Aflower, also known as abloom orblossom,[1] is the reproductive structure found inflowering plants (plants of the divisionAngiospermae). Flowers consist of a combination of vegetative organs –sepals that enclose and protect the developing flower. Petals attract pollinators, and reproductive organs that producegametophytes, which in flowering plants producegametes. The male gametophytes, which produce sperm, are enclosed withinpollen grains produced in theanthers. The female gametophytes are contained within theovules produced in theovary.[2][3] In some plants, multiple flowers occur singly on apedicel (flower stalk), and some are arranged in a group (inflorescence) on apeduncle (inflorescence stalk).[2]
Most flowering plants depend on animals, such as bees, moths, and butterflies, to transfer their pollen between different flowers, and have evolved to attract thesepollinators by various strategies, including brightly colored, large petals with patterns only visible to under ultraviolet light, attractive scents, and the production ofnectar, a food source for pollinators.[4][2] In this way, many flowering plants haveco-evolved with pollinators to be mutually dependent on services they provide to one another—in the plant's case, a means of reproduction; in the pollinator's case, a source of food.[5]
When pollen from the anther of a flower is transferred to thestigma to another, it is called pollination. Some flowers mayself-pollinate, producing seed using pollen from a different flower of the same plant, but others have mechanisms to prevent self-pollination and rely oncross-pollination, when pollen is transferred from the anther of one flower to the stigma of another flower on a different individual of the same species. Self-pollination happens in flowers where thestamen andcarpel mature at the same time, and are positioned so that the pollen can land on the flower's stigma. This pollination does not require an investment from the plant to provide nectar and pollen as food for pollinators.[6] Some flowers producediaspores without fertilization (parthenocarpy).[7] After fertilization, the ovary of the flower develops intofruit containingseeds.[2]
Flowers have long been appreciated for their beauty and pleasant scents, and also hold cultural significance as religious, ritual, or symbolic objects, or sources ofmedicine and food.
Etymology
Flower is from theMiddle Englishflour, which referred to both the ground grain and the reproductive structure in plants, before splitting off in the 17th century. It comes originally from the Latin name of the Italian goddess of flowers,Flora. The early word for flower in English wasblossom,[8] though it now refers to flowers only offruit trees.[9]
Themorphology of a flower, or its form and structure,[10] can be considered in two parts: the vegetative part, consisting of non-reproductive structures such aspetals; and the reproductive or sexual parts. A stereotypical flower is made up of four kinds of structures arranged in whorls around the tip of a short stalk or axis, called areceptacle.[11][2] The four main whorls (starting from the base of the flower or lowest node and working upwards) are thecalyx,corolla,androecium, andgynoecium. Together the calyx and corolla make up the non-reproductive part of the flower called theperianth, and in monocotyledons, may not be differentiated. If this is the case, then they are described astepals.[2][12]
Thesepals, collectively called the calyx, are modified leaves that occur on the outermost whorl of the flower.[2] They are leaf-like, in that they have a broad base,stomata andchlorophyll and may havestipules. Sepals are often waxy and tough, and grow quickly to protect the flower as it develops.[13][14] They may bedeciduous, but will more commonly grow on to assist in fruit dispersal. If the calyx is fused it is called gamosepalous.[13]
Corolla
Thepetals, collectively called corolla, are almost or completely fiberless leaf-like structures that form the innermost whorl of the perianth. They are often delicate and thin and are usually colored, shaped, or scented to encourage pollination.[2][15] Although similar to leaves in shape, they are more comparable to stamens in that they form almost simultaneously with one another, but their subsequent growth is delayed. If the corolla is fused together it is calledsympetalous.[16] Inmonocotyledonous flowers (e.g.,Lilium sp.), petals and sepals are indistinguishable and are individually calledtepals. Petals also tend to have patterns only visible under ultraviolet light, which are visible to pollinators but not to humans.[2]
Reproductive parts of easter lily (Lilium longiflorum). 1. Stigma, 2. Style, 3. Stamens, 4. Filament, 5. Petal
Androecium
Theandroecium, consisting of stamens, is the whorl of pollen-producing male parts. Stamens consist typically of ananther, made up of four pollen sacs arranged in twothecae, connected to afilament, or stalk.[2] The anther contains microsporocytes which becomepollen, the malegametophyte, after undergoingmeiosis. Although they exhibit the widest variation among floral organs, the androecium is usually confined just to one whorl and to two whorls only in rare cases. Stamens range in number, size, shape, orientation, and in their point of connection to the flower.[15][16]
In general, there is only one type of stamen, but there are plant species where the flowers have two types; a "normal" one and one with anthers that produce sterile pollen meant to attract pollinators.[17]
Gynoecium
Thegynoecium, consisting of one or morecarpels, is the female part of the flower found on the innermost whorl. Each carpel consists of astigma, which receives pollen, astyle, which acts as a stalk, and anovary, which contains the ovules.[2] Carpels may occur in one to several whorls, and when fused are often described as apistil. Inside the ovary, theovules are attached to theplacenta by structures calledfuniculi.[18][19]
Variation
Although this arrangement is considered "typical", plant species show a wide variation in floral structure.[20] The four main parts of a flower are generally defined by their positions on the receptacle and not by their function. Many flowers lack some parts or parts may be modified into other functions or look like what is typically another part.[21] In some families, such as thegrasses, the petals are greatly reduced; in many species, the sepals are colorful and petal-like. Other flowers have modified petal-like stamens; the double flowers ofpeonies androses are mostly petaloid stamens.[22]
Many flowers have symmetry. When theperianth is bisected through the central axis from any point and symmetrical halves are produced, the flower is said to beactinomorphic or regular. This is an example ofradial symmetry. When flowers are bisected and produce only one line that produces symmetrical halves, the flower is said to be irregular orzygomorphic. If, in rare cases, they have no symmetry at all they are called asymmetric.[23][24]
