Flower
Flowers, also known as blossoms and blooms, are the reproductive structures of flowering plants. Typically, they are structured in four circular levels around the end of a stalk. These include: sepals, which are modified leaves that support the flower; petals, often designed to attract pollinators; male stamens, where pollen is presented; and female gynoecia, 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 plants 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 orchids and major crop plants to large trees.
Etymology
In botany, flowers are defined as the reproductive structures of angiosperms, while cones are regarded as the gymnosperm equivalent. Bloom is similarly defined, but may also be used to describe the collective of flowers on a plant, as in the phrase: covered with bloom. Flower is also commonly used to describe the whole of a plant that produces flowers.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 cognates and are derived from the Proto-Indo-European word bʰleh₃ōs. Both bloom and blossom refer to flowers as well as the state of flowering; as in the phrases: in bloom or in blossom.
Function
The main purpose of a flower is reproduction of the individual, aiding in the survival of the species. Flowers not only produce spores, which become gametophytes that produce sex cells, leading to fertilised cells, but also develop and help disseminate seeds. Sexual reproduction between plants results in evolutionary adaptation, which improves species survival. Plants favour cross-pollination because it promotes the joining of sex cells from genetically distinct plants of the same species, thereby increasing genetic diversity. Facilitating this process is a key function of flowers and is often reflected in their form and structure. Features designed to attract pollinators are among the most common adaptations.Structure
The structure of a flower, termed its morphology, 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, or complete, flower is made up of four kinds of structures arranged in sets called whorls. They grow around the tip of a short stalk or axis, called a receptacle. The four main whorls are the calyx, petals, androecium, and gynoecium.Vegetative
The non-reproductive or vegetative part of the flower, known collectively as the perianth, consists of calyx, and the petals. The receptacle is the thickened part of the flower stalk, called the pedicel, which supports all of the other flower structures.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, pores, green pigment, and may have analogous outgrowths from the stem. Sepals are often waxy, tough, and grow quickly to protect the flower as it develops. Although they sometimes fall off at maturity, sepals more commonly persist to protect the fruit and aid in its dispersal. The sepals in some flowers may be partially or completely fused together.Petals
The petals, collectively called the corolla, are almost or completely fibreless leaf-like structures that form the innermost whorl of the perianth. They are often delicate and thin and are usually coloured, shaped, or scented, to encourage and facilitate pollination. The petals may be fused together. Petals also tend to have patterns only visible under ultraviolet light, which is visible to pollinators but not to humans. In some flowers, petals and sepals are indistinguishable from one another.Reproductive
All flowering plants are heterosporous, that is, every individual plant produces two types of spores. Spores are formed from mature plants, which contain two sets of chromosomes, and are divided into microspores and megaspores—the precursors to pollen and embryo sacs respectively. Pollen and embryo sacs are the male and female gametophytes, sex cell-producing structures, and contain just one set of chromosomes. Microspores are produced by meiosis inside anthers, the male part of flowers, and megaspores are produced inside ovules contained within the ovary. As with all heterosporous plants, the gametophytes also develop inside the spores.Male
The androecium is the whorl of male parts called stamens, which produce pollen. Stamens consist typically of an anther, made up of four pollen sacs arranged in two sheaths called thecae, connected to a filament, or stalk. The anther contains microspores 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.Female
The gynoecium, consisting of one or more carpels, is the female part of the flower and found on the innermost whorl. Each carpel consists of: a stigma, which receives pollen; a style, the stalk; and an ovary, which contains the ovules, and the female gametophytes by extension. Carpels may be fused together and are often described collectively as a pistil. Inside the ovary, the ovules are attached to the placenta by structures called funiculi.Variation
Although most plants have flowers with four whorls—protective leaves, petals, male parts, and female parts—and their typical sub-structures, they vary greatly between flowering plants. This variation encompasses all aspects of flowers, including size, shape, and colour. Flowers range in size from to in diameter. Additionally, the four main parts of a flower are generally defined by their positions and not by their function. Many flowers lack some parts, have parts that are modified for other functions, or contain parts that look like what is typically another part. In some flowers, organs such as stamens, stigmas, and sepals are modified to resemble petals. This is most common in cultivation, where flowers with many additional "petals" are found to be more attractive.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 regular. This is an example of radial symmetry. If there is only one plane of symmetry, 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 — such 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.File:Paardenbloem 26-04-2020. 01.jpg|alt=A yellow dandelion "flower", really a group of small flowers or florets. In the centre, the florets are clustered and extend upwards. On the outside they consist of just a single petal, creating the flower-like appearance.|thumb|A dandelion inflorescence is made up of many small flowers grouped closely together to look like a single flower.|upright=0.8