Petals are modified leaves that surround the reproductive parts of flowers. They are often brightly colored or unusually shaped to attract pollinators. Together, all of the petals of a flower are called a corolla. Petals are usually accompanied by another set of special leaves called sepals, that collectively form the calyx and lie just beneath the corolla. When the petals and sepals of a flower look similar they are called tepals. Examples of plants in which the term tepal is appropriate include genera such as Aloe and Tulipa. Conversely, genera such as Rosa and Phaseolus have well-distinguished sepals and petals.
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 (see Symmetry in biology and Floral symmetry). If all of the petals are essentially identical in size and shape, the flower is said to be regular or actinomorphic (meaning "ray-formed"). Many flowers are symmetrical in only one plane (i.e., symmetry is bilateral) and are termed irregular or zygomorphic (meaning "yoke-" or "pair-formed"). In irregular flowers, other floral parts may be modified from the regular form, but the petals show the greatest deviation from radial symmetry. Examples of zygomorphic flowers may be seen in orchids and members of the pea family.
In many plants of the aster family such as the sunflower, Helianthus annuus, the circumference of the flower head is 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.
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 etc.
Although petals are usually the most conspicuous parts of animal-pollinated flowers, those of wind-pollinated species such as the grasses, have either very small petals or lack them entirely.
Similar structures 
Sometimes, botanically different structures have the appearance of petals. In the genus Canna the true petals are tiny while the stamens are large and brightly colored. A number of plants have bracts that resemble petals, for example in Bougainvillea, Cornus florida (flowering dogwood) and members of the Euphorbiaceae family.
The collective term for all of the petals is the corolla. The calyx (all of the sepals) and the corolla together make up the perianth. 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.
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 together, it is gamopetalous or sympetalous.
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.
- Soltis, Pamela S.; Douglas E. Soltis (2004). "The origin and diversification of angiosperms". American Journal of Botany 91 (10): 1614–1626. doi:10.3732/ajb.91.10.1614. PMID 21652312.
- L. Anders Nilsson (1988). Analysis of theory of evolution of corolla elongation involving pollinating species "The evolution of flowers with deep corolla tubes". Nature 334 (6178): 147–149.
- Rasmussen, D. A.; Kramer, E. M.; Zimmer, E. A. (2008). "One size fits all? Molecular evidence for a commonly inherited petal identity program in Ranunculales". American Journal of Botany 96 (1): 96–109. doi:10.3732/ajb.0800038. PMID 21628178.
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