Flowers, also known asblossoms andblooms, are the reproductive structures offlowering plants. Typically, they are structured in four circular levels around the end of a stalk. These include:sepals, which are modified leaves that support the flower;petals, often designed to attract pollinators; malestamens, wherepollen is presented; and femalegynoecia, where pollen is received and its movement is facilitated to the egg. When flowers are arranged in a group, they are known collectively as aninflorescence.
The development of flowers is a complex and important part in the life cycles of flowering plants. In most plants, flowers are able to producesex cells of both sexes. Pollen, which can produce the male sex cells, is transported between the male and female parts of flowers inpollination. Pollination can occur between different plants, as incross-pollination, or between flowers on the same plant or even the same flower, as inself-pollination. Pollen movement may be caused by animals, such as birds and insects, or non-living things like wind and water. The colour and structure of flowers assist in the pollination process.
After pollination, the sex cells are fused together in the process offertilisation, which is a key step insexual reproduction. Through cellular and nuclear divisions, the resulting cell grows into aseed, which contains structures to assist in the future plant's survival and growth. At the same time, the female part of the flower forms into afruit, and the other floral structures die. The function of fruit is to protect the seed and aid in its dispersal away from the mother plant. Seeds can be dispersed by living things, such as birds who eat the fruit and distribute the seeds when theydefecate. Non-living things like wind and water can also help to disperse the seeds.
Flowers first evolved between 150 and 190million years ago, in theJurassic. Plants with flowers replaced non-floweringplants in many ecosystems, as a result of flowers' superior reproductive effectiveness. In thestudy of plant classification, flowers are a key feature used to differentiate plants. For thousands of years humans have used flowers for a variety of other purposes, including: decoration, medicine, food, and perfumes. In human cultures, flowers are used symbolically and feature in art, literature, religious practices, ritual, and festivals. All aspects of flowers, including size, shape, colour, and smell, show immense diversity across flowering plants. They range in size from 0.1 mm (1⁄250 inch) to 1 metre (3.3 ft), and in this way range from highly reduced and understated, to dominating the structure of the plant. Plants with flowers dominate the majority of the world's ecosystems, and themselves range from tinyorchids and majorcrop plants to large trees.
Etymology
Inbotany, flowers are defined as the reproductive structures of angiosperms (flowering plants),[1] whilecones are regarded as thegymnosperm equivalent.[2][note 1]Bloom is similarly defined, but may also be used to describe the collective of flowers on a plant, as in the phrase:covered with bloom.[4]Flower is also commonly used to describe the whole of a plant that produces flowers.[4]
Flower enteredMiddle English viaOld Frenchflor from earlierLatinflōs,flōris and before thatProto-Italic*flōs, all of which had the same meaning 'flower'.[5][6] The spellingflour was more common in English until the 17th century, when it became specialised to mean "ground grain" — originally an instance of figurativeflower meaning "best part; finest".[7] TheOld English word for flower wasblossom,[8] which is still used today, but refers especially to the flowers of ediblefruit trees, and not to the whole flowering plant.[4]Flower,bloom, andblossom are allcognates and are derived from theProto-Indo-European word*bʰleh₃ōs ('blossoming').[6] Bothbloom andblossom refer to flowers as well as the state of flowering; as in the phrases:in bloom orin blossom.[4]
Function