Flowers may be directly attached to the plant at their base (sessile—the supporting stalk or stem is highly reduced or absent).[2][25] The stem or stalk subtending a flower, or aninflorescence of flowers, is called apeduncle. If a peduncle supports more than one flower, the stems connecting each flower to the main axis are calledpedicels.[2][26] The apex of a flowering stem forms a terminal swelling which is called the torus or receptacle.[2][24]
In the majority of species, individual flowers have both carpels and stamens. These flowers are described by botanists as being perfect, bisexual, orhermaphrodite. In some species of plants, the flowers are imperfect or unisexual: having only either male (stamen) or female (carpel) parts. If unisexual male and female flowers appear on the same plant, the species is calledmonoecious.[27] However, if an individual plant is either female or male, the species is calleddioecious. Many flowers have floralnectaries, which are glands that produce a sugary fluid (nectar) used to attract pollinators. They are not considered as an organ on their own.[28]
Thecalla lily is not a single flower. It is aninflorescence of tiny flowers pressed together on a central stalk that is surrounded by a large petal-likebract.[29]
In those species that have more than one flower on an axis, the collective cluster of flowers is called aninflorescence. Some inflorescences are composed of many small flowers arranged in a formation that resembles a single flower. A common example of this is most members of the very large composite (Asteraceae) group. A singledaisy orsunflower, for example, is not a flower but a flowerhead—an inflorescence composed of numerous flowers (or florets).[2][30] An inflorescence may include specialized stems and modified leaves known asbracts.[2][31]
Afloral formula is a way to represent the structure of a flower using specific letters, numbers, and symbols, presenting substantial information about the flower in a compact form. It can represent ataxon, usually giving ranges of the numbers of different organs, or particular species. Floral formulae have been developed in the early 19th century and their use has declined since. Prenneret al. (2010) devised an extension of the existing model to broaden the descriptive capability of the formula.[32] The format of floral formulae differs in different parts of the world, yet they convey the same information.[33][34][35][36]
The structure of a flower can also be expressed by the means offloral diagrams. The use of schematic diagrams can replace long descriptions or complicated drawings as a tool for understanding both floral structure and evolution. Such diagrams may show important features of flowers, including the relative positions of the various organs, including the presence of fusion and symmetry, as well as structural details.[37]
Development
A flower develops on a modified shoot oraxis from a determinate apicalmeristem (determinate meaning the axis grows to a set size). It has compressed internodes, bearing structures that in classical plant morphology are interpreted as highly modifiedleaves.[38] Detailed developmental studies, however, have shown that stamens are often initiated more or less likemodified stems (caulomes) that in some cases may even resemblebranchlets.[39][20] Taking into account the whole diversity in the development of the androecium of flowering plants, we find a continuum between modified leaves (phyllomes), modified stems (caulomes), and modified branchlets (shoots).[40][41]
Transition
The transition to flowering is one of the major phase changes that a plant makes during its life cycle. The transition must take place at a time that is favorable forfertilization and the formation ofseeds, hence ensuring maximalreproductive success. To meet these needs a plant can interpret important endogenous and environmental cues such as changes in levels ofplant hormones and seasonabletemperature andphotoperiod changes.[42] Manyperennial and mostbiennial plants requirevernalization to flower. The molecular interpretation of these signals is through the transmission of a complex signal known asflorigen, which involves a variety ofgenes, including Constans, Flowering Locus C, and Flowering Locus T. Florigen is produced in the leaves in reproductively favorable conditions and acts inbuds and growing tips to induce several different physiological and morphological changes.[43]
The ABC model of flower development
The first step of the transition is the transformation of the vegetative stem primordia into floral primordia. This occurs as biochemical changes take place to change the cellular differentiation of leaf, bud and stem tissues into tissue that will grow into the reproductive organs. Growth of the central part of the stem tip stops or flattens out and the sides develop protuberances in a whorled or spiral fashion around the outside of the stem end. These protuberances develop into the sepals, petals, stamens, andcarpels. Once this process begins, in most plants, it cannot be reversed and the stems develop flowers, even if the initial start of the flower formation event was dependent on some environmental cue.[44]
The ABC model is a simple model that describes thegenes responsible for the development of flowers. Three gene activities interact in a combinatorial manner to determine the developmental identities of theprimordia organ within the floralapical meristem. These gene functions are called A, B, and C. Genes are expressed in only the outer and lower most section of the apical meristem, which becomes a whorl of sepals. In the second whorl, both A and B genes are expressed, leading to the formation of petals. In the third whorl, B and C genes interact to form stamens and in the center of the flower C genes alone give rise to carpels. The model is based upon studies of aberrant flowers and mutations inArabidopsis thaliana and the snapdragon,Antirrhinum majus. For example, when there is a loss of B gene function, mutant flowers are produced with sepals in the first whorl as usual, but also in the second whorl instead of the normal petal formation. In the third whorl, the lack of the B function but the presence of the C function mimics the fourth whorl, leading to the formation of carpels also in the third whorl.[45]
The principal purpose of a flower is the reproduction[46] of the individual and the species. All flowering plants areheterosporous, that is, every individual plant produces two types ofspores. Microspores are produced bymeiosis inside anthers and megaspores are produced inside ovules that are within an ovary. Anthers typically consist of four microsporangia and an ovule is an integumented megasporangium. Both types of spores develop intogametophytes inside sporangia. As with all heterosporous plants, the gametophytes also develop inside the spores, i.e., they are endosporic.
