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For other uses, see Flower (disambiguation).
"Floral" redirects here. For other uses, see Floral (disambiguation).

Yellow and blue flower, shaped like a bird of paradise
Pink rose in front of a black background
Small pink and white orchid
Large inflorescence of many small pink flowers
Clockwise from top: Strelitzia reginae, a pink rose, Epipactis palustris, and inflorescence of Dracophyllum traversii

Flowers, also known as blooms and blossoms, are the reproductive structures of flowering plants (angiosperms). Typically, they are structured in four sets, called whorls, around the end of a stalk. These whorls include: calyx, modified leaves; corolla, the petals; androecium, the male reproductive unit consisting of stamens and pollen; and gynoecium, the female part, containing style and stigma, which receives the pollen, and ovary, which contains the ovules. When arranged in groups, with our without modified leaves (bracts), they are known collectively as an inflorescence. Flowers can be described systematically using both formulae and diagrams.

Flowers grow out of an apical meristem (stem tip) and are controlled by the presence MADS-box genes. Simple models are used to describe this development. Flowers are heterospourous, and so produce both microspores and megaspores, which generally create male and female gametophytes (organism that leads to creation of sex cells) respectively. Pollination mediates the transport of pollen to the ovules in the ovaries, to facilitate sexual reproduction. It can occur between different plants, as in cross-pollination, or between flowers on the same plant–or even the same flower, as in self-pollination. Vectors transport the pollen between stamen and stigma. They may be living animals, such as birds and insects, or non-living factors such as wind and water. Pollen, especially from wind-dispersing plants, is a large contributer to asthma

After pollination, fertilisation occurs. It involves both plasmogamy (fusion of cells excluding the cell wall) and karyogamy (fusion of the nuclei). The result is a diploid (two copies of each chromosome) cell called a zygote. Through cell and nuclear devision (mitosis) the zygote grows into a seed, which contains structures to assist in the future plants survival and success. At the same time, the ovary forms into a fruit, and the other floral structures die. Its function is to protect the seed and aid in dispersal. This dispersal is divided into external vectors (allochory or by the plant itself (autochory). External vectors include both living things, such as animals and insects, and non-living things, which includes wind and water.

Flowers evolved between 150 and 190 million years ago, during the later part of the Jurassic era and early Cretaceous. As a subgroup of seed plants, angiosperms used the flower to out compete them, as a result of greater efficiency. The colour of flowers assist in pollination and are the result of potosynthetic pigments. In taxonomy, which is the study of plant classification, flowers are a key tool used to differentiate plants. For thousands of years humans have used flowers for a variety of purposes including: decoration, medicine, food, and perfumes. In human cultures, flowers are used symbolically and feature in art, literature, religious practises, ritual, and festivals.

Etymology

Flower is from the Middle English flour, 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 was blossom,[1] which is still in use.[2]

Morphology

Main article: Floral morphology
Diagram of flower parts.

The morphology of a flower, or its form and structure,[3] can be considered in two parts: the vegetative part, consisting of non-reproductive structures such as petals; 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 a receptacle.[4] The four main whorls (starting from the base of the flower or lowest node and working upwards) are the calyx, corolla, androecium, and gynoecium.[5]

Perianth

Main article: Perianth

Calyx

The sepals, collectively called the calyx, are modified leaves that occur on the outermost whorl of the flower. They are leaf-like, in that they have a broad base, stomata and chlorophyll and may have stipules. Sepals are often waxy and tough, and grow quickly to protect the flower as it develops.[6][7] They may be deciduous, but will more commonly grow on to assist in fruit dispersal. If the calyx is fused it is called gamosepalous.[6]

Corolla

The petals, 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.[8] 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 called sympetalous.[9] In monocotyledonous flowers (e.g., Lilium sp.), petals and sepals are indistinguishable and are individually called tepals. Petals also tend to have patterns only visible under ultraviolet light, which are visible to pollinators but not to humans.

