Botany, also calledplant science, is the branch ofnatural science andbiology studyingplants, especiallytheir anatomy,taxonomy, andecology.^[1] Abotanist orplant scientist is ascientist who specialises in this field. "Plant" and "botany" may be defined more narrowly to include onlyland plants and their study, which is also known asphytology. Phytologists or botanists (in the strict sense) study approximately 410,000species ofland plants, including some 391,000 species ofvascular plants (of which approximately 369,000 areflowering plants)^[2] and approximately 20,000bryophytes.^[3]

Botany originated in prehistory asherbalism with the efforts of early humans to identify – and later cultivate – plants that were edible, poisonous, and possibly medicinal, making it one of the first endeavours of human investigation. Medievalphysic gardens, often attached tomonasteries, contained plants possibly having medicinal benefit. They were forerunners of the firstbotanical gardens attached touniversities, founded from the 1540s onwards. One of the earliest was thePadua botanical garden. These gardens facilitated the academic study of plants. Efforts to catalogue and describe their collections were the beginnings ofplant taxonomy and led in 1753 to thebinomial system of nomenclature ofCarl Linnaeus that remains in use to this day for the naming of all biological species.

In the 19th and 20th centuries, new techniques were developed for the study of plants, including methods ofoptical microscopy andlive cell imaging,electron microscopy, analysis ofchromosome number,plant chemistry and the structure and function ofenzymes and otherproteins. In the last two decades of the 20th century, botanists exploited the techniques ofmolecular genetic analysis, includinggenomics andproteomics andDNA sequences to classify plants more accurately.

Modern botany is a broad subject with contributions and insights from most other areas of science and technology. Research topics include the study of plantstructure,growth and differentiation,reproduction,biochemistry andprimary metabolism, chemical products,development,diseases,evolutionary relationships,systematics, andplant taxonomy. Dominant themes in 21st-century plant science aremolecular genetics andepigenetics, which study the mechanisms and control of gene expression during differentiation ofplant cells andtissues. Botanical research has diverse applications in providingstaple foods, materials such astimber,oil, rubber,fibre and drugs, in modernhorticulture,agriculture andforestry,plant propagation,breeding andgenetic modification, in the synthesis of chemicals and raw materials for construction and energy production, inenvironmental management, and the maintenance ofbiodiversity.

The term "botany" comes from theAncient Greek wordbotanē (βοτάνη) meaning "pasture", "herbs" "grass", or "fodder";^[4]Botanē is in turn derived fromboskein (Greek:βόσκειν), "to feed" or "tograze".^[5][6][7] Traditionally, botany has also included the study offungi andalgae bymycologists andphycologists respectively, with the study of these three groups of organisms remaining within the sphere of interest of theInternational Botanical Congress.

Botany originated asherbalism, the study and use of plants for theirpossible medicinal properties.^[8] The early recorded history of botany includes many ancient writings and plant classifications. Examples of early botanical works have been found in ancient texts from India dating back to before 1100 BCE,^[9][10]Ancient Egypt,^[11] in archaicAvestan writings, and in works from China purportedly from before 221 BCE.^[9][12]

Modern botany traces its roots back toAncient Greece specifically toTheophrastus (c. 371–287 BCE), a student ofAristotle who invented and described many of its principles and is widely regarded in thescientific community as the "Father of Botany".^[13] His major works,Enquiry into Plants andOn the Causes of Plants, constitute the most important contributions to botanical science until theMiddle Ages, almost seventeen centuries later.^[13][14]

Another work from Ancient Greece that made an early impact on botany isDe materia medica, a five-volume encyclopedia aboutpreliminary herbal medicine written in the middle of the first century by Greek physician and pharmacologistPedanius Dioscorides.De materia medica was widely read for more than 1,500 years.^[15] Important contributions from themedieval Muslim world includeIbn Wahshiyya'sNabatean Agriculture,Abū Ḥanīfa Dīnawarī's (828–896) theBook of Plants, andIbn Bassal'sThe Classification of Soils. In the early 13th century,Abu al-Abbas al-Nabati, andIbn al-Baitar (d. 1248) wrote on botany in a systematic and scientific manner.^[16][17][18]

In the mid-16th century,botanical gardens were founded in a number of Italian universities. ThePadua botanical garden in 1545 is usually considered to be the first which is still in its original location. These gardens continued the practical value of earlier "physic gardens", often associated with monasteries, in which plants were cultivated for suspected medicinal uses. They supported the growth of botany as an academic subject. Lectures were given about the plants grown in the gardens. Botanical gardens came much later to northern Europe; the first in England was theUniversity of Oxford Botanic Garden in 1621.^[19]

German physicianLeonhart Fuchs (1501–1566) was one of "the three German fathers of botany", along with theologianOtto Brunfels (1489–1534) and physicianHieronymus Bock (1498–1554) (also called Hieronymus Tragus).^[20][21] Fuchs and Brunfels broke away from the tradition of copying earlier works to make original observations of their own. Bock created his own system of plant classification.

PhysicianValerius Cordus (1515–1544) authored a botanically and pharmacologically important herbalHistoria Plantarum in 1544 and apharmacopoeia of lasting importance, theDispensatorium in 1546.^[22] NaturalistConrad von Gesner (1516–1565) and herbalistJohn Gerard (1545 –c. 1611) published herbals covering the supposed medicinal uses of plants. NaturalistUlisse Aldrovandi (1522–1605) was considered thefather of natural history, which included the study of plants. In 1665, using an early microscope,PolymathRobert Hooke discoveredcells (a term he coined) incork, and a short time later in living plant tissue.^[23]

During the 18th century, systems ofplant identification were developed comparable todichotomous keys, where unidentified plants are placed intotaxonomic groups (e.g. family, genus and species) by making a series of choices between pairs ofcharacters. The choice and sequence of the characters may be artificial in keys designed purely for identification (diagnostic keys) or more closely related to the natural orphyletic order of thetaxa in synoptic keys.^[24] By the 18th century, new plants for study were arriving in Europe in increasing numbers from newly discovered countries and the European colonies worldwide. In 1753,Carl Linnaeus published hisSpecies Plantarum, a hierarchical classification of plant species that remains the reference point formodern botanical nomenclature. This established a standardised binomial or two-part naming scheme where the first name represented thegenus and the second identified thespecies within the genus.^[25] For the purposes of identification, Linnaeus'sSystema Sexualeclassified plants into 24 groups according to the number of their male sexual organs. The 24th group,Cryptogamia, included all plants with concealed reproductive parts,mosses,liverworts,ferns,algae andfungi.^[26]

