re: Pistachio Trivet

Generally true, George, but the most important thing is that the root stock has to be adapted to the soil conditions.  In that case they may use the closest related species that will grow in the those conditions and they often find a tree native tot he area.  Most peach varieties for example need relatively acid soil so if you want to grow them where you have alkaline soil, you may have to chose a root stock from another native tree adapted to the local soil conditions.   It also needs to grow in diameter at a similar rate.  I had a white cultivar of redbud tree that was grafted onto native redbud root stock.  The tree ultimately died (<10 years) because the root stock grew much faster than than the graft.  When I cut it down, I cut the trunk in half and I could see where the cambium layer had pretty much severed at the graft.  The base was nearly twice as large below the graft as it was above.   I know that the root stock was a redbud because it sprouted and I now have true redbud tree growing there.  

The reason I asked is because the grain of the wood is so different above and below the graft it made me wonder what they used.