Grains of pollen sticking to this bee will be transferred to the next flower it visits.
Since the flowers are the reproductive organs of the plant, they mediate the joining of the sperm, contained within pollen, to the ovules — contained in the ovary.[14] Pollination is the movement of pollen from the anthers to the stigma.[47] Normally pollen is moved from one plant to another, known ascross-pollination, but many plants can self-pollinate. Cross-pollination is preferred because it allows forgenetic variation, which contributes to the survival of the species.[48] Many flowers depend on external factors for pollination, such as the wind, water, animals, and especiallyinsects. Larger animals such as birds, bats, and even somepygmy possums,[49] however, can also be employed.[50][51] To accomplish this, flowers have specific designs which encourage the transfer of pollen from one plant to another of the same species. The period during which this process can take place (when the flower is fully expanded and functional) is calledanthesis,[52] hence the study of pollination biology is calledanthecology.[53]
Flowering plants usually faceevolutionary pressure to optimize the transfer of theirpollen, and this is typically reflected in the morphology of the flowers and the behavior of the plants.[54] Pollen may be transferred between plants via several 'vectors,' or methods. Around 80% of flowering plants make use of biotic or living vectors. Others use abiotic, or non-living, vectors and some plants make use of multiple vectors, but most are highly specialized.[55]
Though some fit between or outside of these groups,[56] most flowers can be divided between the following two broad groups of pollination methods:
Biotic pollination
Flowers that use biotic vectors attract and useinsects,bats,birds, or other animals to transfer pollen from one flower to the next. Often they arespecialized in shape and have an arrangement of the stamens that ensures that pollen grains are transferred to the bodies of the pollinator when it lands in search of its attractant (such as nectar, pollen, or a mate).[57] In pursuing this attractant from many flowers of the same species, the pollinator transfers pollen to the stigmas—arranged with equally pointed precision—of all of the flowers it visits.[58] Many flowers rely on simple proximity between flower parts to ensure pollination, while others have elaborate designs to ensure pollination and preventself-pollination.[48] Flowers use animals including: insects (entomophily), birds (ornithophily), bats (chiropterophily), lizards,[51] and even snails and slugs (malacophilae).[59]
Attraction methods
Ophrys apifera, a bee orchid, which has evolved over many generations to mimic a female bee.[60]
Plants cannot move from one location to another, thus many flowers have evolved to attract animals to transfer pollen between individuals in dispersed populations. Most commonly, flowers are insect-pollinated, known asentomophilous; literally "insect-loving" in Greek.[61] To attract these insects flowers commonly have glands callednectaries on various parts that attract animals looking for nutritiousnectar.[62] Some flowers have glands calledelaiophores, which produce oils rather than nectar.[63] Birds and bees havecolor vision, enabling them to seek out colorful flowers.[64] Some flowers have patterns, callednectar guides, that show pollinators where to look for nectar; they may be visible only underultraviolet light, which is visible to bees and some other insects.[65]
Flowers also attract pollinators byscent, though not all flower scents are appealing to humans; several flowers are pollinated by insects that are attracted to rotten flesh and have flowers that smell like dead animals. These are often calledcarrion flowers, including plants in the genusRafflesia, and thetitan arum.[64] Flowers pollinated by night visitors, including bats and moths, are likely to concentrate on scent to attract pollinators and so most such flowers are white.[66] Some plants pollinated by bats have a sonar-reflecting petal above its flowers, which helps the bat find them,[67] and one species, the cactusEspostoa frutescens, has flowers that are surrounded by an area of sound-absorbent and woolly hairs called the cephalium, which absorbs the bat's ultrasound instead.[68]
Flowers are also specialized in shape and have an arrangement of thestamens that ensures that pollen grains are transferred to the bodies of the pollinator when it lands in search of its attractant. Other flowers use mimicry orpseudocopulation to attract pollinators. Many orchids, for example, produce flowers resembling female bees or wasps in color, shape, and scent. Males move from one flower to the next in search of a mate, pollinating the flowers.[69][70]
Many flowers have close relationships with one or a few specific pollinating organisms. Many flowers, for example, attract only one specific species of insect and therefore rely on that insect for successful reproduction. This close relationship is an example ofcoevolution, as the flower and pollinator have developed together over a long period to match each other's needs.[71] This close relationship compounds the negative effects ofextinction, however, since the extinction of either member in such a relationship would almost certainly mean the extinction of the other member as well.[72]
A grass flower with its long, thin filaments and a large feathery stigma.
