Petal

The collection of all petals in a flower is referred to as the corolla. The role of the corolla in plant evolution has been studied extensively since Charles Darwin postulated a theory of the origin of elongated corollae and corolla tubes. A corolla of separate petals, without fusion of individual segments, is apopetalous. If the petals are free from one another in the corolla, the plant is polypetalous or choripetalous; while if the petals are at least partially fused, it is gamopetalous or sympetalous. In the case of fused tepals, the term is syntepalous. Fused petals may form a tube, which is then known as a 'corolla tube'. == Petals, sepals, and tepals ==

Petals are usually surrounded by an outer whorl of modified leaves called sepals, that collectively form the calyx and lie just beneath the corolla. The calyx and the corolla together make up the perianth, the non-reproductive portion of a flower. When the petals and sepals of a flower are difficult to distinguish, they are collectively called tepals (e.g. Aloe and Tulipa species). Conversely, genera such as Rosa and Phaseolus have well-distinguished sepals and petals. When the undifferentiated tepals resemble petals, they are referred to as "petaloid", as in petaloid monocots, orders of monocots with brightly coloured tepals. Since they include Liliales, an alternative name is lilioid monocots. == Variations ==

'' flowers resemble those of geraniums, but are conspicuously zygomorphic. '' (the garden pea) an example of a zygomorphic flower '' showing (from bend to tip of flower) spathe, floral cup, tepals, and corona '' Petals can differ dramatically in different species. The number of petals in a flower may hold clues to a plant's classification. For example, flowers on eudicots (the largest group of dicots) most frequently have four or five petals while flowers on monocots have three or six petals, although there are many exceptions to this rule. The petal whorl or corolla may be either radially or bilaterally symmetrical. If all of the petals are essentially identical in size and shape, the flower is said to be regular separated from each other at the limb. Claws are distinctly developed in petals of some flowers of the family Brassicaceae, such as Erysimum cheiri. The inception and further development of petals show a great variety of patterns. Petals of different species of plants vary greatly in colour or colour pattern, both in visible light and in ultraviolet. Such patterns often function as guides to pollinators and are variously known as nectar guides, pollen guides, and floral guides. ==Genetics==

The genetics behind the formation of petals, in accordance with the ABC model of flower development, are that sepals, petals, stamens, and carpels are modified versions of each other. It appears that the mechanisms to form petals evolved very few times (perhaps only once), rather than evolving repeatedly from stamens. ==Significance of pollination==

Pollination is an important step in the sexual reproduction of higher plants. Pollen is produced by the male flower or by the male organs of hermaphroditic flowers. Pollen does not move on its own and thus requires wind or animal pollinators to disperse the pollen to the stigma of the same or nearby flowers. However, pollinators are rather selective in determining the flowers they choose to pollinate. This develops competition between flowers and as a result flowers must provide incentives to appeal to pollinators (unless the flower self-pollinates or is involved in wind pollination). Petals play a major role in competing to attract pollinators. Henceforth pollination dispersal could occur and the survival of many species of flowers could prolong. ==Types of pollination==

Wind pollination The flowers of wind-pollinated plants, such as the grasses, often have small, dull petals and produce little or no scent. Some examples are apetalous, having no petals at all. Flowers that depend on wind pollination will produce large amounts of pollen because most of the pollen scattered by the wind tends to not reach other flowers. Attracting insects '', emerging from tubular calyx and bearing long points Flowers have various regulatory mechanisms to attract insects. One such helpful mechanism is the use of colour guiding marks. Insects such as the bee or butterfly can see the ultraviolet marks which are contained on these flowers, acting as an attractive mechanism which is not visible towards the human eye. Many flowers contain a variety of shapes acting to aid with the landing of the visiting insect and also influence the insect to brush against anthers and stigmas (parts of the flower). One such example of a flower is the pōhutukawa (Metrosideros excelsa), which acts to regulate colour in a different way. The pōhutukawa contains small petals also having bright large red clusters of stamens. Another attractive mechanism for flowers is the use of scents which are highly attractive to humans. One such example is the rose. On the other hand, some flowers produce the smell of rotting meat and are attractive to insects such as flies. Darkness is another factor that flowers have adapted to as nighttime conditions limit vision and colour-perception. Fragrancy can be especially useful for flowers that are pollinated at night by moths and other flying insects. == References ==