Scanning electron micrograph of the upper surface of a petal of a hybrid garden rose. The picture shows closely packed epidermal cells in the form of short blunt papillae, 20-30 microns wide. At the apex of each, the outer cell wall has folded to produce a series of narrow radiating ridges (yellow). The function of petals is to capture the attention of pollinators, whether invertebrates or birds. The sap of the epidermal cells contains pigments, and may also develop chemical fragrances. The fine "nanoridges" - less than an micron wide - at the apex of each papilla scatter light; this has the effect of producing a brighter and more shining visual effect. They may also provide a secure foothold for a visiting insect, but their true biological purpose remains uncertain. Developmentally, petals are modified sterile stamens. Sepals, enclosing the unopened flower bud, are modified leaves.