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Flowers, also known as blossoms and blooms, are the reproductive structures of Flowering plant. Typically, they are structured in four circular levels around the end of a stalk. These include: , which are modified leaves that support the flower; , often designed to attract pollinators; male , where pollen is presented; and female gynoecium, where pollen is received and its movement is facilitated to the egg. When flowers are arranged in a group, they are known collectively as an inflorescence.
The development of flowers is a complex and important part in the life cycles of flowering plants. In most plants, flowers are able to produce sex cells of both sexes. Pollen, which can produce the male sex cells, is transported between the male and female parts of flowers in pollination. Pollination 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. 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 of fertilisation, which is a key step in sexual reproduction. Through cellular and nuclear divisions, the resulting cell grows into a seed, 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 a fruit, 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 they defecate. Non-living things like wind and water can also help to disperse the seeds.
Flowers first evolved between 150 and 190 million years ago, in the Jurassic. Plants with flowers replaced non-flowering Seed plant in many ecosystems, as a result of flowers' superior reproductive effectiveness. In the study 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 to , 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 tiny Orchid and major crop plants to large trees.
Flower entered Middle English via Old French flor from earlier Latin flōs, flōris and before that Proto-Italic flōs, all of which had the same meaning 'flower'. The spelling flour was more common in English until the 17th century, when it became specialised to mean "ground grain" — originally an instance of figurative flower meaning "best part; finest". The Old English word for flower was blossom, which is still used today, but refers especially to the flowers of edible fruit trees, and not to the whole flowering plant. Flower, bloom, and blossom are all and are derived from the Proto-Indo-European word bʰleh₃ōs ('blossoming'). Both bloom and blossom refer to flowers as well as the state of flowering; as in the phrases: in bloom or in blossom.
Most flowers have symmetry. When the flower is bisected through the central axis from any point and symmetrical halves are produced, the flower is said to be Actinomorphic (as in Cyperaceae). This is an example of radial symmetry. If there is only one plane of symmetry (as in ), the flower is said to be irregular. If, in very rare cases, they have no symmetry at all they are called asymmetric. 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.
In the majority of species, individual flowers have both female parts and male parts. These flowers are described as being perfect, bisexual, or hermaphrodite. 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 called monoecious. However, if an individual plant is either female or male, the species is called dioecious. Many flowers have nectaries, which are glands that produce nectar: a sugary fluid used to attract pollinators. Their shape varies between different plants, are they not considered as an organ on their own.
Some flowers are lacking or have only a highly reduced stalk, and so are attached directly to the plant. There are several structures, found in some plants, that resemble flowers or floral organs. These include: coronas, crown-like outgrowths; and pseudonectaries, that look like nectaries but do not contain nectar. In plants where disease has taken hold, phyllody—leafy flower parts—may occur. inflorescence is made up of many small flowers grouped closely together to look like a single flower (a pseudanthium).|upright=0.8]]
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.
Colour, or colour effects, may also be produced by structural coloration, in which colour is produced by tiny surface structures interfering with waves of light. This includes iridescence (as in some ) and (as in edelweiss), which diffract light using tiny grooves. The colour of flowers can also change; sometimes this acts as a signal to pollinators (as in Viola cornuta). Change may also occur as a result of temperature; pH, as in the found in Hydrangea; metals; sugars; and cell shape.
The ABC model was the first unifying principle in the development of flowers, and its major tenets have been found to hold in most flowering plants. It describes how three groups of Gene—A, B, and C—are responsible for the development of flowers. These three gene groups' activities interact together to determine the developmental identities of the primordia organ within the floral apical 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. 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.
