Petals attract bees and other insects that pollinate the plant. Evidence for this function lies in the noticeable differences in petal structure depending on pollination method.
Petals are leaves with modifications to attract pollinating organisms; adaptations include shape, color and scent. Large petals attract pollinators from a distance. Large petals also attract large pollinators; plants that rely largely on birds for pollination tend to have larger petals. Petals that produce a noticeable scent are important in attracting nocturnal pollinators, such as moths or even bats. Plants that depend upon flies for pollination produce a scent similar to rotting carrion. Showy colors help attract pollinators with color vision, such as some birds; hummingbirds, for instance, prefer red flowers. Many petals also have nectar guides, markings that reflect ultraviolet light. These markings are invisible to humans but, for many insects, provide a highly visible path to the nectar-containing parts of the flower. Through pollination, plants and pollinators form a mutualistic relationship in which plants provide nourishment and pollinators fertilize the plants.
Some plants rely solely on the wind for pollination. These plants, having no need to attract pollinators, lack special adaptations to petals. Wind-pollinated plants often have nondescript petals or lack petals entirely. They also produce higher amounts of pollen, as much of their pollen scatters in the wind before reaching another flower.