The main purpose of a flower isreproduction of the individual,[9] aiding in the survival of the species.[10] Flowers not only producespores, which becomegametophytes that producesex cells, leading tofertilised cells, but also develop and help disseminateseeds.[11]Sexual reproduction between plants results inevolutionary adaptation, which improves species survival. Plants favourcross-pollination because it promotes the joining of sex cells from genetically distinct plants of the same species, thereby increasinggenetic diversity. Facilitating this process is a key function of flowers and is often reflected in their form and structure.[11] Features designed to attractpollinators are among the most common adaptations.[12]
The structure of a flower, termed itsmorphology,[13] can be considered in two parts: the vegetative part, consisting of non-reproductive structures such aspetals; and the reproductive or sexual parts. A stereotypical, or complete,[14] flower is made up of four kinds of structures arranged in sets called whorls. They grow around the tip of a short stalk or axis, called areceptacle.[15] The four main whorls (starting from the base of the flower and working upwards) are thecalyx,petals,androecium, andgynoecium.[16]
The non-reproductive or vegetative part of the flower, known collectively as theperianth, consists of calyx (the modified outer leaves), and the petals. The receptacle is the thickened part of the flower stalk, called the pedicel, which supports all of the other flower structures.[14][17]
Calyx
Thesepals, collectively called the calyx, are modified leaves that occur on the outermost whorl of the flower. They are leaf-like,[18] in that they have a broad base,pores,green pigment, and may have analogousoutgrowths from the stem. Sepals are often waxy, tough, and grow quickly to protect the flower as it develops.[17][19] Although they sometimesfall off at maturity, sepals more commonly persist to protect the fruit and aid in its dispersal.[20] The sepals in some flowers may be partially or completelyfused together.[19][21]
Petals
Thepetals, collectively called the corolla,[20] are almost or completely fibreless leaf-like structures that form the innermost whorl of the perianth. They are often delicate and thin and are usually coloured, shaped, or scented, to encourage and facilitate pollination.[22] The petals may befused together.[23] Petals also tend to have patterns only visible underultraviolet light, which is visible to pollinators but not to humans.[22] In some flowers, petals and sepals areindistinguishable from one another.[24]
Reproductive
Diagram of an anther in cross section. 1: Filament; 2: Theca; 3: Connective (the conducting vessels in red); 4: Pollen sac (also called sporangium)
All flowering plants areheterosporous, that is, every individual plant produces two types ofspores. Spores are formed frommature plants, which containtwo sets of chromosomes, and are divided into microspores and megaspores—the precursors to pollen and embryo sacs respectively. Pollen and embryo sacs are the male and femalegametophytes, sex cell-producing structures, and contain just one set of chromosomes. Microspores are produced bymeiosis inside anthers, the male part of flowers,[20] and megaspores are produced insideovules contained within theovary.[25][26] As with all heterosporous plants, the gametophytes also developinside the spores.[25]
Male
Theandroecium is the whorl of male parts called stamens, which producepollen. Stamens consist typically of ananther, made up of four pollen sacs arranged in two sheaths calledthecae, connected to afilament, or stalk.[20][23] The anther contains microspores which become pollen, the malegametophyte, after undergoingmeiosis.[25] Although they exhibit the widest variation among floral organs,[note 2] the androecium is usually confined just to one whorl and to two whorls only in rare cases.[23]
Female
Thegynoecium, consisting of one or morecarpels, is the female part of the flower and found on the innermost whorl.[20] Each carpel consists of: astigma, which receives pollen; astyle, the stalk; and anovary, which contains theovules, and the female gametophytes by extension. Carpels may be fused together and are often described collectively as a pistil. Inside the ovary, the ovules are attached to theplacenta by structures called funiculi.[28][29]
Variation
Diagram showing that: hermaphrodite flowers have both sexes, monoecious plants have sexes on different flowers, and dioecious plants have either just female or just male flowers.