Increasing knowledge ofplant anatomy,morphology and life cycles led to the realisation that there were more natural affinities between plants than the artificial sexual system of Linnaeus.Adanson (1763),de Jussieu (1789), andCandolle (1819) all proposed various alternative natural systems of classification that grouped plants using a wider range of shared characters and were widely followed. TheCandollean system reflected his ideas of the progression of morphological complexity and the laterBentham & Hooker system, which was influential until the mid-19th century, was influenced by Candolle's approach.Darwin's publication of theOrigin of Species in 1859 and his concept of common descent required modifications to the Candollean system to reflect evolutionary relationships as distinct from mere morphological similarity.^[27]

In the 19th century botany was a socially acceptable hobby for upper-class women. These women would collect and paint flowers and plants from around the world with scientific accuracy. The paintings were used to record many species that could not be transported or maintained in other environments.Marianne North illustrated over 900 species in extreme detail with watercolor and oil paintings.^[28] Her work and many other women's botany work was the beginning of popularizing botany to a wider audience.

Botany was greatly stimulated by the appearance of the first "modern" textbook,Matthias Schleiden'sGrundzüge der Wissenschaftlichen Botanik, published in English in 1849 asPrinciples of Scientific Botany.^[29] Schleiden was a microscopist and an early plant anatomist who co-founded thecell theory withTheodor Schwann andRudolf Virchow and was among the first to grasp the significance of thecell nucleus that had been described byRobert Brown in 1831.^[30] In 1855,Adolf Fick formulatedFick's laws that enabled the calculation of the rates ofmolecular diffusion in biological systems.^[31]

Building upon the gene-chromosome theory of heredity that originated withGregor Mendel (1822–1884),August Weismann (1834–1914) proved that inheritance only takes place throughgametes. No other cells can pass on inherited characters.^[32] The work ofKatherine Esau (1898–1997) on plant anatomy is still a major foundation of modern botany. Her booksPlant Anatomy andAnatomy of Seed Plants have been key plant structural biology texts for more than half a century.^[33][34]

The discipline ofplant ecology was pioneered in the late 19th century by botanists such asEugenius Warming, who produced the hypothesis that plants formcommunities, and his mentor and successorChristen C. Raunkiær whose system for describingplant life forms is still in use today. The concept that the composition of plant communities such astemperate broadleaf forest changes by a process ofecological succession was developed byHenry Chandler Cowles,Arthur Tansley andFrederic Clements. Clements is credited with the idea ofclimax vegetation as the most complex vegetation that an environment can support and Tansley introduced the concept ofecosystems to biology.^[35][36][37] Building on the extensive earlier work ofAlphonse de Candolle,Nikolai Vavilov (1887–1943) produced accounts of thebiogeography,centres of origin, and evolutionary history of economic plants.^[38]

Particularly since the mid-1960s there have been advances in understanding of the physics ofplant physiological processes such astranspiration (the transport of water within plant tissues), the temperature dependence of rates of waterevaporation from the leaf surface and themolecular diffusion of water vapour and carbon dioxide throughstomatal apertures. These developments, coupled with new methods for measuring the size of stomatal apertures, and the rate ofphotosynthesis have enabled precise description of the rates ofgas exchange between plants and the atmosphere.^[39][40] Innovations instatistical analysis byRonald Fisher,^[41]Frank Yates and others atRothamsted Experimental Station facilitated rational experimental design and data analysis in botanical research.^[42] The discovery and identification of theauxin plant hormones byKenneth V. Thimann in 1948 enabled regulation of plant growth by externally applied chemicals.Frederick Campion Steward pioneered techniques ofmicropropagation andplant tissue culture controlled byplant hormones.^[43] The synthetic auxin2,4-dichlorophenoxyacetic acid or 2,4-D was one of the first commercial syntheticherbicides.^[44]

20th century developments in plant biochemistry have been driven by modern techniques oforganic chemical analysis, such asspectroscopy,chromatography andelectrophoresis. With the rise of the related molecular-scale biological approaches ofmolecular biology,genomics,proteomics andmetabolomics, the relationship between the plantgenome and most aspects of the biochemistry, physiology, morphology and behaviour of plants can be subjected to detailed experimental analysis.^[45] The concept originally stated byGottlieb Haberlandt in 1902^[46] that all plant cells aretotipotent and can be grownin vitro ultimately enabled the use ofgenetic engineering experimentally to knock out a gene or genes responsible for a specific trait, or to add genes such asGFP thatreport when a gene of interest is being expressed. These technologies enable the biotechnological use of whole plants or plant cell cultures grown inbioreactors to synthesisepesticides,antibiotics or otherpharmaceuticals, as well as the practical application ofgenetically modified crops designed for traits such as improved yield.^[47]

Modern morphology recognises a continuum between the major morphological categories of root, stem (caulome), leaf (phyllome) andtrichome.^[48] Furthermore, it emphasises structural dynamics.^[49] Modern systematics aims to reflect and discoverphylogenetic relationships between plants.^{[50][51][52][53]} Modernmolecular phylogenetics largely ignores morphological characters, relying on DNA sequences as data. Molecular analysis ofDNA sequences from most families of flowering plants enabled theAngiosperm Phylogeny Group to publish in 1998 aphylogeny of flowering plants, answering many of the questions about relationships amongangiosperm families and species.^[54] The theoretical possibility of a practical method for identification of plant species and commercial varieties byDNA barcoding is the subject of active current research.^[55][56]

Botany is divided along several axes.

