Subject: Why can't any plant graft onto any other plant?

Why can't any plant graft onto any other plant? Plants seem to have a
"preference" as to the taxonomic groups of plants they like to graft onto.
Apples seem to like cherries,plums and fruit trees like that,but I have yet to
see an apple grafted to an oak tree. I've never seen an oak grafted to anything,
not even another oak. Shouldn't it be possible to graft an walnut to  
a hickory nut tree? I suppose the grafts apple/oak,Black oak/Pin oak,and 
walnut/hickory are possible,but are some species combination grafts impossible?
  why? what about oak/pine? I know there is a vascular thing involved,and those
channels need to be lined up, but aren't most of these these trees able grow so
they get that straight? Does grafting incompatibility have a chemical cause? Is
it because of one of those chemicals which no one has discovered? Does any know
why trees are selective in what kind of tree at they will graft to? Why are
fruit trees really good at grafting? Please answer. Thank you.           
 
                     

Re: Why can't any plant graft onto any other plant?