Reproductive

Main article: Plant reproductive morphology
Reproductive parts of easter lily (Lilium longiflorum). 1. Stigma, 2. Style, 3. Stamens, 4. Filament, 5. Petal

Androecium

The androecium, consisting of stamens, is the whorl of pollen-producing male parts. Stamens consist typically of an anther, made up of four pollen sacs arranged in two thecae, connected to a filament, or stalk. The anther contains microsporocytes which become pollen, the male gametophyte, after undergoing meiosis. 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.[8][9]

In general, there is 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. These plants are called heterantherous.[10]

Gynoecium

The gynoecium, consisting of one or more carpels, is the female part of the flower found on the innermost whorl. Each carpel consists of a stigma, which receives pollen, a style, which acts as a stalk, and an ovary, which contains the ovules. Carpels may occur in one to several whorls, and when fused are often described as a pistil. Inside the ovary, the ovules are attached to the placenta by structures called funiculi.[11][12]

Variation

Although this arrangement is considered "typical", plant species show a wide variation in floral structure.[13] 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.[14] In some families, such as the grasses, the petals are greatly reduced; in many species, the sepals are colourful and petal-like. Other flowers have modified petal-like stamens; the double flowers of peonies and roses are mostly petaloid stamens.[15]

Many flowers have symmetry. When the perianth is bisected through the central axis from any point and symmetrical halves are produced, the flower is said to be actinomorphic or regular. This is an example of radial symmetry. When flowers are bisected and produce only one line that produces symmetrical halves, the flower is said to be irregular or zygomorphic. If, in rare cases, they have no symmetry at all they are called asymmetric.[16][17]

Flowers may be directly attached to the plant at their base (sessile—the supporting stalk or stem is highly reduced or absent).[18] The stem or stalk subtending a flower, or an inflorescence of flowers, is called a peduncle. If a peduncle supports more than one flower, the stems connecting each flower to the main axis are called pedicels.[19] The apex of a flowering stem forms a terminal swelling which is called the torus or receptacle.[17]

In the majority of species, individual flowers have both carpels and stamens. These flowers are described by botanists as being perfect, bisexual, or hermaphrodite. 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 called monoecious.[20] However, if an individual plant is either female or male, the species is called dioecious. Many flowers have floral nectaries, which are glands that produce a sugary fluid (nectar) used to attract pollinators. They are not considered as an organ on their own.[21]

Inflorescence

Main article: Inflorescence
The yellow inflorescence of Zantedeschia aethiopica

In those species that have more than one flower on an axis, the collective cluster of flowers is called an inflorescence. 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 single daisy or sunflower, for example, is not a flower but a flower head—an inflorescence composed of numerous flowers (or florets).[22] An inflorescence may include specialized stems and modified leaves known as bracts.[23]

Floral diagrams and formulae

Main articles: Floral formula and Floral diagram

A floral 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 a taxon, usually giving ranges of the numbers of different organs, or particular species. The format of floral formulae differs in different parts of the world, yet they convey the same information.[24][25]

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 fusion and symmetry, and structural details.[26]

Development

A flower develops on a modified shoot or axis from a determinate apical meristem (determinate meaning the axis grows to a set size). It has compressed internodes, bearing structures that in classical plant morphology are interpreted as highly modified leaves.[27] Developmental studies have shown that floral organs can originate from modified stems (caulomes), leaves (phyllomes), and branchlets (shoots).[13][28] This suggests a continuum in floral evolution.[29][30] All aspects of flower morphology and function are controlled by the presence of different genes, called MADS-box genes.[31]

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 favourable for fertilisation and the formation of seeds, hence ensuring maximal reproductive success. To meet these needs a plant can interpret important endogenous and environmental cues such as changes in levels of plant hormones and seasonable temperature and photoperiod changes.[32] Many perennial and most biennial plants require vernalisation to flower. The molecular interpretation of these signals is through the transmission of a complex signal known as florigen, which involves a variety of genes. Florigen is produced in the leaves in reproductively favourable conditions and acts in buds and growing tips to induce several different physiological and morphological changes.[33]

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, and carpels. 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.[34]

Diagram of the ABC model of development

Organ development

Main article: ABC model of flower development

The ABC model describes how three genes–A, B, and C–are responsible for the development of flowers. These three gene activities interact in a combinatorial manner to determine the developmental identities of the primordia organ within the floral apical meristem. Alone, A produces sepals in the first whorl. Together, A and B produce the petals in the second whorl. C alone produces carpels in the centre of the flower. C and B together produce the stamens in the third whorl.[35]

Function

See also: Plant reproductive morphology

The principal purpose of a flower is reproduction,[36] both of the individual and of the species. All flowering plants are heterosporous, that is, every individual plant produces two types of spores. Microspores are produced by meiosis 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 into gametophytes inside sporangia. As with all heterosporous plants, the gametophytes also develop inside the spores, i.e., they are endosporic.[37]