Some subfields of botany relate to particular groups of organisms. Divisions related to the broader historical sense of botany includebacteriology,mycology (orfungology), andphycology – respectively, the study of bacteria, fungi, and algae – withlichenology as a subfield of mycology. The narrower sense of botany as the study ofembryophytes (land plants) is calledphytology.Bryology is the study ofmosses (and in the broader sense alsoliverworts andhornworts).Pteridology (orfilicology) is the study offerns and allied plants. A number of other taxa of ranks varying from family to subgenus have terms for their study, includingagrostology (orgraminology) for the study ofgrasses,synantherology for the study of composites, andbatology for the study ofbrambles.

Study can also be divided byguild rather thanclade orgrade. For example,dendrology is the study of woody plants.

Many divisions ofbiology have botanical subfields. These are commonly denoted by prefixing the word plant (e.g.plant taxonomy,plant ecology,plant anatomy,plant morphology,plant systematics), or prefixing or substituting the prefix phyto- (e.g.phytochemistry,phytogeography). The study offossil plants is calledpalaeobotany. Other fields are denoted by adding or substituting the word botany (e.g.systematic botany).

Phytosociology is a subfield of plant ecology that classifies and studies communities of plants.

The intersection of fields from the above pair of categories gives rise to fields such asbryogeography, the study of the distribution of mosses.

Different parts of plants also give rise to their own subfields, includingxylology,carpology (orfructology), andpalynology, these being the study of wood, fruit and pollen/spores respectively.

Botany also overlaps on the one hand with agriculture, horticulture and silviculture, and on the other hand with medicine and pharmacology, giving rise to fields such asagronomy,horticultural botany,phytopathology, andphytopharmacology.

The study of plants is vital because they underpin almost all animal life on Earth by generating a large proportion of theoxygen and food that provide humans and other organisms withaerobic respiration with the chemical energy they need to exist. Plants,algae andcyanobacteria are the major groups of organisms that carry outphotosynthesis, a process that uses the energy of sunlight to convert water andcarbon dioxide^[57] into sugars that can be used both as a source of chemical energy and of organic molecules that are used in the structural components of cells.^[58] As a by-product of photosynthesis, plants releaseoxygen into the atmosphere, a gas that is required bynearly all living things to carry out cellular respiration. In addition, they are influential in the globalcarbon andwater cycles and plant roots bind and stabilise soils, preventing soilerosion.^[59] Plants are crucial to the future of human society as they provide food, oxygen,biochemicals, and products for people, as well as creating and preserving soil.^[60]

Historically, all living things were classified as either animals or plants^[61] and botany covered the study of all organisms not considered animals.^[62] Botanists examine both the internal functions and processes within plantorganelles, cells, tissues, whole plants, plant populations and plant communities. At each of these levels, a botanist may be concerned with the classification (taxonomy),phylogeny andevolution, structure (anatomy andmorphology), or function (physiology) of plant life.^[63]

The strictest definition of "plant" includes only the "land plants" orembryophytes, which includeseed plants (gymnosperms, including thepines, andflowering plants) and the free-sporingcryptogams includingferns,clubmosses,liverworts,hornworts andmosses. Embryophytes are multicellulareukaryotes descended from an ancestor that obtained its energy from sunlight byphotosynthesis. They have life cycles withalternating haploid anddiploid phases. The sexualhaploid phase of embryophytes, known as thegametophyte, nurtures the developing diploid embryosporophyte within its tissues for at least part of its life,^[64] even in the seed plants, where the gametophyte itself is nurtured by its parent sporophyte.^[65] Other groups of organisms that were previously studied by botanists include bacteria (now studied inbacteriology), fungi (mycology) – includinglichen-forming fungi (lichenology), non-chlorophytealgae (phycology), and viruses (virology). However, attention is still given to these groups by botanists, and fungi (including lichens) and photosyntheticprotists are usually covered in introductory botany courses.^[66][67]

Palaeobotanists study ancient plants in the fossil record to provide information about theevolutionary history of plants.Cyanobacteria, the first oxygen-releasing photosynthetic organisms on Earth, are thought to have given rise to the ancestor of plants by entering into anendosymbiotic relationship with an early eukaryote, ultimately becoming thechloroplasts in plant cells. The new photosynthetic plants (along with their algal relatives) accelerated the rise in atmosphericoxygen started by thecyanobacteria,changing the ancient oxygen-free,reducing, atmosphere to one in which free oxygen has been abundant for more than 2 billion years.^[68][69]

Among the important botanical questions of the 21st century are the role of plants as primary producers in the global cycling of life's basic ingredients: energy, carbon, oxygen, nitrogen and water, and ways that our plant stewardship can help address the global environmental issues ofresource management,conservation,human food security,biologically invasive organisms,carbon sequestration,climate change, andsustainability.^[70]

Virtually all staple foods come either directly fromprimary production by plants, or indirectly from animals that eat them.^[71] Plants and other photosynthetic organisms are at the base of mostfood chains because they use the energy from the sun and nutrients from the soil and atmosphere, converting them into a form that can be used by animals. This is what ecologists call the firsttrophic level.^[72] The modern forms of the majorstaple foods, such ashemp,teff, maize, rice, wheat and other cereal grasses,pulses,bananas and plantains,^[73] as well ashemp,flax andcotton grown for their fibres, are the outcome of prehistoric selection over thousands of years from amongwild ancestral plants with the most desirable characteristics.^[74]

Botanists study how plants produce food and how to increase yields, for example throughplant breeding, making their work important to humanity's ability to feed the world and providefood security for future generations.^[75] Botanists also study weeds, which are a considerable problem in agriculture, and the biology and control ofplant pathogens in agriculture and naturalecosystems.^[76]Ethnobotany is the study of the relationships between plants and people. When applied to the investigation of historical plant–people relationships ethnobotany may be referred to as archaeobotany orpalaeoethnobotany.^[77] Some of the earliest plant-people relationships arose between theindigenous people of Canada in identifying edible plants from inedible plants. This relationship the indigenous people had with plants was recorded by ethnobotanists.^[78]

Plant biochemistry is the study of the chemical processes used by plants. Some of these processes are used in theirprimary metabolism like the photosyntheticCalvin cycle andcrassulacean acid metabolism.^[79] Others make specialised materials like thecellulose andlignin used to build their bodies, andsecondary products likeresins andaroma compounds.

