The term petal refers to the often colorful, modified leaf structure that forms the corolla, or inner whorl, of a flower. These structures are a defining characteristic of angiosperms, or flowering plants, positioned between the protective sepals and the reproductive organs. The primary role of the petal is to participate in the reproductive cycle by attracting animal pollinators, which facilitates pollen transfer. Petals achieve this through visual signals, scent, and specialized mechanical structures.
Anatomy and Composition of Petals
Petals are fundamentally modified leaves, and their structure is optimized for display and interaction. The outermost layer is the epidermis, which is covered by a protective cuticle and often features specialized cells called papillae. These cone-shaped or rounded epidermal cells scatter and reflect light in specific ways, which enhance the color saturation and create a velvety texture that influences the flower’s visual signal.
Beneath the epidermis lies the mesophyll, a layer of parenchyma cells that contains the pigments responsible for the petal’s color. The petal’s thin structure is supported by vascular tissue, or venation, which transports nutrients and water. Unlike the extensive venation in typical leaves, a petal often has a simpler, single vascular bundle, reflecting its role as a temporary, display-focused organ. A petal is often structurally divided into a broad, upper blade and a narrow, lower claw, resembling the leaf blade and petiole of a conventional leaf.
Visual and Olfactory Signals for Pollination
The primary function of petals is communication, utilizing a combination of visual and olfactory signals to attract specific animal pollinators. The colors that make a flower attractive are determined by major pigment groups stored in the mesophyll, such as anthocyanins, carotenoids, and flavonoids. Anthocyanins produce reds, purples, and blues, while carotenoids are responsible for yellows and oranges.
These color signals are specialized to match the visual capabilities of the target pollinator. Bees possess trichromatic vision that extends into the ultraviolet (UV) range, making them attracted to blue and UV-reflecting flowers. Many flowers that appear uniformly colored to the human eye display complex UV patterns, known as nectar guides, which direct the pollinator toward the pollen and nectar rewards at the flower’s center. Conversely, birds, which have excellent red-light perception, are drawn to red flowers, a color insects cannot easily perceive.
Petals also produce complex olfactory signals through volatile organic compounds, which are emitted by specialized glandular cells called osmophores. The fragrance of a flower is a long-distance attractant, especially when the flower is partially concealed by foliage. These scents, which can include terpenoids and phenylpropanoids, work in coordination with the visual display, providing a dual-sensory signal that helps pollinators identify and remember rewarding flowers. The visual signal may be dominant for short-distance choice, but the scent remains a cue for initial long-range detection.
Protective and Specialized Petal Roles
Beyond their role in attraction, petals serve protective and mechanical functions for the flower’s reproductive structures. During the bud stage, the petals enclose and shield the delicate stamens and carpels from damage by weather, desiccation, and herbivores. The outermost petals, sometimes referred to as guard petals, are tougher and more resilient, forming a physical barrier until the flower is ready to bloom.
Once the flower opens, the petals can be modified to restrict or facilitate access for particular pollinators. In many species, petals are fused to form a corolla tube, which limits nectar access only to pollinators with long mouthparts, such as hummingbirds or certain moths. Other petals are modified into specialized structures, such as the large, lower petal of an orchid, called a labellum, which functions as an intricate landing platform for insects. Nectaries, which produce the sugary reward, are strategically located at the base of the petals, requiring the animal to brush against the reproductive organs to access the food source.
The Evolution of Petal Diversity
The diversity in petal form across angiosperms reflects co-evolution with animal pollinators. Petals are believed to have originated through the modification of pre-existing floral organs, either from sterile stamens (staminodes) or from outer perianth parts like sepals. Molecular studies suggest that the mechanism for petal development, regulated by B-class MADS-box genes, arose very early in the evolutionary history of flowering plants.
This evolutionary history has resulted in a wide range of corolla structures, categorized by the degree of petal fusion. Flowers with separate, unfused petals are classified as polypetalous, such as roses and buttercups. Conversely, flowers with petals that are partially or completely fused into a cup or tube are called sympetalous, a characteristic seen in morning glories and bellflowers. In wind-pollinated species, such as grasses, the petals are often significantly reduced in size or entirely absent, demonstrating the selective pressure of pollination strategy on petal morphology.