The calyx of the lavender flower is an even series of tiny ridges and grooves which shelter the secretor glands which produce the essence. We still have much to learn before we totally understand how these structures, which are like tiny factories for producing essential oils, function.
It appears that the essential oil plays a role of chemical defence, protecting the plant from micro-organisms, and/or a role of attracting pollinating insects !

Clichés au microscope électronique du Laboratoire de Biotechnologies Végétales
Université Jean Monet Saint-Etienne