Plants and various other groups of photosynthetic eukaryotes collectively known as "algae" have unique organelles known aschloroplasts. Chloroplasts are thought to be descended fromcyanobacteria that formedendosymbiotic relationships with ancient plant and algal ancestors. Chloroplasts and cyanobacteria contain the blue-green pigmentchlorophylla.^[80] Chlorophylla (as well as its plant and green algal-specific cousinchlorophyllb)^[a] absorbs light in the blue-violet and orange/red parts of thespectrum while reflecting and transmitting the green light that we see as the characteristic colour of these organisms. The energy in the red and blue light that these pigments absorb is used by chloroplasts to make energy-rich carbon compounds from carbon dioxide and water byoxygenic photosynthesis, a process that generatesmolecular oxygen (O₂) as a by-product.

The light energy captured bychlorophylla is initially in the form of electrons (and later aproton gradient) that is used to make molecules ofATP andNADPH which temporarily store and transport energy. Their energy is used in thelight-independent reactions of the Calvin cycle by the enzymerubisco to produce molecules of the 3-carbon sugarglyceraldehyde 3-phosphate (G3P). Glyceraldehyde 3-phosphate is the first product of photosynthesis and the raw material from whichglucose and almost all other organic molecules of biological origin are synthesised. Some of the glucose is converted to starch which is stored in the chloroplast.^[84] Starch is the characteristic energy store of most land plants and algae, whileinulin, a polymer offructose is used for the same purpose in the sunflower familyAsteraceae. Some of the glucose is converted tosucrose (common table sugar) for export to the rest of the plant.

Unlike in animals (which lack chloroplasts), plants and their eukaryote relatives have delegated many biochemical roles to theirchloroplasts, including synthesising all theirfatty acids,^[85][86] and mostamino acids.^[87] The fatty acids that chloroplasts make are used for many things, such as providing material to buildcell membranes out of and making the polymercutin which is found in theplant cuticle that protects land plants from drying out.^[88]

Plants synthesise a number of uniquepolymers like thepolysaccharide moleculescellulose,pectin andxyloglucan^[89] from which the land plant cell wall is constructed.^[90]Vascular land plants makelignin, a polymer used to strengthen thesecondary cell walls of xylemtracheids andvessels to keep them from collapsing when a plant sucks water through them under water stress. Lignin is also used in other cell types likesclerenchyma fibres that provide structural support for a plant and is a major constituent of wood.Sporopollenin is a chemically resistant polymer found in the outer cell walls of spores and pollen of land plants responsible for the survival of early land plant spores and the pollen of seed plants in the fossil record. It is widely regarded as a marker for the start of land plant evolution during theOrdovician period.^[91]The concentration of carbon dioxide in the atmosphere today is much lower than it was when plants emerged onto land during theOrdovician andSilurian periods. Manymonocots likemaize and thepineapple and somedicots like theAsteraceae have since independently evolved^[92] pathways likeCrassulacean acid metabolism and theC₄ carbon fixation pathway for photosynthesis which avoid the losses resulting fromphotorespiration in the more commonC₃ carbon fixation pathway. These biochemical strategies are unique to land plants.

Phytochemistry is a branch of plant biochemistry primarily concerned with the chemical substances produced by plants duringsecondary metabolism.^[93] Some of these compounds are toxins such as thealkaloidconiine fromhemlock. Others, such as theessential oilspeppermint oil and lemon oil are useful for their aroma, as flavourings and spices (e.g.,capsaicin), and in medicine as pharmaceuticals as inopium fromopium poppies. Manymedicinal andrecreational drugs, such astetrahydrocannabinol (active ingredient incannabis),caffeine,morphine andnicotine come directly from plants. Others are simplederivatives of botanical natural products. For example, the pain killeraspirin is the acetylester ofsalicylic acid, originally isolated from thebark ofwillow trees,^[94] and a wide range ofopiatepainkillers likeheroin are obtained by chemical modification ofmorphine obtained from theopium poppy.^[95] Popularstimulants come from plants, such ascaffeine from coffee, tea and chocolate, andnicotine from tobacco. Most alcoholic beverages come fromfermentation ofcarbohydrate-rich plant products such asbarley (beer), rice (sake) and grapes (wine).^[96]Native Americans have used various plants as ways of treating illness or disease for thousands of years.^[97] This knowledge Native Americans have on plants has been recorded byenthnobotanists and then in turn has been used bypharmaceutical companies as a way ofdrug discovery.^[98]

Plants can synthesise coloured dyes and pigments such as theanthocyanins responsible for the red colour ofred wine, yellowweld and bluewoad used together to produceLincoln green,indoxyl, source of the blue dyeindigo traditionally used to dye denim and the artist's pigmentsgamboge androse madder.

Sugar,starch, cotton,linen,hemp, some types ofrope, wood andparticle boards,papyrus and paper,vegetable oils,wax, andnatural rubber are examples of commercially important materials made from plant tissues or their secondary products.Charcoal, a pure form of carbon made bypyrolysis of wood, has a longhistory as a metal-smelting fuel, as a filter material andadsorbent and as an artist's material and is one of the three ingredients ofgunpowder.Cellulose, the world's most abundant organic polymer,^[99] can be converted into energy, fuels, materials and chemical feedstock.Products made from cellulose includerayon andcellophane,wallpaper paste,biobutanol andgun cotton.Sugarcane,rapeseed andsoy are some of the plants with a highly fermentable sugar or oil content that are used as sources ofbiofuels, important alternatives tofossil fuels, such asbiodiesel.^[100] Sweetgrass was used by Native Americans to ward off bugs likemosquitoes.^[101] These bug repelling properties of sweetgrass were later found by theAmerican Chemical Society in the moleculesphytol andcoumarin.^[101]

