dicot stems with primary growth have pith in the center, with vascular bundles forming a distinct ring visible when the stem is viewed in cross section. the outside of the stem is covered with an epidermis, which is covered by a waterproof cuticle. the epidermis also may contain stomata for gas exchange and multicellular stem hairs called trichomes. a cortex consisting of hypodermis (collenchyma cells) and endodermis (starch containing cells) is present above the pericycle and vascular bundles.