The female flower ofEnhalus acoroides, which is pollinated through a combination of hyphydrogamy and ephydrogamy.
Flowers that use abiotic, or non-living, vectors use thewind or, much less commonly,water, to move pollen from one flower to the next.[55] In wind-dispersed (anemophilous) species, the tiny pollen grains are carried, sometimes many thousands of kilometers,[73] by the wind to other flowers. Common examples include thegrasses,birch trees, along with many other species in the orderFagales,[74]ragweeds, and manysedges. They do not need to attract pollinators and therefore tend not to grow large, showy, or colorful flowers, and do not have nectaries, nor a noticeable scent. Because of this, plants typically have many thousands of tiny flowers which have comparatively large, feathery stigmas; to increase the chance of pollen being received.[69] Whereas the pollen of entomophilous flowers is usually large, sticky, and rich inprotein (to act as a "reward" for pollinators), anemophilous flower pollen is typically small-grained, very light, smooth, and of little nutritional value toinsects.[75][76] In order for the wind to effectively pick up and transport the pollen, the flowers typically have anthers loosely attached to the end of long thin filaments, or pollen forms around acatkin which moves in the wind. Rarer forms of this involve individual flowers being moveable by the wind (pendulous), or even less commonly; the anthers exploding to release the pollen into the wind.[75]
Pollination through water (hydrophily) is a much rarer method, occurring in only around 2% of abiotically pollinated flowers.[55] Common examples of this includeCalitriche autumnalis,Vallisneria spiralis and somesea-grasses. One characteristic which most species in this group share is a lack of anexine, or protective layer, around the pollen grain.[77] Paul Knuth identified two types of hydrophilous pollination in 1906 and Ernst Schwarzenbach added a third in 1944. Knuth named his two groups 'Hyphydrogamy' and the more common 'Ephydrogamy'.[78] In hyphydrogamy pollination occurs below the surface of the water and so the pollen grains are typically negativelybuoyant. For marine plants that exhibit this method, the stigmas are usually stiff, while freshwater species have small and feathery stigmas.[79] In ephydrogamy pollination occurs on the surface of the water and so the pollen has a low density to enable floating, though many also use rafts, and arehydrophobic. Marine flowers have floating thread-like stigmas and may have adaptations for the tide, while freshwater species create indentations in the water.[79] The third category, set out by Schwarzenbach, is those flowers which transport pollen above the water through conveyance. This ranges from floating plants, (Lemnoideae), tostaminate flowers (Vallisneria). Most species in this group have dry, spherical pollen which sometimes forms into larger masses, and female flowers which form depressions in the water; the method of transport varies.[79]
Mechanisms
Flowers can be pollinated by two mechanisms; cross-pollination and self-pollination. No mechanism is indisputably better than the other as they each have their advantages and disadvantages. Plants use one or both of these mechanisms depending on theirhabitat andecological niche.[80]
Cross-pollination
Cross-pollination is the pollination of the carpel by pollen from a different plant of the same species. Because the genetic make-up of the sperm contained within the pollen from the other plant is different, their combination will result in a new, genetically distinct, plant, through the process ofsexual reproduction. Since each new plant is genetically distinct, the different plants showvariation in their physiological and structural adaptations and so thepopulation as a whole is better prepared for an adverse occurrence in the environment. Cross-pollination, therefore, increases the survival of the species and is usually preferred by flowers for this reason.[48][81]
The principal adaptive function of flowers is the promotion of cross-pollination or outcrossing, a process that allows the masking of deleterious mutations in the genome of progeny. The masking effect of outcrossing sexual reproduction is known as "genetic complementation".[82] This beneficial effect of outcrossing on progeny is also recognized as hybrid vigour or heterosis. Once outcrossing is established due to the benefits of genetic complementation, subsequent switching to inbreeding becomes disadvantageous because it allows the expression of the previously masked deleterious recessive mutations, usually referred to as inbreeding depression. Charles Darwin in his 1889 book The Effects of Cross and Self-Fertilization in the Vegetable Kingdom[83] at the beginning 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."
Self-pollination
Clianthus puniceus, the kakabeak.