Pollination

Main article: Pollination
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.[7] Pollination is the movement of pollen from the anthers to the stigma.[38] It occurs either between flowers (or from one part of a flower to another) of the same plant, as in self-pollination, or between flowers of different plants, as in cross-pollination. Cross-pollination is more common as it increases genetic variation.[39] The period during which this process can take place (when the flower is fully expanded and functional) is called anthesis.[40]

Flowering plants usually face evolutionary pressure to optimise the transfer of their pollen, and this is typically reflected in the morphology of the flowers and the behaviour of the plants.[41] Pollen may be transferred between plants via several 'vectors'–agents that transport pollen. Around 80% of flowering plants make use of biotic or living vectors. Others use abiotic (non-living) vectors and some plants make use of multiple vectors, but most are highly specialised.[42]

Biotic pollination

Flowers that use biotic vectors attract and use insects, bats, birds, or other animals to transfer pollen from one flower to the next. Often they are specialised 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).[43] 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.[44] Many flowers rely on simple proximity between flower parts to ensure pollination, while others have elaborate designs to ensure pollination and prevent self-pollination.[39] Flowers use animals including: insects (entomophily), birds (ornithophily), bats (chiropterophily), lizards,[45] and even snails and slugs (malacophilae).[46]

Attraction methods

Ophrys apifera, a bee orchid, which has evolved over many generations to mimic a female bee[47]

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 as entomophilous; literally "insect-loving" in Greek.[48] To attract these insects flowers commonly have glands called nectaries on various parts that attract animals looking for nutritious nectar.[49] Some flowers have glands called elaiophores, which produce oils rather than nectar.[50] Birds and bees have colour vision, enabling them to seek out colourful flowers.[51] Some flowers have patterns, called nectar guides, that show pollinators where to look for nectar; they may be visible only under ultraviolet light, which is visible to bees and some other insects.[52]

Flowers also attract pollinators by scent.[51] 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.[53]

Pollinator relationships

Further information: Pollination syndrome

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 of coevolution, as the flower and pollinator have developed together over a long period to match each other's needs.[54] This close relationship compounds the negative effects of extinction, however, since the extinction of either member in such a relationship would almost certainly mean the extinction of the other member as well.[55]

Abiotic pollination

Main articles: Anemophily and Hydrophily
The male cones of Pinus nigra, which use the wind to transport pollen to female cones
The female flower of Enhalus acoroides, which is pollinated through a combination of hyphydrogamy and ephydrogamy

Flowers that use abiotic (non-living) vectors use the wind or, much less commonly, water, to move pollen from one flower to the next.[42] Wind-dispersed (anemophilous) 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.[56] Whereas the pollen of entomophilous flowers is usually large, sticky, and rich in protein (to act as a "reward" for pollinators), anemophilous flower pollen is typically small-grained, very light, smooth, and of little nutritional value to insects.[57][58]

Pollination through water (hydrophily) is a much rarer method, occurring in only around 2% of abiotically pollinated flowers.[42] This can be either above the surface of the water (ephydrogamy) or below the surface (hyphydrogamy).[59]

Mechanisms

Main article: Pollination § Mechanism

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 their habitat and ecological niche.[60]

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 of sexual reproduction. Since each new plant is genetically distinct, the different plants show variation in their physiological and structural adaptations and so the population 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.[39][61]

Self-pollination

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,[39] leading to the creation of a genetic clone through asexual reproduction. This increases the reliability of producing seeds, the rate at which they can be produced, and lowers the amount energy needed.[62] But, most importantly, it limits genetic variation. In addition, self-pollination causes inbreeding depression, due largely to the expression of recessive deleterious mutations.[63][64]

Many species of plants have ways of preventing self-pollination and hence, self-fertilisation. 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 fertilising its ovules. The latter flower types, which have chemical barriers to their own pollen, are referred to as self-incompatible.[20][65] Self-pollination may also be used strategically as an "insurance policy".[62]

Allergies

Main article: Pollen allergy

Pollen is a large contributor to asthma and other respiratory allergies which combined affect between 10 and 50% of people worldwide. Most of the pollen which causes allergies is that produced by wind-dispersed pollinators such as the grasses, birch trees, oak trees, and ragweeds; the allergens in pollen are proteins which are thought to be necessary in the process of pollination.[66][67]

Fertilisation and seed development

Main articles: Fertilization and Double fertilization
Diagram of a flower, with the pollen tube labeled PG

