Petal

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

A petal (from Ancient Greek pétalon "leaf") 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 corolla (petals) and calyx (the name for all the sepals) make up the outer whorls of a flower; these form what is known as the perianth, which attracts pollinators with its bright color. 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, colored, and surround the reproductive parts. The number of petals in a flower indicates the plant's classification, eudicots (the largest group of dicots) having typically four or five petals and monocots 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 appears that the mechanisms to form petals evolved very few times (perhaps only once), rather than evolving independently from stamens in a large number of plants.^[2]

Variations

Zygomorphic flower

The petals are usually the most conspicuous parts of a flower, and are often colourful and showy. However, 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 forms of petals vary considerably 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 petal whorl or corolla may be either radially or bilaterally symmetrical (see Symmetry in biology). 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.

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. Alternatively, the corolla may be considered as the inner whorl of the perianth structure. 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 way to identify vascular plants. The role of the corolla in plant evolution has been studied extensively since Darwin postulated a theory of the origin of elongated corollae.^[3]

The corolla extends from the calyx outwards. Within it are housed anatomy such as the corolla tube, anthers, stamen, and stigma. The corolla tube is not separate from the corolla, but identifies the base section (some corollas form lobes at the top, which distinguishes the petals from the corolla tube).^[4]

References