Self-pollination is the pollination of the carpel of a flower by pollen from either the same flower or another flower on the same plant,[48] leading to the creation of a genetic clone throughasexual reproduction. This increases the reliability of producing seeds, the rate at which they can be produced, and lowers the amount energy needed.[84] But, most importantly, it limitsgenetic variation. In addition, self-pollination causesinbreeding depression, due largely to the expression of recessive deleteriousmutations.[85][86]
The extreme case of self-fertilization, when the ovule is fertilized by pollen from the same flower or plant, occurs in flowers that always self-fertilize, such as manydandelions.[87] Some flowers are self-pollinated and have flowers that never open or are self-pollinated before the flowers open; these flowers are calledcleistogamous; many species in the genusViola exhibit this, for example.[88]
Conversely, many species of plants have ways of preventing self-pollination and hence, self-fertilization. Unisexual male and female flowers on the same plant may not appear or mature at the same time, or pollen from the same plant may be incapable of fertilizing its ovules. The latter flower types, which have chemical barriers to their own pollen, are referred to as self-incompatible.[27][89] InClianthus puniceus, self-pollination is used strategically as an "insurance policy". When a pollinator, in this case a bird, visitsC. puniceus, it rubs off the stigmatic covering and allows for pollen from the bird to enter the stigma. If no pollinators visit, however, then the stigmatic covering falls off naturally to allow for the flower's own anthers to pollinate the flower through self-pollination.[84]
Pollen is a large contributor toasthma and otherrespiratory allergies which combined affect between 10 and 50% of people worldwide. This number appears to be growing, as the temperature increases due toclimate change mean that plants are producing more pollen[citation needed], which is also more allergenic. Pollen is difficult to avoid, however, because of its small size and prevalence in the natural environment. Most of the pollen which causes allergies is that produced by wind-dispersed pollinators such as thegrasses,birch trees,oak trees, andragweeds; the allergens in pollen are proteins which are thought to be necessary in the process of pollination.[90][91]
Fertilization, also calledSyngamy, is preceded by pollination, which is the movement of pollen from the stamen to the carpel. It encompasses bothplasmogamy, the fusion of theprotoplasts, 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 center-most part of the ovary it enters the egg apparatus and into onesynergid. At this point 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 itsprotoplasm. The sperm's nucleus then fuses with the egg's nucleus, resulting in the formation of azygote, adiploid (two copies of eachchromosome) cell.[2][92]
In Angiosperms (flowering plants) a process known as double fertilization, which involves both karyogamy and plasmogamy, occurs. In double fertilization the second sperm cell subsequently also enters the synergid and fuses with the two polar nuclei of the central cell. Since all three nuclei arehaploid, they result in a largeendosperm nucleus which istriploid.[92]
The fruit of a peach with the seed or stone inside.
Following the formation of zygote it begins to grow through nuclear and cellular divisions, calledmitosis, eventually becoming a small group of cells. One section of it becomes theembryo, while the other becomes the suspensor; a structure which forces the embryo into theendosperm and is later undetectable. Two smallprimordia also form at this time, that later become thecotyledon, which is used as an energy store. Plants which grow out one of these primordia are calledmonocotyledons, while those that grow out two aredicotyledons. The next stage is called the Torpedo stage and involves the growth of several key structures, including: theradicle (embryotic root), theepicotyl (embryotic stem), and thehypocotyl, (the root/shoot junction). In the final stepvascular tissue develops around the seed.[93]
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, sepals, stamens, and petals. The fruit contains three structures: theexocarp, or outer layer, themesocarp, or the fleshy part, and theendocarp, or innermost layer, while the fruit wall is called thepericarp. The size, shape, toughness and thickness varies among differentdry andfleshy fruits. This is because it is directly connected to the method of seed dispersal; that being the purpose of fruit - to encourage or enable the seed's dispersal and protect the seed while doing so.[93]
Following the pollination of a flower, fertilization, and finally the development of a seed and fruit, a mechanism is typically used to disperse the fruit away from the plant.[97] In Angiosperms (flowering plants) seeds are dispersed away from the plant so as to not force competition between the mother and the daughter plants,[98] as well as to enable the colonization of new areas. They are often divided into two categories, though many plants fall in between or in one or more of these:[99]
Allochory
In allochory, plants use an externalvector, or carrier, to transport their seeds away from them. These can be eitherbiotic (living), such as by birds and ants, orabiotic (non-living), such as by the wind or water.[99][100][101]
Many plants use biotic vectors to disperse their seeds away from them. This method falls under the umbrella termzoochory, whileendozoochory, also known asfruigivory, refers specifically to plants adapted to grow fruit in order to attract animals to eat them. Once eaten they go through typically go through animal's digestive system and are dispersed away from the plant.[101] Some seeds are specially adapted either to last in thegizzard of animals or even to germinate better after passing through them.[102][103] They can be eaten by birds (ornithochory), bats (chiropterochory),rodents, primates, ants (myrmecochory),[104] non-bird sauropsids (saurochory), mammals in general(mammaliochory),[102] and evenfish.[105] Typically their fruit are fleshy, have a high nutritional value, and may have chemical attractants as an additional "reward" for dispersers. This is reflectedmorphologically in the presence of morepulp, anaril, and sometimes anelaiosome (primarily for ants), which are other fleshy structures.[106]
Epizoochory occurs in plants whose seeds are adapted to cling on to animals and be dispersed that way, such as many species in the genusAcaena.[107] Typically these plants seed's have hooks or aviscous surface to easier grip to animals, which include birds and animals withfur. Some plants usemimesis, or imitation, to trick animals into dispersing the seeds and these often have specially adapted colors.[106][108]
The final type ofzoochory is calledsynzoochory, which involves neither the digestion of the seeds, nor the unintentional carrying of the seed on the body, but the deliberate carrying of the seeds by the animals. This is usually in the mouth orbeak of the animal (calledStomatochory), which is what is used for many birds and all ants.[109]
In abiotic dispersal plants use the vectors of the wind, water, or a mechanism of their own to transport their seeds away from them.[101][100]Anemochory involves using the wind as a vector to disperse plant's seeds. Because these seeds have to travel in the wind, they are almost always small — sometimes even dust-like, have a highsurface-area-to-volume ratio, and are produced in a large number — sometimes up to a million. Plants such astumbleweeds detach the entire shoot to let the seeds roll away with the wind. Another common adaptation are wings, plumes or balloon-like structures that let the seeds stay in the air for longer and hence travel farther.
