Leaf midrib, coloured scanning electron micrograph (SEM). This is a cross-section through the midrib, the continuation of a leaf's stem along the centre of the leaf. The main vein (midvein) runs through the centre of the midrib. The layers surrounding the central midvein are the large mesophyll cells, and the thin outer layer of epidermal cells. Here, the midvein structure has the larger xylem tubes (which carry water) forming a central bundle that is surrounded by a ring of smaller phloem tubes (carrying nutrients). The tomato plant is an example of this amphicribral vascular anatomy. Magnification: x100 when printed 10cm high.