Plant ecology is the science of the functional relationships between plants and theirhabitats – the environments where they complete theirlife cycles. Plant ecologists study the composition of local and regionalfloras, theirbiodiversity, genetic diversity andfitness, theadaptation of plants to their environment, and their competitive ormutualistic interactions with other species.^[103] Some ecologists even rely onempirical data from indigenous people that is gathered by ethnobotanists.^[104] This information can relay a great deal of information on how the land once was thousands of years ago and how it has changed over that time.^[104] The goals of plant ecology are to understand the causes of their distribution patterns, productivity, environmental impact, evolution, and responses to environmental change.^[105]

Plants depend on certainedaphic (soil) and climatic factors in their environment but can modify these factors too. For example, they can change their environment'salbedo, increaserunoff interception, stabilise mineral soils and develop their organic content, and affect local temperature. Plants compete with other organisms in theirecosystem for resources.^[106][107] They interact with their neighbours at a variety ofspatial scales in groups, populations andcommunities that collectively constitute vegetation. Regions with characteristicvegetation types and dominant plants as well as similarabiotic andbiotic factors,climate, andgeography make upbiomes liketundra ortropical rainforest.^[108]

Herbivores eat plants, but plants candefend themselves and some species areparasitic or evencarnivorous. Other organisms formmutually beneficial relationships with plants. For example,mycorrhizal fungi andrhizobia provide plants with nutrients in exchange for food,ants are recruited byant plants to provide protection,^[109]honey bees,bats and other animalspollinate flowers^[110][111] andhumans andother animals^[112] act asdispersal vectors to spreadspores andseeds.

Plant responses to climate and other environmental changes can inform our understanding of how these changes affect ecosystem function and productivity. For example, plantphenology can be a usefulproxy for temperature inhistorical climatology, and the biologicalimpact of climate change andglobal warming.Palynology, the analysis of fossil pollen deposits in sediments fromthousands or millions of years ago allows the reconstruction of past climates.^[113] Estimates of atmospheric CO₂ concentrations since thePalaeozoic have been obtained fromstomatal densities and the leaf shapes and sizes of ancientland plants.^[114]Ozone depletion can expose plants to higher levels ofultraviolet radiation-B (UV-B), resulting in lower growth rates.^[115] Moreover, information from studies ofcommunity ecology, plantsystematics, andtaxonomy is essential to understandingvegetation change,habitat destruction andspecies extinction.^[116]

Inheritance in plants follows the same fundamental principles of genetics as in other multicellular organisms.Gregor Mendel discovered thegenetic laws of inheritance by studying inherited traits such as shape inPisum sativum (peas). What Mendel learned from studying plants has had far-reaching benefits outside of botany. Similarly, "jumping genes" were discovered byBarbara McClintock while she was studying maize.^[117] Nevertheless, there are some distinctive genetic differences between plants and other organisms.

Species boundaries in plants may be weaker than in animals, and cross specieshybrids are often possible. A familiar example ispeppermint,Mentha ×piperita, asterile hybrid betweenMentha aquatica and spearmint,Mentha spicata.^[118] The many cultivated varieties of wheat are the result of multiple inter- and intra-specific crosses between wild species and their hybrids.^[119]Angiosperms withmonoecious flowers often haveself-incompatibility mechanisms that operate between thepollen andstigma so that the pollen either fails to reach the stigma or fails togerminate and produce malegametes.^[120] This is one of several methods used by plants to promoteoutcrossing.^[121] In many land plants the male and female gametes are produced by separate individuals. These species are said to bedioecious when referring to vascular plantsporophytes anddioicous when referring tobryophytegametophytes.^[122]

Charles Darwin in his 1878 book The Effects of Cross and Self-Fertilization in the Vegetable Kingdom^[123] at the start of chapter XII noted "The first and most important of the conclusions which may be drawn from the observations given in this volume, is that generally cross-fertilisation is beneficial and self-fertilisation often injurious, at least with the plants on which I experimented." An important adaptive benefit of outcrossing is that it allows the masking of deleterious mutations in the genome of progeny. This beneficial effect is also known as hybrid vigor or heterosis. Once outcrossing is established, subsequent switching to inbreeding becomes disadvantageous since it allows expression of the previously masked deleterious recessive mutations, commonly referred to as inbreeding depression.

Unlike in higher animals, whereparthenogenesis is rare,asexual reproduction may occur in plants by several different mechanisms. The formation of stemtubers in potato is one example. Particularly inarctic oralpine habitats, where opportunities for fertilisation of flowersby animals are rare, plantlets orbulbs, may develop instead of flowers, replacingsexual reproduction with asexual reproduction and giving rise toclonal populations genetically identical to the parent. This is one of several types ofapomixis that occur in plants. Apomixis can also happen in aseed, producing a seed that contains an embryo genetically identical to the parent.^[124]

Most sexually reproducing organisms are diploid, with paired chromosomes, but doubling of theirchromosome number may occur due to errors incytokinesis. This can occur early in development to produce anautopolyploid or partly autopolyploid organism, or during normal processes of cellular differentiation to produce some cell types that are polyploid (endopolyploidy), or during gamete formation. Anallopolyploid plant may result from ahybridisation event between two different species. Both autopolyploid and allopolyploid plants can often reproduce normally, but may be unable to cross-breed successfully with the parent population because there is a mismatch in chromosome numbers. These plants that arereproductively isolated from the parent species but live within the same geographical area, may be sufficiently successful to form a newspecies.^[125] Some otherwise sterile plant polyploids can still reproducevegetatively or by seed apomixis, forming clonal populations of identical individuals.^[125]Durum wheat is a fertiletetraploid allopolyploid, whilebread wheat is a fertilehexaploid. The commercial banana is an example of a sterile, seedlesstriploid hybrid.Common dandelion is a triploid that produces viable seeds by apomictic seed.

As in other eukaryotes, the inheritance ofendosymbiotic organelles likemitochondria andchloroplasts in plants is non-Mendelian. Chloroplasts are inherited through the male parent in gymnosperms but often through the female parent in flowering plants.^[126]

A considerable amount of new knowledge about plant function comes from studies of the molecular genetics ofmodel plants such as the Thale cress,Arabidopsis thaliana, a weedy species in the mustard family (Brassicaceae).^[93] Thegenome or hereditary information contained in the genes of this species is encoded by about 135 millionbase pairs of DNA, forming one of the smallest genomes amongflowering plants.Arabidopsis was the first plant to have its genome sequenced, in 2000.^[127] The sequencing of some other relatively small genomes, of rice (Oryza sativa)^[128] andBrachypodium distachyon,^[129] has made them important model species for understanding the genetics, cellular and molecular biology ofcereals,grasses andmonocots generally.