A healthy (left) and infected (right)Hydrangea flower.Phytoplasma has caused the flower to develop leaves in place of petals—an example ofphyllody.[30]
Although most plants have flowers with four whorls—protective leaves, petals, male parts, and female parts—and their typical sub-structures, they vary greatly between flowering plants.[14][31] This variation encompasses all aspects of flowers, including size, shape, and colour.[14] Flowers range in size from0.1 mm (1⁄250 in) (duckweed) to 1 m (3.3 ft) in diameter (corpse flower).[14] Additionally, the four main parts of a flower are generally defined by their positions and not by their function. Many flowers lack some parts, have parts that are modified for other functions, or contain parts that look like what is typically another part.[14][32][33] In some flowers, organs such as stamens, stigmas, and sepals are modified to resemble petals. This is most common in cultivation (such as ofroses), where flowers with many additional "petals" are found to be more attractive.[34][35]
Most flowers have symmetry.[36] When the flower is bisected through the central axis from any point and symmetrical halves are produced,[37] the flower is said to beregular (as insedges). This is an example ofradial symmetry. If there is only one plane of symmetry (as inorchids),[38] the flower is said to be irregular. If, in very rare cases, they have no symmetry at all they are called asymmetric.[39][40] Floral symmetry is a key driver of diversity in flower morphology, because it is one of the main features derived through flower-plant coevolution. Irregular flowers often coevolve with specific pollinators, while radially symmetric flowers tend to attract a wider range of pollinators.[41][note 3]
In the majority of species, individual flowers have both female parts and male parts — such flowers are described as being perfect, bisexual, orhermaphrodite. In some species of plants, the flowers are imperfect or unisexual: having only either male or female parts. If unisexual male and female flowers appear on the same plant, the species is calledmonoecious. However, if an individual plant is either female or male, the species is calleddioecious.[43] Many flowers havenectaries, which are glands that produce nectar: a sugary fluid used to attract pollinators. Their shape varies between different plants,[44] are they not considered as an organ on their own.[44]
Some flowers are lacking or have only a highly reduced stalk, and so areattached directly to the plant.[45] There are several structures, found in some plants, that resemble flowers or floral organs. These include:coronas, crown-like outgrowths;[46] and pseudonectaries, that look like nectaries but do not contain nectar.[47] In plants where disease has taken hold,phyllody—leafy flower parts—may occur.[48]
Adandelion inflorescence is made up of many small flowers grouped closely together to look like a single flower (apseudanthium).
In plants that have more than one flower on an axis, the collective cluster of flowers is called an inflorescence.[14] Some inflorescences are composed of many small flowers arranged in a formation that resembles a single flower. These are known aspseudanthia.[49] A singledaisy orsunflower, for example, is not a flower but an inflorescence composed of numerous florets, or tiny flowers.[50] An inflorescence may include specialised stems and modified leaves known calledbracts, as well as smaller bracteoles.[15]
A floral formula is a way to represent the structure of a flower using letters, numbers, and symbols in a compact way. It can represent both agroup of species or a particular species, and usually gives ranges for the numbers of different organs. The format of floral formulae differs in different parts of the world, but the formulae all convey the same information.[51][52]
Floral diagrams are schematic diagrams that can be used to show important features of flowers, including the relative positions of the various organs, the presence of organ fusion and symmetry, and structural details.[53]
In contrast to the mostly green vegetative parts of plants, flowers are often colourful. This includes the petals and, in some plants, the stamens, anthers, stigmas, ovaries, pollen, styles, and even nectar.[54] These colours are produced mainly bybiological pigments, which are molecules that can absorb and retain energy from light.[55][56] Specific pigments, and so colours, provide different benefits to the plant. These benefits include protecting the plant against degradation and guiding pollinators—both general and specific—to the plant.[57][58]
Colour, or colour effects, may also be produced bystructural coloration, in which colour is produced by tiny surface structures interfering with waves of light.[59] This includesiridescence (as in sometulips) andphotonic crystals (as inedelweiss), which diffract light using tiny grooves.[60][61] The colour of flowers can also change; sometimes this acts as a signal to pollinators (as inViola cornuta). Change may also occur as a result of temperature;pH, as in theanthoxanthins found inHydrangea; metals; sugars; and cell shape.[62]
Diagram showing the floral organs within a developing rose flower, orrose bud
