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Other uses Petals are modified leaves that surround the reproductive parts of flower s. They are often advertising coloration|brightly colored or unusually shaped to attract pollinator s. Together, all of the petals of a flower are called a corolla . Petals are usually accompanied by another set of special leaves called sepal s lying just beneath the corolla. When the petals and sepals of a flower look similar they are called tepal s. Examples of plants in which the term tepal is appropriate include Genus|genera such as Aloe and Tulipa . Conversely, genera such as Rose|Rosa and Phaseolus have well-distinguished sepals and petals.
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Variations
Petals can differ dramatically in different species. The merosity|number of petals in a flower may hold clues to a plant's classification. For example, flowers on eudicots (the largest group of dicotyledon|dicots ) most frequently have four or five petals while flowers on monocotyledon|monocots have three or six petals, although there are many exceptions to this rule.cite journal |title= The origin and diversification of angiosperms |author= Soltis, Pamela S.; Douglas E. Soltis |journal= American Journal of Botany |year= 2004 |volume= 91 | pages= 1614–1626 |url= http://www.amjbot.org/cgi/content/full/91/10/1614 | doi= 10.3732/ajb.91.10.1614 |issue=10 |pmid=21652312
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 Floral symmetry|zygomorphic flowers may be seen in orchid s and members of the Fabaceae|pea family .
In many plants of the Asteraceae|aster family such as the sunflower, Helianthus annuus , the circumference of the Head (botany)|flower head is composed of ray florets. Each ray floret is anatomically an individual flower with a single large petal.
Although petals are usually the most conspicuous parts of a flower, some species, such as Poaceae|grasses , have either very small petals or lack them entirely.
Similar structures
Sometimes botanically different parts have the appearance of petals. In the genus Canna (plant)|Canna the true petals are tiny while the stamen s are large and brightly colored. A number of plants have bract s that resemble petals, for example in Bougainvillea , Cornus florida (flowering dogwood) and members of the Euphorbiaceae family.
Corolla
The collective term for all of the petals is the corolla . The calyx (botany)|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.cite web|author=L. Anders Nilsson|url= http://adsabs.harvard.edu/abs/1988Natur.334..147N Analysis of theory of evolution of corolla elongation involving pollinating species|title=The evolution of flowers with deep corolla tubes|journal=Nature|volume=334|issue=6178|pages=147-149|year=1988
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 .
Genetics
The genetics behind the formation of petals, in accordance with the ABC Model of Flower Development|ABC model of flower development , are that sepals, petals, stamen s, 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.cite journal |doi= 10.3732/ajb.0800038 |title= One size fits all? Molecular evidence for a commonly inherited petal identity program in Ranunculales |year= 2008 |author= Rasmussen, D. A. |journal= American Journal of Botany |last2= Kramer |first2= E. M. |last3= Zimmer |first3= E. A. |volume= 96 |pages= 96–109 |pmid= 21628178 |issue= 1
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References
reflist Commons category|Petals Category:Plant morphology Category:Reproductive system Category:Pollination
