The tissue in the center of the stem (the xylem, colored blue) conducts water and nutrients from the roots to the top of the plant. The tissue of the outer ring (the phloem, colored green) nourishes the roots with sugars and hormones from the leaves. The middle layer (the cambium, colored brown) allows the stem to increase in diameter and makes new xylem and phloem tissue. All three of these layers must match up when grafting the scion to the rootstock in order to make a successful graft.

This is the scary part of grafting tomatoes. Remember that what we’re handling here is tiny tomato plants with stem diameters about the same as a matchstick.

The illustration and caption are from the University of Connecticut Cooperative Extension site which you can see here.