Model plants such asArabidopsis thaliana are used for studying the molecular biology ofplant cells and thechloroplast. Ideally, these organisms have small genomes that are well known or completely sequenced, small stature and short generation times. Corn has been used to study mechanisms ofphotosynthesis andphloem loading of sugar inC₄ plants.^[130] Thesingle celledgreen algaChlamydomonas reinhardtii, while not anembryophyte itself, contains agreen-pigmentedchloroplast related to that of land plants, making it useful for study.^[131] Ared algaCyanidioschyzon merolae has also been used to study some basic chloroplast functions.^[132]Spinach,^[133]peas,^[134]soybeans and a mossPhyscomitrella patens are commonly used to study plant cell biology.^[135]

Agrobacterium tumefaciens, a soilrhizosphere bacterium, can attach to plant cells and infect them with acallus-inducingTi plasmid byhorizontal gene transfer, causing a callus infection called crown gall disease. Schell and Van Montagu (1977) hypothesised that the Ti plasmid could be a natural vector for introducing theNif gene responsible fornitrogen fixation in the root nodules oflegumes and other plant species.^[136] Today, genetic modification of the Ti plasmid is one of the main techniques for introduction oftransgenes to plants and the creation ofgenetically modified crops.

Epigenetics is the study of heritable changes ingene function that cannot be explained by changes in the underlyingDNA sequence^[137] but cause the organism's genes to behave (or "express themselves") differently.^[138] One example of epigenetic change is the marking of the genes byDNA methylation which determines whether they will be expressed or not. Gene expression can also be controlled by repressor proteins that attach tosilencer regions of the DNA and prevent that region of the DNA code from being expressed. Epigenetic marks may be added or removed from the DNA during programmed stages of development of the plant, and are responsible, for example, for the differences between anthers, petals and normal leaves, despite the fact that they all have the same underlying genetic code. Epigenetic changes may be temporary or may remain through successivecell divisions for the remainder of the cell's life. Some epigenetic changes have been shown to beheritable,^[139] while others are reset in the germ cells.

Epigenetic changes ineukaryotic biology serve to regulate the process ofcellular differentiation. Duringmorphogenesis,totipotentstem cells become the variouspluripotentcell lines of theembryo, which in turn become fully differentiated cells. A single fertilised egg cell, thezygote, gives rise to the many differentplant cell types includingparenchyma,xylem vessel elements,phloem sieve tubes,guard cells of theepidermis, etc. as it continues todivide. The process results from the epigenetic activation of some genes and inhibition of others.^[140]

Unlike animals, many plant cells, particularly those of theparenchyma, do not terminally differentiate, remaining totipotent with the ability to give rise to a new individual plant. Exceptions include highly lignified cells, thesclerenchyma and xylem which are dead at maturity, and the phloem sieve tubes which lack nuclei. While plants use many of the same epigenetic mechanisms as animals, such aschromatin remodelling, an alternative hypothesis is that plants set their gene expression patterns using positional information from the environment and surrounding cells to determine their developmental fate.^[141]

Epigenetic changes can lead toparamutations, which do not follow the Mendelian heritage rules. These epigenetic marks are carried from one generation to the next, with one allele inducing a change on the other.^[142]

Thechloroplasts of plants have a number of biochemical, structural and genetic similarities tocyanobacteria, (commonly but incorrectly known as "blue-green algae") and are thought to be derived from an ancientendosymbiotic relationship between an ancestraleukaryotic cell and acyanobacterial resident.^{[143][144][145][146]}

Thealgae are apolyphyletic group and are placed in various divisions, some more closely related to plants than others. There are many differences between them in features such as cell wall composition, biochemistry, pigmentation, chloroplast structure and nutrient reserves. The algal divisionCharophyta, sister to the green algal divisionChlorophyta, is considered to contain the ancestor of true plants.^[147] The Charophyte classCharophyceae and the land plant sub-kingdomEmbryophyta together form themonophyletic group or cladeStreptophytina.^[148]

Nonvascular land plants areembryophytes that lack the vascular tissuesxylem andphloem. They includemosses,liverworts andhornworts.Pteridophytic vascular plants with true xylem and phloem that reproduced by spores germinating into free-living gametophytes evolved during the Silurian period and diversified into several lineages during the lateSilurian and earlyDevonian. Representatives of the lycopods have survived to the present day. By the end of the Devonian period, several groups, including thelycopods,sphenophylls andprogymnosperms, had independently evolved "megaspory" – their spores were of two distinct sizes, largermegaspores and smaller microspores. Their reduced gametophytes developed from megaspores retained within thespore-producing organs (megasporangia) of the sporophyte, a condition known as endospory. Seeds consist of an endosporic megasporangium surrounded by one or two sheathing layers (integuments). The young sporophyte develops within the seed, which ongermination splits to release it. The earliest known seed plants date from the latest DevonianFamennian stage.^[149][150] Following the evolution of the seed habit,seed plants diversified, giving rise to a number of now-extinct groups, includingseed ferns, as well as the modern gymnosperms and angiosperms.^[151]Gymnosperms produce "naked seeds" not fully enclosed in an ovary; modern representatives includeconifers,cycads,Ginkgo, andGnetales.Angiosperms produce seeds enclosed in a structure such as acarpel or anovary.^[152][153] Ongoing research on the molecular phylogenetics of living plants appears to show that the angiosperms are asister clade to the gymnosperms.^[154]

