Inside a bark graft

   Bark grafting is one of the important techniques I use to propagate pecan trees. But have you ever wondered how a bark graft scion actually grows together with the rootstock? I cut open a one-year-old bark graft so you can see exactly how the grafting process works (photo at right).

    In the photo, you can clearly recognize the scion on top but you can also see that it was placed between the bark and the wood along the side of the stock. The stock is identifiable by the flat cut surface made before the scion was inserted. Even three of the staples that I used to hold the graft firmly in place are visible. Look how much wood has grown over the staples in just one year.

    Taking a closer look, we can discover how a tree seals over a graft union. Wood tissue from the scion never actually joins with the wood tissue of the stock.  Note how callus tissue from the scion is rolling over the cut surface of the stock but the wood never actually joins together. The long brown line that marks the scion and stock boundary is another location where wood tissue meets wood tissue and the two never fuse. 
     It is only where cambium tissue from the scion meets cambium from the stock that the tree can grown new wood and bark to cover over the wounds created by grafting. At first, most of the new wood and bark created by the fusion of cambium tissues is located outside the scion-stock boundary. But as the stock and scion continue to grow, new wood is created to cover the cut surface of the stock.
    This photo also illustrates how trees work to block the spread of wood decay that results from tree wounding. On the right side of the tree, a new layer of wood has grown since the graft was made. Note the faint brown line that marks a boundary between this new layer and the discolored wood associated with the pruning wound. Trees develop a specialized boundary layer (the brown line) to wall off decay organisms from reaching new wood tissues.
     A photo of the inside of a much older bark graft can been seen here.