Diagram of the ABC model of development
Floral development begins with the transformation of vegetative growth into floral growth.[63] This is regulated by both genetic and environmental factors.[64] The eventual formation of a flower starts with a shoot apicalmeristem (SAM): a group of dividing cells responsible for leaves and buds. The organs which make up a flower—in most cases the sepals, petals, male parts, and female parts—grow out of agrowth-limited floral meristem, which a SAM creates.[63] TheABC model of flower development can be used, for many plants, to describe how groups of genes come together to induce each organ being produced.[65] In general, all aspects of floral development are controlled by agene regulatory network of specialisedMADS-box genes—which includes the ABC genes—and associatedproteins.[66][67] For plants, the transition into flowering is a major change and must occur at the right time so as to ensure reproductive success. Plants determine this time by interpreting both internal and environmental cues, such asday length.[64]
The ABC model was the first unifyingprinciple in the development of flowers, and its major tenets have been found to hold in most flowering plants.[68] It describes how three groups ofgenes—A, B, and C—are responsible for the development of flowers. These three gene groups' activities interact together to determine the developmental identities of theprimordia organ within the floralapical meristem. Alone, A genes produce sepals in the first whorl. Together, A and B produce the petals in the second whorl. C genes alone produce carpels in the centre of the flower. C and B together produce the stamens in the third whorl.[65] This can also be extended to the more complex ABCDE model, which adds an additional two gene groups to explain the development of structures like ovules.[69]
The transition to flowering is one of the major phase changes that a plant makes during its life cycle.[14] The transition must take place at a time that is favourable forfertilisation and the formation ofseeds, hence ensuring maximalreproductive success. To meet these needs a plant can interpret importantinternal and environmental cues such as: changes in levels ofplant hormones (such asgibberellins),[70] seasonabletemperature, andday length changes.[64] Many plants, including many of those that havemore than two-year lifespans and justtwo-year lifespans, requirecold exposure to flower.[70][71][72] These cues are interpreted molecularly through a complex signal calledflorigen, which involves a variety ofgenes. Florigen is produced in the leaves in reproductively favourable conditions and acts in stem tips to force switching from developing leaves to flowers.[73] Once developed, flowers mayselectively open and close their flowers at different times of day; usually arounddusk anddawn.[74] They may alsotrack the path of the sun to remain warm—potentially both for their own benefit and to attract pollinators. Both of these mechanisms are controlled by a plant'scircadian rhythm and in response to environmental changes.[75]
Since the flowers are the reproductive organs of the plant, they mediate the joining of the sperm, contained within pollen, to the eggs in the ovules—contained in the ovary.[77] Pollination is this movement of pollen from the male parts to the female parts.[78] It occurs either between flowers (or from one part of a flower to another) of the same plant, as inself-pollination, or between flowers of different plants, as in cross-pollination. Cross-pollination is more common in flowering plants as it increasesgenetic variation.[11][79] Pollination typically only takes place when the flower isfully expanded and functional.[80]
Flowering plants usually faceevolutionary pressure to optimise the transfer of theirpollen, and this is typically reflected in the morphology of their flowers and their reproductive strategies.[81][82] Agents that transport pollen between plants are called vectors. Around 80% of flowering plants make use of biotic or living vectors. Others use abiotic or non-living vectors, or some combination of the two.[83][84]
Flowers that use biotic vectors attract and use animals to transfer pollen from one flower to the next. Often they are shaped and designed to both attract pollinators and ensure pollen is transferred effectively.[12][85] Flowers most commonlyemploy insects,[86][87] but also:birds, bats, lizards,[88] other mammals,[89] snails and slugs,[86] and, in rare cases,crustaceans and worms.[89] Rewards given to pollinators by flowers to encourage pollination include: food (such aspollen, starch, ornectar), mates, shelter, a place to raise their young, andpseudocopulation (sexual deception).[85] In the latter, the flower is scented or shaped so as to encouragesexual arousal and pollination from the subsequentintercourse.[90] They may also be attracted by various stimuli such as size and scent (as incarrion flowers). Colour is also a factor, and includesnectar guides, which show pollinators where to look for nectar; they may be visible only underultraviolet light.[12][91][92]