Plantphysiology encompasses all the internal chemical and physical activities of plants associated with life.^[155] Chemicals obtained from the air, soil and water form the basis of allplant metabolism. The energy of sunlight, captured by oxygenic photosynthesis and released bycellular respiration, is the basis of almost all life.Photoautotrophs, including all green plants, algae andcyanobacteria gather energy directly from sunlight by photosynthesis.Heterotrophs including all animals, all fungi, all completely parasitic plants, and non-photosynthetic bacteria take in organic molecules produced by photoautotrophs and respire them or use them in the construction of cells and tissues.^[156]Respiration is the oxidation of carbon compounds by breaking them down into simpler structures to release the energy they contain, essentially the opposite of photosynthesis.^[157]

Molecules are moved within plants by transport processes that operate at a variety ofspatial scales. Subcellular transport of ions, electrons and molecules such as water andenzymes occurs acrosscell membranes. Minerals and water are transported from roots to other parts of the plant in thetranspiration stream.Diffusion,osmosis, andactive transport andmass flow are all different ways transport can occur.^[158] Examples ofelements that plants need to transport arenitrogen,phosphorus,potassium,calcium,magnesium, andsulfur. In vascular plants, these elements are extracted from the soil as soluble ions by the roots and transported throughout the plant in the xylem. Most of the elements required forplant nutrition come from the chemical breakdown of soil minerals.^[159]Sucrose produced by photosynthesis is transported from the leaves to other parts of the plant in the phloem andplant hormones are transported by a variety of processes.

Plants are not passive, but respond toexternal signals such as light, touch, and injury by moving or growing towards or away from the stimulus, as appropriate. Tangible evidence of touch sensitivity is the almost instantaneous collapse of leaflets ofMimosa pudica, the insect traps ofVenus flytrap andbladderworts, and the pollinia of orchids.^[161]

The hypothesis that plant growth and development is coordinated byplant hormones or plant growth regulators first emerged in the late 19th century. Darwin experimented on the movements of plant shoots and roots towardslight^[162] andgravity, and concluded "It is hardly an exaggeration to say that the tip of the radicle . . acts like the brain of one of the lower animals . . directing the several movements".^[163] About the same time, the role ofauxins (from the Greekauxein, to grow) in control of plant growth was first outlined by the Dutch scientistFrits Went.^[164] The first known auxin,indole-3-acetic acid (IAA), which promotes cell growth, was only isolated from plants about 50 years later.^[165] This compound mediates the tropic responses of shoots and roots towards light and gravity.^[166] The finding in 1939 that plantcallus could be maintained in culture containing IAA, followed by the observation in 1947 that it could be induced to form roots and shoots by controlling the concentration of growth hormones were key steps in the development of plant biotechnology and genetic modification.^[167]

Cytokinins are a class of plant hormones named for their control of cell division (especiallycytokinesis). The natural cytokininzeatin was discovered in corn,Zea mays, and is a derivative of thepurineadenine. Zeatin is produced in roots and transported to shoots in the xylem where it promotes cell division, bud development, and the greening of chloroplasts.^[168][169] Thegibberelins, such asgibberelic acid arediterpenes synthesised fromacetyl CoA via themevalonate pathway. They are involved in the promotion of germination and dormancy-breaking in seeds, in regulation of plant height by controlling stem elongation and the control of flowering.^[170]Abscisic acid (ABA) occurs in all land plants except liverworts, and is synthesised fromcarotenoids in the chloroplasts and other plastids. It inhibits cell division, promotes seed maturation, and dormancy, and promotes stomatal closure. It was so named because it was originally thought to controlabscission.^[171]Ethylene is a gaseous hormone that is produced in all higher plant tissues frommethionine. It is now known to be the hormone that stimulates or regulates fruit ripening and abscission,^[172][173] and it, or the synthetic growth regulatorethephon which is rapidly metabolised to produce ethylene, are used on industrial scale to promote ripening of cotton,pineapples and otherclimacteric crops.

Another class ofphytohormones is thejasmonates, first isolated from the oil ofJasminum grandiflorum^[174] which regulates wound responses in plants by unblocking the expression of genes required in thesystemic acquired resistance response to pathogen attack.^[175]

In addition to being the primary energy source for plants, light functions as a signalling device, providing information to the plant, such as how much sunlight the plant receives each day. This can result in adaptive changes in a process known asphotomorphogenesis.Phytochromes are thephotoreceptors in a plant that are sensitive to light.^[176]

Plant anatomy is the study of the structure of plant cells and tissues, whereasplant morphology is the study of their external form.^[177]All plants are multicellular eukaryotes, their DNA stored in nuclei.^[178][179] The characteristic features ofplant cells that distinguish them from those of animals and fungi include a primarycell wall composed of the polysaccharidescellulose,hemicellulose andpectin,^[180] largervacuoles than in animal cells and the presence ofplastids with unique photosynthetic and biosynthetic functions as in the chloroplasts. Other plastids contain storage products such as starch (amyloplasts) or lipids (elaioplasts). Uniquely,streptophyte cells and those of the green algal orderTrentepohliales^[181] divide by construction of aphragmoplast as a template for building acell plate late incell division.^[84]

The bodies ofvascular plants includingclubmosses,ferns andseed plants (gymnosperms andangiosperms) generally have aerial and subterranean subsystems. Theshoots consist ofstems bearing green photosynthesisingleaves and reproductive structures. The underground vascularisedroots bearroot hairs at their tips and generally lack chlorophyll.^[183] Non-vascular plants, theliverworts,hornworts andmosses do not produce ground-penetrating vascular roots and most of the plant participates in photosynthesis.^[184] Thesporophyte generation is nonphotosynthetic in liverworts but may be able to contribute part of its energy needs by photosynthesis in mosses and hornworts.^[185]

The root system and the shoot system are interdependent – the usually nonphotosynthetic root system depends on the shoot system for food, and the usually photosynthetic shoot system depends on water and minerals from the root system.^[183] Cells in each system are capable of creating cells of the other and producingadventitious shoots or roots.^[186]Stolons andtubers are examples of shoots that can grow roots.^[187] Roots that spread out close to the surface, such as those of willows, can produce shoots and ultimately new plants.^[188] In the event that one of the systems is lost, the other can often regrow it. In fact it is possible to grow an entire plant from a single leaf, as is the case with plants inStreptocarpus sect.Saintpaulia,^[189] or even a singlecell – which can dedifferentiate into acallus (a mass of unspecialised cells) that can grow into a new plant.^[186]In vascular plants, the xylem and phloem are the conductive tissues that transport resources between shoots and roots. Roots are often adapted to store food such as sugars orstarch,^[183] as insugar beets and carrots.^[188]