Inhydrochory plants are adapted to disperse their seeds through bodies of water and so typically are buoyant and have a lowrelative density with regards to the water. Commonly seeds are adapted morphologically with hydrophobic surfaces, small size, hairs, slime,oil, and sometimes air spaces within the seeds.[106] These plants fall into three categories: ones where seeds are dispersed on the surface of water currents, under the surface of water currents, and by rain landing on a plant.[110]
Inautochory, plants create their own vectors to transport the seeds away from them. Adaptations for this usually involve the fruits exploding and forcing the seeds awayballistically, such as inHura crepitans,[111] or sometimes in the creation of creepingdiaspores.[106] Because of the relatively small distances that these methods can disperse their seeds, they are often paired with an external vector.[108]
While land plants have existed for about 425 million years, the first onesreproduced by a simple adaptation of their aquatic counterparts:spores. In the sea, plants—and some animals—can simply scatter out geneticclones of themselves to float away and grow elsewhere. This is how early plants reproduced. But plants soon evolved methods of protecting these copies to deal with drying out and other damage which is even more likely on land than in the sea. The protection became theseed, though it had not yet evolved the flower. Early seed-bearing plants include theginkgo andconifers.
Archaefructus liaoningensis, one of the earliest known flowering plants
Several groups of extinctgymnosperms, particularlyseed ferns, have been proposed as the ancestors of flowering plants but there is no continuous fossil evidence showing exactly how flowers evolved. The apparently sudden appearance of relatively modern flowers in the fossil record posed such a problem for the theory of evolution that it was called an "abominable mystery" byCharles Darwin.
Recently discovered angiosperm fossils such asArchaefructus, along with further discoveries of fossil gymnosperms, suggest how angiosperm characteristics may have been acquired in a series of steps. An early fossil of a flowering plant,Archaefructus liaoningensis from China, is dated about 125 million years old.[112][113] Even earlier from China is the 125–130 million years oldArchaefructus sinensis. In 2015 a plant (130 million-year-oldMontsechia vidalii, discovered in Spain) was claimed to be 130 million years old.[114] In 2018, scientists reported that the earliest flowers began about 180 million years ago.[115]
Amborella trichopoda may have characteristic features of the earliest flowering plants
RecentDNA analysis (molecular systematics)[116] shows thatAmborella trichopoda, found on the Pacific island of New Caledonia, is the only species in thesister group to the rest of the flowering plants, and morphological studies suggest that it has features which may have been characteristic of the earliest flowering plants.[117]
Besides the hard proof of flowers in or shortly before theCretaceous,[118][119] there is some circumstantial evidence of flowers as much as 250 million years ago. A chemical used by plants to defend their flowers,oleanane, has been detected in fossil plants that old, includinggigantopterids,[120] which evolved at that time and bear many of the traits of modern, flowering plants, though they are not known to be flowering plants themselves, because only their stems and prickles have been found preserved in detail; one of the earliest examples ofpetrification.
The similarity inleaf andstem structure can be very important, because flowers are genetically just an adaptation of normal leaf and stem components on plants, a combination of genes normally responsible for forming new shoots.[121] The most primitive flowers are thought to have had a variable number of flower parts, often separate from (but in contact with) each other. The flowers would have tended to grow in a spiral pattern, to bebisexual (in plants, this means both male and female parts on the same flower), and to be dominated by theovary (female part). As flowers grew more advanced, some variations developed parts fused together, with a much more specific number and design, and with either specific sexes per flower or plant, or at least "ovary inferior".
The general assumption is that the function of flowers, from the start, was to involve animals in the reproduction process. Pollen can be scattered without bright colors and obvious shapes, which would therefore be a liability, using the plant's resources, unless they provide some other benefit. One proposed reason for the sudden, fully developed appearance of flowers is that they evolved in an isolated setting like an island, or chain of islands, where the plants bearing them were able to develop a highly specialized relationship with some specific animal (a wasp, for example), the way many island species develop today. This symbiotic relationship, with a hypothetical wasp bearing pollen from one plant to another much the wayfig wasps do today, could have eventually resulted in both the plant(s) and their partners developing a high degree of specialization.Island genetics is believed to be a common source ofspeciation, especially when it comes to radical adaptations which seem to have required inferior transitional forms. Note that the wasp example is not incidental; bees, apparently evolved specifically for symbiotic plant relationships, are descended from wasps.
