A skeletonised leaf of common ivy, Hedera helix, showing vascular tissue. The picture shows the vascular system branching repeatedly from the main mid-rib of the leaf down to small vessels; a pattern typical of dicotyledons. The green background is artificial. Broadly, a leaf has three tissue types. The epidermis is a waterproof covering equipped with pores called stomata. Remaining fragments are visible to top, and top left. The interior of the leaf consists of green tissues - mesophyll and parenchyma. These are absent here. 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 makes it more resistant to decay than the rest of the leaf