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 internal and environmental cues such as: changes in levels of plant hormones (such as Gibberellin), seasonable temperature, and Photoperiodism changes. Many plants, including many of those that have Perennial and just Biennial plant, require Vernalization to flower. These cues are interpreted molecularly through a complex signal called florigen, which involves a variety of . Florigen is produced in the leaves in reproductively favourable conditions and acts in stem tips to force switching from developing leaves to flowers. Once developed, flowers may Nyctinasty their flowers at different times of day; usually around dusk and dawn. They may also Heliotropism to remain warm—potentially both for their own benefit and to attract pollinators. Both of these mechanisms are controlled by a plant's circadian rhythm and in response to environmental changes.
Flowering plants usually face evolutionary pressure to optimise the transfer of their pollen, and this is typically reflected in the morphology of their flowers and their reproductive strategies. 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.
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. This close relationship is an example of coevolution, as the plant and pollinator have developed together over a long period to match each other's needs.
In flowering plants, fertilisation is preceded by pollination, which is the movement of pollen from the stamen to the carpel. It encompasses both plasmogamy, the fusion of the Protoplast (cell without cell wall), and karyogamy, the fusion of the Cell nucleus. 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 centre-most part of the ovary it enters the egg apparatus and is guided by a synergid.
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 its cell membrane and much of the Protoplasm that fills its cells. The sperm's nucleus then fuses with the egg's nucleus, resulting in the formation of a zygote; a diploid cell, containing two copies of each chromosome. Flowering plants undergo double 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 are haploid, they result in a large endosperm nucleus which is triploid.
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, stamens, petals, and sepals. This process is called floral senescence; 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. The fruit contains three main structures: the outer layer of Exocarp; the Mesocarp; and the Pyrena, or innermost layer. The pericarp, 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 different Dry fruits and Fleshy fruit. 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.
The flower was the angiosperms' most significant evolutionary innovation, granting the ability to effectively take advantage of animal pollinators. Other 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 pioneered double fertilisation, which allows energy investment (into endosperm) 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. The net effect of these features was greater reproductive security and efficiency. This allowed the angiosperms to replace many other —such as Pinales, , Gnetophyta and Ginkgoales—in the majority of .
A key driving force in the evolution of flowers is coevolution, where pollinator and flower evolve with one another, 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. However, this close interdependence increases the risk of extinction, since the extinction of either member almost certainly means the extinction of the other member as well. 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). For example, Japanese honeysuckle flowers strategically open during the night to attract nocturnal moths, which are more efficient pollinators than diurnality bees. With the innovation of the flower—and other adaptations—angiosperms rapidly diversified. Approximately 90% of all living land plant species are angiosperms. This is attributed, in part, to coevolution, which caused specialisation and so speciation; where populations diverge into separate species. Both the strength of close pollinator-flower relationships and the survival of either species are effected by climate change. Reducing numbers of pollinators have led to the extinction of many flowering plants.
In 1963, the biologists Robert Sokal and Peter Sneath created the method of numerical taxonomy, which differentiates taxa 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. While this and earlier methods, such as Linnaeus's, used morphological features, many botanists today employ genetic sequencing, the study of cells, and the study of pollen. These come as a result of advancements in DNA-related science. Despite this, morphological characteristics such as the nature of the flower and inflorescence still make up the bedrock of plant taxonomy.
Most crop plants have flowers, and they produce much of the most common crop products—such as seeds and fruits; around half of all cropland is used to grow three flowering plants: rice, wheat, and corn. Flowers are steeped to make teas, either alone, as in Herbal tea, or in combination with the tea plant. Essential oils and other flower extracts are widely used in and because they contain phytochemicals and may have anti-microbial effects. Flowers from many plants are also used in the production of , such as Cannabis sativa, Clivia miniata, and Madagascar periwinkle . Some flowers are used in cooking as spices, these include saffron and Clove; derived from Crocus and Syzygium aromaticum respectively.
Flowers feature extensively in art across a variety of mediums, and different flowers are ascribed symbolic meanings. For example, violets may represent modesty, virtue, or affection. 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. In literature, flowers feature in imagery of places and as Metaphor for pleasure, beauty, and life.
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