Fertilisation, also called syngamy, is preceded by pollination, which is the movement of pollen from the stamen to the carpel. It encompasses both plasmogamy, the fusion of the protoplasts, and karyogamy, the fusion of the nuclei. When pollen lands on the stigma of the flower it begins creating a pollen 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 one synergid. 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 its cell membrane and much of its protoplasm. The sperm's nucleus then fuses with the egg's nucleus, resulting in the formation of a zygote, a diploid (two copies of each chromosome) cell.[68]

In angiosperms (flowering plants) a process known as double fertilisation, which involves both karyogamy and plasmogamy, occurs. In double fertilisation the second sperm cell subsequently also enters the synergid and fuses with the two polar nuclei of the central cell. Since all three nuclei are haploid, they result in a large endosperm nucleus which is triploid.[68]

Seed development

Main article: Seed development
The fruit of a peach with the seed or stone inside

Following the formation of a zygote it begins to grow through nuclear and cellular divisions, called mitosis, eventually becoming a small group of cells. One section of it becomes the embryo, while the other becomes the suspensor; a structure which forces the embryo into the endosperm and is later undetectable. Two small primordia also form at this time, that later become the cotyledon, which is used as an energy store. Plants which grow out one of these primordia are called monocotyledons, while those that grow out two are dicotyledons. The next stage is called the Torpedo stage and involves the growth of several key structures, including: the radicle (embryotic root), the epicotyl (embryotic stem), and the hypocotyl, (the root/shoot junction). In the final step vascular tissue develops around the seed.[69]

Fruit development

Main article: Fruit § Development

The ovary, inside which the seed is forming from the ovule, grows into a fruit. 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: the exocarp, or outer layer, the mesocarp, or the fleshy part, and the endocarp, or innermost layer, while the fruit wall is called the pericarp. The size, shape, toughness and thickness varies among different dry and fleshy 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.[69]

Seed dispersal

Clockwise from top: The Kererū, an important disperser of seeds in New Zealand; a maple seed; Acaena novae-zelandiae, which uses epizoochory to disperse its seeds;[70] Hura crepitans, which disperses its seeds ballistically.[71]
Main articles: Biological dispersal and Seed dispersal

Following the pollination of a flower, fertilisation, and finally the development of a seed and fruit, a mechanism is typically used to disperse the fruit away from the plant.[72] In angiosperms (flowering plants) seeds are dispersed away from the plant so as to not force competition between the mother and the daughter plants,[73] as well as to enable the colonization of new areas. They are often divided into the two categories of allochory and autochory, though many plants fall in between or in one or more of these:[74]

Allochory

In allochory, plants use an external vector, or carrier, to transport their seeds away from them. These can be either biotic (living), such as by birds and ants, or abiotic (non-living), such as by the wind or water.[74][75][76]

Biotic vectors

Main article: Seed dispersal § Animals: epi- and endozoochory

Many plants use biotic vectors to disperse their seeds away from them. This method falls under the umbrella term zoochory, while endozoochory, also known as fruigivory, 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.[76] Some seeds are specially adapted either to last in the gizzard of animals or even to germinate better after passing through them.[77][78] They can be eaten by birds (ornithochory), bats (chiropterochory), rodents, primates, ants (myrmecochory),[79] non-bird sauropsids (saurochory), mammals in general (mammaliochory),[77] and even fish.[80] Typically their fruit are fleshy, have a high nutritional value, and may have chemical attractants as an additional "reward" for dispersers. This is reflected morphologically in the presence of more pulp, an aril, and sometimes an elaiosome (primarily for ants), which are other fleshy structures.[81]

Epizoochory occurs in plants whose seeds are adapted to cling on to animals and be dispersed that way, such as many species in the genus Acaena.[82] Typically these plants seed's have hooks or a viscous surface to easier grip to animals, which include birds and animals with fur. Some plants use mimesis, or imitation, to trick animals into dispersing the seeds and these often have specially adapted colors.[81][83]

The final type of zoochory is called synzoochory, 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 or beak of the animal (called Stomatochory), which is what is used for many birds and all ants.[84]

Abiotic vectors

Main articles: Anemochory and Seed dispersal § Water

In abiotic dispersal plants use the vectors of the wind, water, or a mechanism of their own to transport their seeds away from them.[76][75] 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 high surface-area-to-volume ratio, and are produced in a large number—sometimes up to a million. Adaptations include wings, plumes, or balloon-like structures that let the seeds stay in the air for longer and hence travel farther.[85]

In hydrochory plants are adapted to disperse their seeds through bodies of water and so typically are buoyant and have a low relative 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.[81] These plants fall into three categories: ones where seeds are dispersed on the surface of water currents, either because of their low specific gravity, or by using surface tension; and one that move under the surface of water currents.[85]

