Vividly coloured poppy flowers due to dense pigmentation and strong scattering in thin petals.

The flowers of poppies (Papaveraceae) exhibit bright colours, despite their thin and floppy petals. We investigated the optical properties of flowers of Papaver rhoeas, P. dubium, Meconopsis cambrica and Argemone polyanthemos using a combined approach of anatomy, spectrophotometry and optical modelling. The petals of Papaver flowers are composed of only three cell layers, an upper and lower epidermal layer, which are densely filled with pigment, and an unpigmented mesophyll layer. Dense pigmentation together with strong scattering structures, composed of serpentine cell walls and air cavities, cause the striking poppy colours. We discuss how various aspects of the optical signal contribute to the flower's visibility to pollinators.