Well, this is a very useful thread. I’ve always wondered about this issue and now I think we have the answer.
But it does make me wonder just how thick (not very!) those two cells of cambium make the layer which has now been identified as a grafter’s pay dirt. Would you have to use a microscope to see the individual cells, or would a good jeweler’s loupe do it? I imagine somebody knows how to stain the cells to make them stand out better. That would be interesting, and might even be useful in practice.