A skeletonised leaf of common ivy, Hedera helix, showing the pattern of vascular tissues. These tissues appear as the "veins" of an intact leaf. The pattern of r branching into a mosaic of thinner strands of cells is typical of dicotyledons. Broadly, a leaf has three tissue types. The epidermis is a waterproof covering with pores called stomata on its underside. Remains of the epidermis are visible to the left edge of the picture. The interior of a leaf consists of green tissues - mesophyll and parenchyma. These are absent in this picture. 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 presence of the dense cell wall of the vascular system means that it is more resistant to decay than other tissues.