Leaf midrib. Coloured scanning electron micrograph (SEM) of a section through the midrib of a leaf from the Common Box (Buxus sempervirens). The midrib (midvein) is the continuation of a leaf's stem along the centre of the leaf. At lower centre is a vascular bundle, which consists of xylem (dark brown) and phloem (orange within brown) tissues. Xylem transports water and mineral nutrients from the roots throughout the plant and phloem transports carbohydrate and hormones around the plant. The surface (epidermis) of the leaf is covered in a waxy cuticle (top) that helps to prevent water loss. Magnification: x200 when printed 10 centimetres wide.