Petal
In botany, the petal is an anthophile that forms part of the corolla of a flower. It is the inner part of the perianth, which comprises the sterile parts of a flower. In a "typical" the petals are conspicuous and colored, and surround the reproductive parts. The number of petals on a flower is indicative of the plant's classification: dicots, which typically have four or five petals; and monocots, which have three or some multiple of three petals. The main function of the petals or corolla is to attract pollinators.
There is considerable variation in petal shape between plants. The petals may be united at the base, forming a floral tube. In some flowers, the entire perianth forms a cup (called the calyx tube) surrounding the gynoecium, with the sepals, petals, and stamens attached to the edge of the calyx.
The flowers of some species are missing or have greatly reduced petals. They are called apétalas. Examples of flowers with greatly reduced perianths can be seen among the grasses. This is characteristic of wind-pollinated plants that do not need to attract pollinators.
Anatomy
The structure of tepals and petals is similar to that of sepals. Epidermal cell walls are often convex or papillose, especially on the adaxial side. On many petals, such as those of Brassica napus, the papillae are conical, with a marked cuticular thickening at the apex, and radial striations towards the base. It has been suggested that these thickenings allow for an even diffusion of emerging light, such that the petals shine uniformly at any angle of illumination.
Some epidermal cells of the petals are osmophores, containing essential oils that impart the characteristic fragrance to the flowers. The mesophyll generally does not present chlorophyll parenchyma, but fundamental parenchyma.
Petal color
The color of the petals results from the presence of pigments. In many flowers the cells present chromoplasts with carotenoid pigments (red, orange, yellow). The most important pigments are flavonoids, mainly anthocyanins, which are dissolved in the cell juice; the basic pigments are pelargonidin (red), cyanidin (violet), and delphinidin (blue), the flavonols (yellow or ivory). The color of the anthocyanin pigments depends on the pH of the cell juice: in Brunfelsia australis (mountain lily) the flowers are purplish, when they age they turn white due to a change in pH.
The white color of many flowers, such as Magnolia grandiflora, is due to the phenomenon of total light reflection. The petals may have air spaces in a subepidermal position or a layer of cells with abundant starch grains, and in both cases light is reflected. The dark colors are due to a total absorption of light operated by complementary pigments. In Tulipa gesneriana (black tulip) there is blue anthocyanin in the epidermal cells and yellow carotene in the subepidermal cells.
In some species the basal parts of the petals contain a flavonol glucoside called chalcone, which absorbs ultraviolet light, turning them into "nectar guides" for pollinating insects. This particular colour, visible only to insects, is called 'bee purple'.