Autochory

Main article: Autochory

In autochory, plants create their own vectors to transport the seeds away from them. Adaptations for this usually involve the fruits exploding and forcing the seeds away ballistically, such as in Hura crepitans, or sometimes in the creation of creeping diaspores.[81] Because of the relatively small distances that these methods can disperse their seeds, they are often paired with an external vector.[83]

Evolution

Further information: Evolutionary history of plants § Flowers, and Floral biology
Fossil of an early flowering plant
Archaefructus liaoningensis, one of the earliest known flowering plants
A plant with small white flowers
Amborella trichopoda may have characteristic features of the earliest flowering plants.[86]

Flowers originated between 150 and 190 million years ago, during the later part of the Jurassic era and early Cretaceous. Their development allowed angiosperms to outcompete most other seed plants, as a result of several traits resulting in greater efficiency.[87] There is debate over whether the evolutionary process of forming the components which eventually became the first flower, can be entirely described as gradualistic. Evidence suggests, however, that at least some of the process can be attributed to sudden, random changes such as homeotic mutations that change the position of an appendage. This concept is called saltationism.[88]

An early fossil of a flowering plant, Archaefructus liaoningensis from China, is dated about 125 million years old.[89] Even earlier from China is the 125–130 million years old Archaefructus sinensis. In 2015 a plant (130 million-year-old Montsechia vidalii, discovered in Spain) was claimed to be 130 million years old.[90]

Angiosperms, or flowering plants, are a sub-group of spermatophytes (seed plants), and of these, the living species are regarded as a more recent ancestor of the older angiophytes.[91] Aspects of flower morphology can be split into three evolutionary groups based on when they first occurred: pre-anigophyte, angiophyte, and angiosperm. Pre-angiophyte features include the ovules, seeds, microsporangia, and the outer male and inner female organs. The first three of these are shared by all seed plants, while the latter two must be considered pre-angiophyte because they exist in Gnetophyta.[92]

The main evolutionary developments of the angiophytes were the carpel and thecal organisation. It is a covering for the ovule or ovules that protects them through either secretion or postgenital fusion, the combining of different organs. The aspect of angiosperm carpels that is unique to them is postgenital fusion, which seals the carpel shut. Thecal organisation refers to the fact that angiosperm stamens, with their four microsporangia, are almost always divided into two thecae (sheaths). The evolutionary reason for this is uncertain, but may be because it is more efficient at presenting pollen than a single sporangium.[93] Within the angiosperms, some plants have flowers with features not found in other angiosperms. These include: petals; syncarpy; sympetaly and floral tubes; floral spurs; tenuinucellar, and unitegmic ovules.[94]

Colour

White flower with deep purple inner colour
Yellow flower with red inner colour
Flower with very fluffy white petals and yellow reproductive structures in the middle
Purple flower with white stamens at centre
Clockwise from top: Hibiscus trionum which exhibits irridescence, Mentzelia lindleyi which does as well, Leontopodium alpinum with its photonic structures, and purple-coloured Clematis 'Etoile Violette'.

In contrast to the mostly green vegetative parts of plants, flowers often exhibit colour. This includes the petals and, in some plants, the stamens, anthers, stigmas, ovaries, pollen, styles and even nectar.[95] These colours are produced principally by photosynthetic pigments, which are molecules that can absorb and retain energy from light, for just a few microseconds at maximum.[96] That is, they reflect only specific wavelengths of light.[97] As colour applies to the success of flowers, it helps both plants and animals to detect either the environment or flowers, thus increasing pollination.[98]

Specific pigments, and so colours, provide different benefits to the plant. Tetrapyrroles (green, blue-green, red) assist in both flower detection and photosynthesis; carotenoids (yellow, orange, red) assist in both signalling and detection, as well as storage and preventing degradation by photo-oxidation; flavonoids (colourless, white, pale yellow, red, purple, blue) provide detection and defence against ultraviolet (UV) damage; betalains (red, purple, blue) which provide benefits similar to some flavonoids.[98] The largest contributors to flower pollination come from bees, of which are there more than 20,000 species. Bees cannot see as humans do, instead they see within the ultraviolet range. As a result, many petals contain carotenoids and flavonoids, because of their ultraviolet reflective and absorptive properties respectively. This means they stand out against green colours, which bees perceive as a wash of greys.[99]