Stems mainly provide support to the leaves and reproductive structures, but can store water in succulent plants such ascacti, food as in potatotubers, orreproduce vegetatively as in thestolons ofstrawberry plants or in the process oflayering.^[190] Leaves gather sunlight and carry outphotosynthesis.^[191] Large, flat, flexible, green leaves are called foliage leaves.^[192]Gymnosperms, such asconifers,cycads,Ginkgo, andgnetophytes are seed-producing plants with open seeds.^[193]Angiosperms areseed-producing plants that produce flowers and have enclosed seeds.^[152] Woody plants, such asazaleas andoaks, undergo a secondary growth phase resulting in two additional types of tissues: wood (secondaryxylem) and bark (secondaryphloem andcork). All gymnosperms and many angiosperms are woody plants.^[194] Some plants reproduce sexually, some asexually, and some via both means.^[195]

Although reference to major morphological categories such as root, stem, leaf, and trichome are useful, one has to keep in mind that these categories are linked through intermediate forms so that a continuum between the categories results.^[196] Furthermore, structures can be seen as processes, that is, process combinations.^[49]

Systematic botany is part of systematic biology, which is concerned with the range and diversity of organisms and their relationships, particularly as determined by their evolutionary history.^[197] It involves, or is related to, biological classification, scientific taxonomy andphylogenetics. Biological classification is the method by which botanists group organisms into categories such asgenera orspecies. Biological classification is a form ofscientific taxonomy. Modern taxonomy is rooted in the work ofCarl Linnaeus, who grouped species according to shared physical characteristics. These groupings have since been revised to align better with theDarwinian principle ofcommon descent – grouping organisms by ancestry rather thansuperficial characteristics. While scientists do not always agree on how to classify organisms,molecular phylogenetics, which usesDNA sequences as data, has driven many recent revisions along evolutionary lines and is likely to continue to do so. The dominant classification system is calledLinnaean taxonomy. It includes ranks andbinomial nomenclature. The nomenclature of botanical organisms is codified in theInternational Code of Nomenclature for algae, fungi, and plants (ICN) and administered by theInternational Botanical Congress.^[198][199]

KingdomPlantae belongs toDomainEukaryota and is broken down recursively until each species is separately classified. The order is:Kingdom;Phylum (or Division);Class;Order;Family;Genus (pluralgenera);Species. The scientific name of a plant represents its genus and its species within the genus, resulting in a single worldwide name for each organism.^[199] For example, the tiger lily isLilium columbianum.Lilium is the genus, andcolumbianum thespecific epithet. The combination is the name of the species. When writing the scientific name of an organism, it is proper to capitalise the first letter in the genus and put all of the specific epithet in lowercase. Additionally, the entire term is ordinarily italicised (or underlined when italics are not available).^{[200][201][202]}

The evolutionary relationships and heredity of a group of organisms is called itsphylogeny. Phylogenetic studies attempt to discover phylogenies. The basic approach is to use similarities based on shared inheritance to determine relationships.^[203] As an example, species ofPereskia are trees or bushes with prominent leaves. They do not obviously resemble a typical leaflesscactus such as anEchinocactus. However, bothPereskia andEchinocactus have spines produced fromareoles (highly specialised pad-like structures) suggesting that the two genera are indeed related.^[204][205]

Judging relationships based on shared characters requires care, since plants may resemble one another throughconvergent evolution in which characters have arisen independently. Someeuphorbias have leafless, rounded bodies adapted to water conservation similar to those of globular cacti, but characters such as the structure of their flowers make it clear that the two groups are not closely related. Thecladistic method takes a systematic approach to characters, distinguishing between those that carry no information about shared evolutionary history – such as those evolved separately in different groups (homoplasies) or those left over from ancestors (plesiomorphies) – and derived characters, which have been passed down from innovations in a shared ancestor (apomorphies). Only derived characters, such as the spine-producing areoles of cacti, provide evidence for descent from a common ancestor. The results of cladistic analyses are expressed ascladograms: tree-like diagrams showing the pattern of evolutionary branching and descent.^[206]

From the 1990s onwards, the predominant approach to constructing phylogenies for living plants has beenmolecular phylogenetics, which uses molecular characters, particularlyDNA sequences, rather than morphological characters like the presence or absence of spines and areoles. The difference is that the genetic code itself is used to decide evolutionary relationships, instead of being used indirectly via the characters it gives rise to.Clive Stace describes this as having "direct access to the genetic basis of evolution."^[207] As a simple example, prior to the use of genetic evidence, fungi were thought either to be plants or to be more closely related to plants than animals. Genetic evidence suggests that the true evolutionary relationship of multicelled organisms is as shown in the cladogram below – fungi are more closely related to animals than to plants.^[208]

In 1998, theAngiosperm Phylogeny Group published aphylogeny for flowering plants based on an analysis of DNA sequences from most families of flowering plants. As a result of this work, many questions, such as which families represent the earliest branches ofangiosperms, have now been answered.^[54] Investigating how plant species are related to each other allows botanists to better understand the process of evolution in plants.^[209] Despite the study of model plants and increasing use of DNA evidence, there is ongoing work and discussion among taxonomists about how best to classify plants into varioustaxa.^[210] Technological developments such as computers andelectron microscopes have greatly increased the level of detail studied and speed at which data can be analysed.^[211]

A few symbols are in current use in botany. A number of others are obsolete; for example, Linnaeus used planetary symbols⟨♂⟩ (Mars) for biennial plants,⟨♃⟩ (Jupiter) for herbaceous perennials and⟨♄⟩ (Saturn) for woody perennials, based on the planets' orbital periods of 2, 12 and 30 years; and Willd used⟨♄⟩ (Saturn) for neuter in addition to⟨☿⟩ (Mercury) for hermaphroditic.^[212] The following symbols are still used:^[213]

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