Tomato stem. Coloured scanning electron micrograph (SEM) of the stem of a tomato plant (Solanum 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, such as the one seen at the start of the clip. The hairs help to protect the plant from insects and other pests.