct dwellings and to manufacture clothing.Wood is used for buildings, boats, and furniture, and for smaller items such asmusical instruments and sports equipment. Wood ispulped to makepaper andcardboard.[117] Cloth is often made fromcotton,flax,ramie or synthetic fibres such asrayon, derived from plant cellulose.Thread used to sew cloth likewise comes in large part from cotton.[118]
Thousands of plant species are cultivated for their beauty and to provide shade, modify temperatures, reduce wind, abate noise, provide privacy, and reduce soil erosion. Plants are the basis of a multibillion-dollar per year tourism industry, which includes travel tohistoric gardens,national parks,rainforests,forests with colourful autumn leaves, and festivals such asJapan's[119] andAmerica's cherry blossom festivals.[120]
Flowers are often used as memorials, gifts and to mark special occasions such as births, deaths, weddings and holidays. Flower arrangements may be used to sendhidden messages.[135] Plants and especially flowers form the subjects of many paintings.[136][137]
Weeds are commercially or aesthetically undesirable plants growing in managed environments such as in agriculture and gardens.[138] People have spread many plants beyond their native ranges; some of these plants have becomeinvasive, damaging existing ecosystems by displacing native species, and sometimes becoming serious weeds of cultivation.[139]
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