Likewise, mostfruit used in plant reproduction comes from the enlargement of parts of the flower. This fruit is frequently a tool which depends upon animals wishing to eat it, and thus scattering the seeds it contains.
While many suchsymbiotic relationships remain too fragile to survive competition with mainland organisms, flowers proved to be an unusually effective means of production, spreading (whatever their actual origin) to become the dominant form of land plant life.
Flower evolution continues to the present day; modern flowers have been so profoundly influenced by humans that many of them cannot be pollinated in nature. Many modern, domesticated flowers used to be simple weeds, which only sprouted when the ground was disturbed. Some of them tended to grow with human crops, and the prettiest did not get plucked because of their beauty, developing a dependence upon and special adaptation to human affection.[122]
Reflectance spectra for the flowers of several varieties ofrose. A red rose absorbs about 99.7% of light across a broad area below the red wavelengths of the spectrum, leading to an exceptionallypure red. A yellow rose will reflect about 5% of blue light, producing an unsaturated yellow (a yellow with a degree of white in it).
Many flowering plants reflect as muchlight as possible within the range of visiblewavelengths of the pollinator the plant intends to attract. Flowers that reflect the full range of visible light are generally perceived aswhite by a human observer. An important feature of white flowers is that they reflect equally across the visible spectrum. While many flowering plants use white to attract pollinators, the use of color is also widespread (even within the same species). Color allows a flowering plant to be more specific about the pollinator it seeks to attract.[citation needed] The color model used by human color reproduction technology (CMYK) relies on the modulation of pigments that divide the spectrum into broad areas of absorption. Flowering plants by contrast are able to shift the transition point wavelength between absorption and reflection. If it is assumed that the visual systems of most pollinators view the visible spectrum ascircular then it may be said that flowering plants produce color by absorbing the light in one region of the spectrum and reflecting the light in the other region. With CMYK, color is produced as a function of the amplitude of the broad regions of absorption. Flowering plants by contrast produce color by modifying the frequency (or rather wavelength) of the light reflected. Most flowers absorb light in the blue to yellow region of the spectrum and reflect light from the green to red region of the spectrum. For many species of flowering plant, it is the transition point that characterizes the color that they produce. Color may be modulated by shifting the transition point between absorption and reflection and in this way a flowering plant may specify which pollinator it seeks to attract.[citation needed] Some flowering plants also have a limited ability to modulate areas of absorption. This is typically not as precise as control over wavelength. Humans observers will perceive this as degrees ofsaturation (the amount ofwhite in the color).
Carl Linnaeus's method for classifying plants focused solely on the structure and nature of the flowers.[123]
Inplant taxonomy, which is the study of the classification and identification of plants, themorphology of plant's flowers are used extensively – and have been for thousands of years. Although the history of plant taxonomy extends back to at least around 300B.C. with the writings ofTheophrastus,[124] the foundation of the modern science is based on works in the 18th and 19th centuries.[125]
Carl Linnaeus (1707–1778), was a Swedish botanist who spent most of his working life as a professor of natural history. His landmark 1757 bookSpecies Plantarum lays out his system of classification as well as the concept ofbinomial nomenclature, the latter of which is still used around the world today.[125][note 1] He identified 24 classes, based mainly on the number, length and union of thestamens.
The first ten classes follow the number of stamens directly (Octandria have 8 stamens etc.),[123] while class eleven has 11–20 stamens and classes twelve and thirteen have 20 stamens; differing only in their point of attachment. The next five classes deal with the length of the stamens and the final five with the nature of the reproductive capability of the plant; where the stamen grows; and if the flower is concealed or exists at all (such as inferns). This method of classification, despite being artificial,[123] was used extensively for the following seven decades, before being replaced by the system of another botanist.[126]
Antoine Laurent de Jussieu (1748–1836) was a French botanist whose 1787 workGenera plantarum: secundum ordines naturales disposita set out a new method for classifying plants; based instead on natural characteristics. Plants were divided by the number, if any, ofcotyledons, and the location of the stamens.[126]
The next most major system of classification came in the late 19th century from the botanistsJoseph Dalton Hooker (1817–1911) andGeorge Bentham (1800–1884). They built on the earlier works of de Jussieu andAugustin Pyramus de Candolle and devised a system which is still used in many of the world'sherbaria.
Plants were divided at the highest level by the number of cotyledons and the nature of the flowers, before falling into orders (families),genera, andspecies. This system of classification was published in theirGenera plantarum in three volumes between 1862 and 1883.[127] It is the most highly regarded and deemed the "best system of classification," in some settings.[128]
Many flowers have importantsymbolic meanings in Western culture.[130] The practice of assigning meanings to flowers is known asfloriography. Some of the more common examples include:
Redroses are given as a symbol of love, beauty, and passion.[131]
Poppies are a symbol of consolation in time of death. In the United Kingdom, New Zealand, Australia and Canada,red poppies are worn to commemorate soldiers who have died in times of war.
