A skeletonised leaf of common ivy, Hedera helix, overlaying an intact leaf.The picture shows how the vascular system branches repeatedly from the main mid-rib of the leaf down to small vessels; a pattern typical of dicotyledonous plants.Broadly, a leaf as three tissue types. The epidermis is a waterproof covering with pores on its underside called stomata. Remaining fragments of epidermis on the skeleton leaf are visible to lower left in the picture. The interior tissue (green) of the intact leaf in the background consists of mesophyll and parenchyma. The vascular system supplies water and minerals to the leaf, via cells with thickened walls containing lignin, called xylem. Photosynthetic products of the leaf (sugars) are conducted to the roots by vascular cells called phloem. The dense cell wall of the vascular system means that it is more resistant to decay than the rest of the leaf