Whenever I remove a pecan tree, I like to save the portion of the trunk that contains the graft union. After drying a few weeks, I slice open the trunk with a band saw hoping to reveal the anatomy of the graft. This winter I got lucky. In the photo above, you can see every step I took in placing a bark graft on this tree.
As you look at the photo, note that the color of the wood is distinctively different between scion and stock. Wood growing from the scion is lighter in color than the wood of the stock. A distinctive color boundary between scion and stock extends across the entire trunk. Even though scion and stock grow together seamlessly, they remain genetically separate and never mix. This is graphically illustrated in the photo by the sliver of light-colored scionwood that dives deep into the stock. This inclusion of scion into the stock is exactly where I inserted the grafting wood under the bark of the stock (even the staples are visible).
I've always been fascinated by the grafting process and how that process is reflected in the grain of the wood. The photo above shows a hall table I built using pecan boards cut to reveal a graft union. The left side of the table is Giles rootstock while the right side is the cultivar, Osage. Note the distinctive brown line that cuts across the table indicating the boundary between scion and stock. In this example, the rootstock wood is lighter in color than the Osage wood grafted on top.