Petal

For other uses, seePetal (disambiguation).

Petalsare modifiedleavesthat surround the reproductive parts offlowers.They are oftenbrightly colouredor unusually shaped to attractpollinators.All of the petals of a flower are collectively known as thecorolla.Petals are usually accompanied by another set of modified leaves calledsepals,that collectively form thecalyxand lie just beneath the corolla. The calyx and the corolla together make up theperianth,the non-reproductive portion of a flower. When the petals and sepals of a flower are difficult to distinguish, they are collectively calledtepals.Examples of plants in which the termtepalis appropriate includegenerasuch asAloeandTulipa.Conversely, genera such asRosaandPhaseolushave well-distinguished sepals and petals. When the undifferentiated tepals resemble petals, they are referred to as "petaloid", as inpetaloid monocots,orders of monocots with brightly coloured tepals. Since they includeLiliales,an alternative name is lilioid monocots.

Diagram showing the parts of a mature flower. In this example, the perianth is separated into a calyx (sepals) and corolla (petals)

Although petals are usually the most conspicuous parts of animal-pollinated flowers, wind-pollinated species, such as thegrasses,either have very small petals or lack them entirely (apetalous).

Tetramericflower of a primrosewillowherb(Ludwigia octovalvis) showing petals and sepals

Atulip's actinomorphic flower with three petals and three sepals, that collectively present a good example of an undifferentiated perianth. In this case, the word "tepals"is used.

Corolla

Apopetalous corolla

Tubular-campanulate corolla, bearing long points and emergent from tubular calyx (Brugmansia aurea,Golden Angel's Trumpet, familySolanaceae).

The collection of all petals in a flower is referred to as the corolla. The role of the corolla in plantevolutionhas been studied extensively sinceCharles Darwinpostulated a theory of the origin of elongated corollae and corolla tubes.

A corolla of separate petals, without fusion of individual segments, isapopetalous.If the petals are free from one another in the corolla, the plant ispolypetalousorchoripetalous;while if the petals are at least partially fused, it isgamopetalousorsympetalous.In the case of fused tepals, the term issyntepalous.The corolla in some plants forms a tube.

Variations

Pelargonium peltatum,the Ivy-leaved Pelargonium: its floral structure is almost identical to that of geraniums, but it is conspicuously zygomorphic

Geranium incanum,with an actinomorphic flower typical of the genus

The white flower ofPisum sativum,the Garden Pea: an example of a zygomorphic flower.

Narcissus pseudonarcissus,the Wild Daffodil, showing ( from bend to tip of flower )spathe,floral cup,tepals,corona

The petals ofCombretum indicumgrow on the sepals of the flower.

Petals can differ dramatically in different species. Thenumber of petals in a flowermay hold clues to a plant's classification. For example, flowers oneudicots(the largest group ofdicots) most frequently have four or five petals while flowers onmonocotshave three or six petals, although there are many exceptions to this rule.

The petal whorl or corolla may be either radially or bilaterally symmetrical (seeSymmetry in biologyandFloral symmetry). If all of the petals are essentially identical in size and shape, the flower is said to beregularoractinomorphic(meaning "ray-formed" ). Many flowers are symmetrical in only one plane (i.e., symmetry is bilateral) and are termedirregularorzygomorphic(meaning "yoke-" or "pair-formed" ). Inirregularflowers, other floral parts may be modified from theregularform, but the petals show the greatest deviation from radial symmetry. Examples ofzygomorphicflowers may be seen inorchidsand members of thepea family.

In many plants of theaster familysuch as the sunflower,Helianthus annuus,the circumference of theflower headis composed of ray florets. Each ray floret is anatomically an individual flower with a single large petal. Florets in the centre of the disc typically have no or very reduced petals. In some plants such asNarcissus,the lower part of the petals or tepals are fused to form a floral cup (hypanthium) above the ovary, and from which the petals proper extend.

A petal often consists of two parts: the upper broader part, similar to a leaf blade, also called theblade;and the lower narrower part, similar to a leafpetiole,called theclaw,separated from each other at thelimb.Claws are distinctly developed in petals of some flowers of the familyBrassicaceae,such asErysimum 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 asnectar guides,pollen guides, and floral guides.

Genetics

The genetics behind the formation of petals, in accordance with theABC model of flower development,are that sepals, petals,stamens,andcarpelsare 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 ofhermaphroditicflowers.

Pollen does not move on its own and thus requires wind or animal pollinators to disperse the pollen to thestigmaof 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.

Functions and purposes

Petals have various functions and purposes depending on the type of plant. In general, petals operate to protect some parts of the flower and attract/repel specific pollinators.

Function

This is where the positioning of the flower petals are located on the flower is the corolla e.g. the buttercup having shiny yellow flower petals which contain guidelines amongst the petals in aiding the pollinator towards the nectar. Pollinators have the ability to determine specific flowers they wish to pollinate.Using incentives, flowers draw pollinators and set up a mutual relation between each other in which case the pollinators will remember to always guard and pollinate these flowers (unless incentives are not consistently met and competition prevails).

Scent

The petals could produce different scents to allure desirable pollinatorsor repel undesirable pollinators.Some flowers will also mimic the scents produced by materials such as decaying meat, to attract pollinators to them.

Colour

Various colour traits are used by different petals that could attract pollinators that have poor smelling abilities, or that only come out at certain parts of the day. Some flowers can change the colour of their petals as a signal to mutual pollinators to approach or keep away.

Shape and size

Furthermore, the shape and size of the flower/petals are important in selecting the type of pollinators they need. For example, large petals and flowers will attract pollinators at a large distance or that are large themselves. Collectively, the scent, colour, and shape of petals all play a role in attracting/repelling specific pollinators and providing suitable conditions for pollinating. Some pollinators include insects, birds, bats, and wind. In some petals, a distinction can be made between a lower narrowed, stalk-like basal part referred to as the claw, and a wider distal part referred to as the blade (or limb). Often, the claw and blade are at an angle with one another.

Types of pollination

Main article:Pollination syndrome

Wind pollination

Main article:Anemophily

Wind-pollinated flowers often have small, dull petals and produce little or no scent. Some of these flowers will often have 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

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 pohutukawa (Metrosideros excelsa), which acts to regulate colour in a different way. The pohutukawa 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.

Attracting birds

Flowers are also pollinated by birds and must be large and colourful to be visible against natural scenery. In New Zealand, such bird–pollinated native plants include: kowhai (Sophoraspecies), flax (Phormium tenax) and kaka beak (Clianthus puniceus). Flowers adapt the mechanism on their petals to change colour in acting as a communicative mechanism for the bird to visit. An example is the tree fuchsia (Fuchsia excorticata), which are green when needing to be pollinated and turn red for the birds to stop coming and pollinating the flower.

Bat-pollinated flowers

Flowers can be pollinated by short-tailed bats. An example of this is the dactylanthus (Dactylanthus taylorii). This plant has its home under the ground acting the role of a parasite on the roots of forest trees. The dactylanthus has only its flowers pointing to the surface and the flowers lack colour but have the advantage of containing much nectar and a strong scent. These act as a useful mechanism in attracting the bat.

Bibliography

• Simpson, Michael G. (2011).Plant Systematics.Academic Press.ISBN978-0-08-051404-8.
• Foster, Tony."Botany Word of the Day".Phytography.Retrieved27 November2014.