Many flowers have close relationships with just one or a few specific pollinators. They may be structured to allow or encourage pollination from these organisms. This increases efficiency, because there is a higher chance pollination comes from pollen of the same species of plant.[93] This close relationship is an example ofcoevolution, as the plant and pollinator have developed together over a long period to match each other's needs.[58]
Flowers that use abiotic, or non-living, vectors use thewind or, much less commonly,water, to move pollen from one flower to the next.[84]Wind-dispersed species do not need to attract pollinators and therefore tend not to grow large, showy, or colourful flowers, and do not have nectaries, nor a noticeable scent.[93] Whereas the pollen of insect-pollinated flowers is usually large, sticky, and rich inprotein to act as a "reward", wind-pollinated flowers' pollen is typically small, very light, smooth, and of little nutritional value.[94][95]
Fertilisation is the fusion of the male and femalesex cells to produce azygote, from which a new organism develops.[10] In humans,sexual intercourse results in the depositing of sperm cells into thevagina. Although not all survive, they travel until one reaches the egg in thefallopian tube, where the male and female sex cells fuse in the process of fertilisation.[96]
In flowering plants, fertilisation is preceded by pollination, which is the movement of pollen from the stamen to the carpel. It encompasses bothplasmogamy, the fusion of theprotoplasts (cell without cell wall), andkaryogamy, the fusion of thenuclei. When pollen lands on the stigma of the flower it begins creating apollen tube, which runs down through the style and into the ovary. After penetrating the centre-most part of the ovary it enters the egg apparatus and is guided by aspecialised cell.[97]
Next, the end of the pollen tube bursts and releases the two sperm cells, one of which makes its way to an egg, while also losing itscell membrane and much of thejelly-like substance that fills its cells. The sperm's nucleus then fuses with the egg's nucleus, resulting in the formation of a zygote; adiploid cell, containing two copies of eachchromosome.[10][97] Flowering plants undergodouble fertilisation, which involves both karyogamy and plasmogamy. In double fertilisation the second sperm cell subsequently also fuses with the two polar nuclei of the central cell. Since all three nuclei arehaploid, they result in a largenutrient tissue nucleus which istriploid.[97]
Development of seeds (peas) and fruit (pod) frompea flower
Following its formation, the zygote begins to grow through nuclear and cellular divisions, calledmitosis, eventually becoming a small group of cells. One section of it becomes theembryo,[98] while the other becomes the suspensor; a structure which forces the embryo into theendosperm and is later undetectable. Two smallgroups of cells also form at this time, which later become thecotyledon, or initial leaf, which is used as an energy store. The next stage involves the growth of several key structures, including: theembryotic root, theembryotic stem, and theroot or shoot junction itself. In the final step,vascular tissue develops around the seed.[99]
The ovary, inside which the seed is forming from the ovule, grows into afruit. All the other main floral parts wither and die during this development, including: the style, stigma, stamens, petals, and sepals. This process is called floralsenescence; it is often accelerated or initiated by the completion of pollination. Death is preferred because flowers are costly to the plant; nevertheless, flowers can last for between a few hours and several months.[100][101] The fruit contains three main structures: the outer layer ofpeel; thefleshy part; and thestone, or innermost layer. Thepericarp, which may include one or more of these structures, represents collectively the fruit wall—everything but the seed. The size, shape, toughness, and thickness of the pericarp varies among differentdry andfleshy fruits. These traits are directly connected to the plant's method of seed dispersal, since the purpose of fruit is to encourage or enable the seed's dispersal and protect the seed while doing so.[99][102]
Following the pollination of a flower, fertilisation, and finally the development of a seed and fruit, a mechanism, or vector, is typically used to disperse the fruit away from the plant.[103] In flowering plants, seeds are dispersed away from the plant so as to not force competition between the mother and daughter plants,[104] as well as to enable the colonisation of new areas. Vectors can generally be divided into two categories: external vectors and internal vectors.[105][106] External vectors include living things like birds or bats, or non-living things such as water and wind.[105][107] Internal vectors, which are derived from the plant itself,[105][106] include, for example, the fruitexploding to release the seeds, as indwarf mistletoes.[108]
Angiosperms now account for most of (land) plant diversity, thanks in large part to flowers.[109][110]
