How weird, my post to this thread has disappeared.

Basically, I had said that it's physiological and genetic, depending on how you use the words. When plants are grafted together, the cambium and phloem cells have to join at the graft. The cells along the graft will even exchange some genetic material. Even plants from different genera have been grafted successfully, but that's much rarer than intraspecific and interspecific grafting.

If I were going to ask a similar question at school, I'd rephrase it something like, "Can you explain the specifics of what determines which species and genera can be grafted together successfully?" While you're at it, you might ask them to cover what makes some grafts fail after an initial, seemingly successful period. If you ask, come back and tell us what they said, so we can discuss their input.

brandon7, your post did not disappear, rather I asked again on a different forum.

I had a citrus grafting class at ASU in 1967 and they may have answered my question and in the short time that has elapsed I just forgot.

I found a University of Missouri publication that offers descriptions in terms of species and genus and family but not a word about anatomy or physiology.

OOOOOOOh. I was wondering if I had lost it (even more so than normal). LOL

There are multiple scientific papers on the subject and some can be found on Google. Unfortunately, many may require paying (boy I hate those pay-to-read sites).

I posted some similar question yesterday just in case it can be useful for your list of compatible plants.