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A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants. The biological function of a flower is to facilitate reproduction, usually by providing a mechanism for the union of sperm with eggs. Flowers may facilitate outcrossing resulting from cross-pollination or allow selfing when self-pollination occurs. The two types of pollination are self-pollination and cross-pollination. Self-pollination happens when the pollen from the anther is deposited on the stigma of the same flower, or another flower on the same plant. Cross-pollination is the transfer of pollen 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 the stamen and carpel 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. Some flowers produce diaspores without fertilization. Flowers contain sporangia and are the site where gametophytes develop. Many flowers have evolved to be attractive to animals, so as to cause them to be vectors for the transfer of pollen. After fertilization, the ovary of the flower develops into fruit-containing seeds. In addition to facilitating the reproduction of flowering plants, flowers have long been admired and used by humans to bring beauty to their environment, and also as objects of romance, ritual, esotericism, witchcraft, religion, medicine, and as a source of food. 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, though it now refers to flowers only of fruit trees. Morphology Parts The flower has two essential parts: the vegetative part, consisting of petals and associated structures in the perianth, and the reproductive or sexual parts. A stereotypical flower consists of four kinds of structures attached to the tip of a short stalk. Each of these kinds of parts is arranged in a whorl on the receptacle. The four main whorls are as follows: Perianth Collectively the calyx and corolla form the perianth. Calyx: The calyx consists of leaf-like structures at the base of a flower that protects the flower during development. The leaf-like structures are individually referred to as sepals. There are often as many of these sepals as there are petals. While most calyces are green, there are exceptions in which the calyx is the same color as the petals of the flower or a different color altogether. The calyx performs a crucial role for the flowering plant. As the flower is forming, it is closed tightly into a bud. The sepals are the outer covering of the flower as it forms and is the only thing you see of the flower while it is still in bud form. It protects the developing flower and prevents it from drying out. Corolla: the next whorl toward the apex, composed of units called petals, which are typically thin, soft, and colored to attract animals that help the process of pollination. 

Perigone: in monocots, the calyx and corolla are indistinguishable thus the whorls of the perianth or perigone are called tepals. Reproductive Androecium: the next whorl, consisting of units called stamens. Stamens consist of two parts: a stalk called a filament, topped by an anther where pollen is produced by meiosis and eventually dispersed. Gynoecium: the innermost whorl of a flower, consisting of one or more units called carpels. The carpel or multiple fused carpels form a hollow structure called an ovary, which produces ovules internally. Ovules are megasporangia and they, in turn, produce megaspores by meiosis which develop into female gametophytes. These give rise to egg cells. The gynoecium of a flower is also described using an alternative terminology wherein the structure one sees in the innermost whorl is called a pistil. A pistil may consist of a single carpel or a number of carpels fused together. The sticky tip of the pistil, the stigma, is the receptor of pollen. The supportive stalk, the style, becomes the pathway for pollen tubes to grow from pollen grains adhering to the stigma. The relationship to the gynoecium on the receptacle is described as hypogynous, perigynous, or epigynous. Structure Although the arrangement described above is considered "typical", plant species show a wide variation in floral structure. These modifications have significance in the evolution of flowering plants and are used extensively by botanists to establish relationships among plant species. 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 that may be modified into other functions and/or look like what is typically another part. In some families, like Ranunculaceae, the petals are greatly reduced and in many species, the sepals are colorful and petal-like. Other flowers have modified stamens that are petal-like; the double flowers of Peonies and Roses are mostly petaloid stamens. Flowers show great variation and plant scientists describe this variation in a systematic way to identify and distinguish species. Specific terminology is used to describe flowers and their parts. Many flower parts are fused together, fused parts originating from the same whorl are connate, while fused parts originating from different whorls are adnate; parts that are not fused are free. When petals are fused into a tube or ring that falls away as a single unit, they are sympetalous. Connate petals may have distinctive regions: the cylindrical base is the tube, the expanding region is the throat and the flaring outer region is the limb. A sympetalous flower, with bilateral symmetry with an upper and lower lip, is bilabiate. Flowers with connate petals or sepals may have various shaped corolla or calyx, including campanulate, funnelform, tubular, urceolate, salverform, or rotate. Referring to "fusion," as it is commonly done, appears questionable because at least some of the processes involved may be non-fusion processes. For example, the addition of intercalary growth at or below the base of the primordia of floral appendages such as sepals, petals, stamens, and carpels may lead to a common base that is not the result of fusion. Many flowers have some form of 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, e.g. rose or trillium. 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, e.g. snapdragon or most orchids Flowers may be directly attached to the plant at their base. The stem or stalk subtending a flower is called a peduncle. If a the peduncle supports more than one flower, the stems connecting each flower to the main axis are called pedicels. The apex of a flowering stem forms a terminal swelling which is called the torus or receptacle Inflorescence In those species that have more than one flower on an axis, the collective cluster of flowers is termed an inflorescence. Some inflorescences are composed of many small flowers arranged in a formation that resembles a single flower. The common example of this is most members of the very large composite group. A single daisy or sunflower, for example, is not a flower but a flower head—an inflorescence composed of numerous flowers. An inflorescence may include specialized stems and modified leaves known as bracts. Diagrams and formulae 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. Floral formulae have been developed in the early 19th century and their use has declined since. Prenner et al. devised an extension of the existing model to broaden the descriptive capability of the formula. The format of floral formulae differs in different parts of the world, yet they convey the same information The structure of a flower can also be expressed by the means of floral 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. Detailed developmental studies, however, have shown that stamens are often initiated more or less like modified stems that in some cases may even resemble branchlets. 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 for fertilization and the formation of seeds, hence ensuring maximal reproductive success. To meet these needs a plant is able to interpret important endogenous and environmental cues such as changes in levels of plant hormones and seasonable temperature and photoperiod changes. Many perennial and most biennial plants require vernalization 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, including Constans, Flowering Locus C and Flowering Locus T. Florigen is produced in the leaves in reproductively favorable conditions and acts in buds and growing tips to induce a number of different physiological and morphological changes. 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 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 of some environmental cue. Organ development The ABC model is a simple model that describes the genes responsible for the development of flowers. Three gene activities interact in a combinatorial manner to determine the developmental identities of the primordia organ within the floral apical meristem. These gene functions are called A. B, and C. A genes are expressed in only 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 in Arabidopsis 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 B function but presence of C function mimics the fourth whorl, leading to the formation of carpels also in the third whorl. Function The principal purpose of a flower is the reproduction of the individual and 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. In the majority of plant species, individual flowers have both functional carpels and stamens. Botanists describe these flowers as perfect or bisexual. and the species as hermaphroditic. In a minority of plant species, their flowers lack one or the other reproductive organ and are described as imperfect or unisexual. If the individual plants of a species each have unisexual flowers of both sexes then the species is monoecious. Alternatively, if each