Flowers originated between 150 and 190million years ago, during theJurassic.[111][112] Althoughmolecular analyses indicate this early appearance of angiosperms—flowering plants, the earliest definitive evidence from the fossil record comes from between 125 and 130 million years ago, during theEarly Cretaceous.[113][114][115][note 5] The exact time at which angiosperms diverged from otherseed plants is a classic open question inevolutionary biology.[112][116][117] Prior to the advent of flowers, plants reproduced usingcones (as ingymnosperms),[118] and spores (as inpteridophytes).[119] The transformation ofspore-producing leaves into structures like stamens and carpels, is the most clear milestone in the complex evolution of flowers.[115] There is debate both over whether these and other changes happened gradually or as sudden shifts likehomeotic mutations, and which aspect of flower morphology came first.[120][121]
The flower was the angiosperms' most significant evolutionary innovation,[109] granting the ability to effectively take advantage of animal pollinators.[115] Otherevolutionary advantages included: being able to have both male and female parts on the same axis; and on this axis have carpels, to protect the ovules; stamens, to present the pollen; and the perianth, to provide protection. In addition, they pioneereddouble fertilisation, which allows energy investment (intoendosperm) to be prolonged until after pollination. The gametophytes, which lead to sex cells, were very reduced, which allowed for greater protection of this critical process.[122] The net effect of these features was greater reproductive security and efficiency.[112] This allowed the angiosperms to replace many otherseed plants—such asPinales,cycads,Gnetophyta andGinkgoales—in the majority ofecosystems.[112]
A key driving force in the evolution of flowers iscoevolution, where pollinator and flower evolve with one another,[123] often to their mutual benefit. This is particularly prominent in insect species such as bees, but is also found in flower-pollinator relationships with birds and bats. Many flowers have evolved in such a way so as to make pollination by specific species easier, thus providing greater efficiency and also ensuring higher rates of pollination. This is because they receive less pollen from other plant species.[58][93] However, this close interdependence increases the risk ofextinction, since the extinction of either member almost certainly means the extinction of the other member as well.[124] Modern-day flowers exhibit a variety of features derived through coevolution including: shape, size, symmetry, timing of flower opening, colour, scent, and pollinator rewards (including pollen, nectar, and oils).[12][92] For example,Japanese honeysuckle flowers strategically open during the night to attractnocturnal moths, which are more efficient pollinators thandiurnal bees.[125] With the innovation of the flower—and other adaptations—angiosperms rapidly diversified.[note 6] Approximately 90% of all living land plant species are angiosperms.[110] This is attributed, in part, to coevolution, which caused specialisation and sospeciation; where populations diverge into separate species.[126] Both the strength of close pollinator-flower relationships and the survival of either species are effected byclimate change. Reducing numbers of pollinators have led to the extinction of many flowering plants.[127]
Classical and modern approaches to angiosperm taxonomy
Inplant taxonomy, which is the study of plant classification and identification, themorphology of plants' flowers is used extensively—and has been since at leastclassical Greece.[128][129] Despite earlier works,Carl Linnaeus's 1753 bookSpecies Plantarum, in which he laid out his system of classification, is regarded as the first taxonomic work to recognise the significance of flowers.[129][130] He identified 24 classes of flowering plants, based mainly on the number, length, and union of thestamens.[130][131][132] Subsequent systems in the 18th and 19th centuries focused more on natural characteristics. This included taking into account the rest of the plant, so that diverse plants weren't put into the same groups, as often happened in Linnaeus's system.[132][133][134]
In 1963, the biologistsRobert Sokal andPeter Sneath created the method ofnumerical taxonomy, which differentiatestaxa based on their tabulated morphological characteristics; such as their flowers. This was an effort to make plant taxonomy more objective, but it remained inconsiderate of evolution, and so not useful in that context.[135] While this and earlier methods, such as Linnaeus's, used morphological features, many botanists today employgenetic sequencing, thestudy of cells, and thestudy of pollen. These come as a result of advancements inDNA-related science.[136] Despite this, morphological characteristics such as the nature of the flower andinflorescence still make up the bedrock of plant taxonomy.[137][138][139]
