Tomato stem. Coloured scanning electron micrograph (SEM) of the stem of a tomato plant (Lycopersicon lycopersicum). The surface layer consists of epidermal cells that have thickened outer walls and secrete a waxy coating, or cuticle, which prevents water loss. Gases are exchanged through openings, or stomata, in the epidermal layer (raised openings, lower left corner for example). The glandular hairs, or trichomes, (vertical spines) bear groups of cells (brown spheres) that secrete oils which are irritating to predatory insects.