Irises/Lily are used in burials as a symbol referring to "resurrection/life". It is also associated with stars (sun) and its petals blooming/shining.
Because of their varied and colorful appearance, flowers have long been a favorite subject of visual artists as well. Some of the most celebrated paintings from well-known painters are of flowers, such asVan Gogh'ssunflowers series orMonet's water lilies. Flowers are also dried, freeze dried and pressed in order to create permanent, three-dimensional pieces offloral art.
Their symbolism in dreams has also been discussed, with possible interpretations including "blossoming potential".[133]
The Roman goddess of flowers, gardens, and the season of Spring isFlora. The Greek goddess of spring, flowers and nature isChloris.
InHindu mythology, flowers have a significant status. Vishnu, one of the three major gods in the Hindu system, is often depicted standing straight on alotus flower.[134] Apart from the association with Vishnu, the Hindu tradition also considers the lotus to have spiritual significance.[135] For example, it figures in the Hindu stories of creation.[136]
History shows that flowers have been used by humans for thousands of years, to serve a variety of purposes. An early example of this is from about 4,500 years ago inAncient Egypt, where flowers would be used to decorate women's hair. Flowers have also inspired art time and time again, such as inMonet'sWater Lilies orWilliam Wordsworth's poem aboutdaffodils entitled: "I Wandered Lonely as a Cloud".[137]
In modern times, people have sought ways to cultivate, buy, wear, or otherwise be around flowers and blooming plants, partly because of their agreeable appearance andsmell. Around the world, people use flowers to mark important events in their lives:
A woman spreading flowers over alingam in a temple inVaranasiFlowers collected for worship of Hindu deities in morning, inWest Bengal.
Flowers likejasmine have been used as a replacement for traditional tea in China for centuries. Most recently many other herbs and flowers used traditionally across the world are gaining importance to preapare a range offloral tea.[citation needed]
Flowers provide less food than other major plant parts (seeds,fruits,roots,stems andleaves), but still provide several importantvegetables andspices. Flower vegetables includebroccoli,cauliflower andartichoke. The most expensive spice,saffron, consists of dried stigmas of acrocus. Other flower spices arecloves andcapers.Hops flowers are used to flavorbeer.Marigold flowers are fed tochickens to give their egg yolks a golden yellow color, which consumers find more desirable; dried and ground marigold flowers are also used as a spice and coloring agent inGeorgian cuisine. Flowers of thedandelion andelder are often made into wine. Beepollen, pollen collected from bees, is considered a health food by some people.Honey consists of bee-processed flower nectar and is often named for the type of flower, e.g.orange blossom honey,clover honey andtupelo honey.
Flowers such as chrysanthemum, rose, jasmine, Japanese honeysuckle, andchamomile, chosen for their fragrance and medicinal properties, are used astisanes, either mixed withtea or on their own.[143]
Flowers have been used since prehistoric times in funeral rituals: traces of pollen have been found on a woman's tomb in theEl Miron Cave in Spain.[144] Many cultures draw a connection between flowers and life and death, and because of their seasonal return flowers also suggest rebirth, which may explain why many people place flowers upon graves. Theancient Greeks, as recorded inEuripides's playThe Phoenician Women, placed a crown of flowers on the head of the deceased;[145] they also covered tombs with wreaths and flower petals. Flowers were widely used inancient Egyptian burials,[146] and theMexicans to this day use flowers prominently in theirDay of the Dead celebrations[147] in the same way that theirAztec ancestors did.
The flower-giving tradition goes back to prehistoric times when flowers often had a medicinal and herbal attributes. Archaeologists found in several grave sites remnants of flower petals. Flowers were first used as sacrificial and burial objects.Ancient Egyptians and later Greeks and Romans used flowers. In Egypt, burial objects from the time around 1540 BC[citation needed] were found, which depictedred poppy,yellow Araun,cornflower andlilies. Records of flower giving appear inChinese writings and Egyptian hieroglyphics, as well as inGreek andRoman mythology. The practice of giving a flower flourished in the Middle Ages when couples showed affection through flowers.
The tradition of flower-giving exists in many forms. It is an important part ofRussian culture and folklore. It is common for students to give flowers to their teachers. To give yellow flowers in a romantic relationship means breakup in Russia. Nowadays, flowers are often given away in the form of aflower bouquet.[148][149][150]
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^abForget, Pierre-Michel (2005).Seed fate : predation, dispersal, and seedling establishment. Wallingford, Oxfordshire, UK: CABI Pub. p. 21.ISBN0-85199-806-2.OCLC54803650.Archived from the original on 2022-02-21. Retrieved2021-07-01.Seeds on plants can be dispersed via a variety of primary dispersal mechanisms, including abiotic factors, like wind or ballistic projection, or biotic factors, like fruit-eating birds.
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