Over millennia, humans have come to use flowers for a variety of purposes around the world, including decoration, medicine, drugs,[140] food,spices,[141] perfumes,[142] andessential oils. Many flowers are edible and are often used in drinks and dishes, such as salads, for taste, scent, and decoration.[143] Inflorescences and the bracts or stems of some flowers are commonly described as vegetables. These include:broccoli,cauliflower, andartichoke. Flowers may be eaten freshly after being picked, or dried and eaten later.[144]Floristry is the production and sale of flowers, and involves preparing freshly cut flowers and arranging them—in abouquet, for example—to the client's liking.[145]
Most crop plants have flowers,[146] and they produce much of the most common crop products—such as seeds and fruits;[10] around half of all cropland is used to grow three flowering plants: rice, wheat, and corn.[147] Flowers are steeped to make teas, either alone, as inherbal teas, or in combination with thetea plant.[148][149] Essential oils and other flower extracts are widely used inherbal medicines anddecoctions because they containphytochemicals and may haveanti-microbial effects.[150][151] Flowers from many plants are also used in the production ofdrugs, such ascannabis,bush lily, andMadagascar periwinkle.[140] Some flowers are used in cooking as spices, these includesaffron andcloves; derived fromCrocus andSyzygium aromaticum respectively.[141]
"I know a bank where the wild thyme blows, Where oxlips and the nodding violet grows, Quite over-canopied with luscious woodbine, With sweet musk-roses and with eglantine: There sleeps Titania sometime of the night, Lull'd in these flowers with dances and delight;"
Flowers are the subject of much symbolism, and feature often in art, ritual, religious practices, and festivals. Plants have been cultivated ingardens for their flowers for around ten thousand years.[152][153] Flowers are associated withburial in many cultures, and are often placed byheadstones to pay respect.[154][155] They are also placed by statues or temples of religious or other figures—sometimes formed into floralwreaths.[156][157] In some places, the dead are buried covered in flowers or on a bed of flowers.[158] They are also associated with love and celebration, and given to others in many places for this reason.[159][151] Economic demand has led to the cultivation of flowers that are longer-lasting, more colourful, and visually appealing.[160]
Flowers feature extensively in art across a variety of mediums, and different flowers are ascribedsymbolic meanings.[161][162] For example,violets may represent modesty,virtue, or affection.[163] In addition to hidden meanings, flowers are used in flags, emblems, and seals. In this way, they represent countries or places. Some countries havenational flowers; for example,Hibiscus × rosa-sinensis is the national flower ofMalaysia.[164] In literature, flowers feature inimagery of places and asmetaphors for pleasure, beauty, and life.[165]
^There are somegymnosperm cones which resemble flowers. The cones ofGinkgo biloba, for example, are mostly considered to be simplestrobili, and not flowers.[3]
^Stamens range in number, size, shape, orientation, and in their point of connection to the flower.[23] In general, plants have only one type of stamen, but there are plant species where the flowers have two types; a typical one, and one with anthers that produce sterile pollen meant to attract pollinators with food (palynivory). These plants are called heterantherous.[27]
^Because animal pollinators are themselves irregular, there is only one comfortable orientation they can have on an irregular flower. Organs can then be arranged to ensure pollen is placed on their bodies in a specific position than ensures pollination of the subsequent flower.[42] Floral symmetry also assists in heat retention, which is required for the growth and effective performance of the floral organs.[41]
^This describes that the flower: (*) is radially symmetric, (K5) has 5 sepals, (C(5)) has 5 fused petals, (A5) has 5 stamens, and (G(2)) has two fused carpels.
^One such early flower isArchaefructus liaoningensis from China; dated to around 125 million years old.[113] Even earlier from China is the 125–130 million years oldArchaefructus sinensis. In 2015Montsechia vidalii, discovered in Spain, was claimed to be 130 million years old.[114]
^These other adaptions include greater density of leaf veins andstomata; smallergenome size, leading to smaller cells; higher rates of photosynthesis; and vessels connected to thexylem.[122]
Buchmann, Stephen L. (2016).The reason for flowers: their history, culture, biology, and how they change our lives (1st ed.). Scribner.ISBN978-1-4767-5552-6.
Leins, Peter (2010).Flower and fruit: morphology, ontogeny, phylogeny, function and ecology. Claudia Erbar. Schweizerbart.ISBN978-3-510-65261-7.OCLC678542669.
Rouhan, Germinal; Gaudeul, Myriam (2021). "Plant taxonomy: A historical perspective, current challenges, and perspectives". In Besse, Pascale (ed.).Molecular plant taxonomy: methods and protocols. Methods in molecular biology (2nd ed.). Humana Press.ISBN978-1-0716-0997-2.
Singer, Rodrigo B.; Marsaioli, Anita Jocelyne; Flach, Adriana; Reis, Mariza G. (2006). "The ecology and chemistry of pollination in Brazilian orchids: recent advances".Floriculture, ornamental and plant biotechnology. Global Science Books.ISBN4-903313-00-X.
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