Another mechanism that flowers use to produce colour, or colour-effects, is structural colour. Some tulips, for example, exhibit iridescence by producing ridged cuticles that split light into their constituent wavelengths by using diffraction grating.[100] Also present in some flowers are photonic structures. For example, edelweiss flowers have small hairs shaped like hollow tubes which diffract light and protect against UV radiation.[101] The colour of flowers can also change; sometimes this is as a signal to pollinators as in Viola cornuta. Change may also occur as a result of temperature; PH, as in the anthoxanthins found in Hydrangea; metals; sugars; and cell shape.[102]

Taxonomy

Further information: Plant taxonomy and Linnaean taxonomy
Linnaeus's diagram of 24 classes of sexual systems, from Systema Naturae

In plant taxonomy, which is the study of the classification and identification of plants, the morphology 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 300 B.C. with the writings of Theophrastus,[103] the foundation of the modern science is based on works in the 18th and 19th centuries.[104]

Carl Linnaeus 1757 book Species Plantarum lays out his system of classification as well as the concept of binomial nomenclature, the latter of which is still used around the world today.[104][note 1] He identified 24 classes, based mainly on the number, length and union of the stamens. The first ten classes follow the number of stamens directly (Octandria have 8 stamens etc.),[105][106] 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 in ferns). This method of classification, despite being artificial,[105] was used extensively for the following seven decades.[107][106]

French botanist Antoine Laurent de Jussieu's 1789 work Genera plantarum set out a new method for classifying plants; based instead on natural characteristics. Plants were divided by the number, if any, of cotyledons, and the location of the stamens.[107][108] The next most major system of classification came in the late 19th century from the botanists Joseph Hooker and George Bentham. They built on the earlier works of de Jussieu and Augustin Pyramus de Candolle, and devised a system in which plants were divided at the highest level by the number of cotyledons and the nature of the flowers, before falling into orders (families), genera, and species. This system of classification was published in their Genera plantarum in three volumes between 1862 and 1883.[109]

In 1963, biologists Robert Sokal and Peter Sneath created the method of numerical taxonomy, which differentiates taxa based on their tabulated morphological characteristics.[110] Following the development in scientific thought after Darwin's On the Origin of Species, many botanists have used more phylogenetic methods and the use of genetic sequencing, cytology, and palynology has become increasingly common.[111] Despite this, morphological characteristics such as the nature of the flower and inflorescence still make up the bedrock of plant taxonomy.[112][113]

Uses

Flowers have been used by humans all over the world for thousands of years for a variety of purposes, including: decoration, medicine, food, perfumes,[114] and essential oils. Many flowers are edible and are often used in drinks and dishes, such as salads, for taste, scent, and decoration.[115] Some flowers are commonly described as vegetables, when in fact they are actually inflorescences, bracts or stems of flowers. These include: broccoli, cauliflower, and artichoke. Flowers may be eaten freshly after being picked, termed floriphagia, or dried and eaten later.[116] Some flowers are steeped with or without Camellia sinensis (tea plant) to produce flower tea.[117] Essential oils and other flower extracts are widely used in herbal medicines and decoctions because they contain phytochemicals and may have anti-microbial effects.[118]

In culture

A Midsummer Night’s Dream

"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;"

William Shakespeare, Act II, Scene I

Flowers are the subject of much symbolism, and feature often in art, ritual, religious practises, and festivals. Plants have been cultivated in gardens for their flowers for around ten thousand years.[119][120] Flowers are associated with burial in many places, and are often placed on headstones to pay respect.[121][122] They are also placed by statues or temples of religious or other figures.[123] In places, the dead are buried covered in flowers or on a bed of flowers.[124] They are also associated with love, and given to others in many places for this reason.[125] As a result of economic forces, plants are bred for longer lasting, more beautiful or colourful flowers.[126]

Flowers feature extensively in art across a variety of mediums, and different flowers are ascribed symbolic meanings.[127][128] For example, violets may represnet modesty, virtue, or affection.[129] In addition to hidden meanings, flowers are used in flags, emblems, and seals. In this way, they represent countries or places. Some countries have national flowers; for example, Hibiscus × rosa-sinensis is the national flower of Malaysia.[130] In literature, flowers feature in imagery of places and as metaphors for pleasure, beauty and life.[131]

"The deep green foliage is quiet and reposeful,
The petals are clad in various shades of red;
The pistil drops with melancholy—
Wondering if spring knows her intimate thoughts."

— Wang Wei

Notes

  1. ^ His earlier works: Systema Naturae (1735) and Genera plantarum (1737) were also influential in the field.[105]

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Bibliography

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