petal

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Tetrameric flower of the Primrose Willowherb (Ludwigia octovalvis) showing petals and sepals.

A petal (from Ancient Greek petalon "leaf", "thin plate") is one member or part of the corolla of a flower. The corolla is the name for all of the petals of a flower; the inner perianth whorl, term used when this is not the same in appearance (color, shape) as the outermost whorl (the calyx) and is used to attract pollinators based on its bright color. It is the inner part of the perianth that comprises the sterile parts of a flower and consists of inner and outer tepals. These tepals are usually differentiated into petals and sepals. The term "tepal" is usually applied when the petals and sepals are similar in shape and color. In a "typical" flower the petals are showy and colored and surround the reproductive parts. The number of petals in a flower (see merosity) is indicative of the plant's classification: eudicots (the largest group of dicots) having typically four or five petals and monocots and magnoliids having three, or some multiple of three, petals.^[1]

Flower with three petals and three sepals

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 seems that the mechanisms to form petals evolved a small number of times (perhaps only once), rather than evolving independently from stamens in a large number of plants.^[2]

Variation

Zygomorphic flower

There exists considerable variation in form of petals among the flowering plants. The petals can be united towards the base, forming a floral tube. In some flowers, the entire perianth forms a cup (called a calyx tube) surrounding the gynoecium, with the sepals, petals, and stamens attached to the rim of the cup.

The flowers of some species lack or have very much reduced petals. These are often referred to as apetalous. Examples of flowers with much reduced perianths are found among the grasses.

The petals are usually the most conspicuous parts of a flower, and 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 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. The petal is the colorful, often showy part of a plant.

Similar structures

Some plants have petaloid stamens, in plants like Canna that have true petals and staminodes, the stamen (staminodes) are modified to look like large showy petals.

A number of plants have bracts that resemble petals for example in Bougainvillea and Cornus florida (flowering dogwood). Petal-like bracts are common features in some plant families like Euphorbiaceae.

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.

Corolla

Fused corolla

Corolla is the collective term for petals of a flower taken as a group within the calyx. Normally the corolla is the most conspicuous part of a flower and of a bright colour other than green. The concept of corolla description is widely used in botany as a primary determinant of vascular plant identification. Alternatively the corolla may be considered as the inner whorl of the perianth structure. The role of the corolla in plant evolution has been studied extensively since Darwin postulated a theory of the origin of elongated corollae.^[3]

References

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