Well, from experience, I can tell you that only a small number of trees, considering the probable hundreds of thousands, if not millions of tree species growing all around the world, only a small percentage will grow a true burl.
Curtis have already explained that not all "bulges" in trees are burls, and if you are going to ask me, what is in fact a true burl, this same subject is been discussed and expended a few times previously, a quick search on the subject will give you plenty of info.
I can also tell you that, in within tree species that are seen producing burls, not all trees from that species will produce burls, either. A very healthy tree doesn't produce burls, as burs are a indication of a tree disease, deformity, damage natural or man made...! Burls are mostly tree "abscesses" that form from a local deformation. The "puss" that we know in a human/animal abscess, is mostly in liquid form, when it becomes more solid is is normally know as cysts or tumours.
Trees, will produce large amounts of "zap" and other natural substances to surround the problem area and will a healing process that works in a half circular way, most times, the best way to explain it is, if you would try to fill a balloon with different colours smoke, the smoke will follow the balloon side walls, while pushed by gravity, and when reaches the balloon's bottom, it starts to come up through its centre and "curl" down again, right and left of the balloon, forming in many small burls a mirror pattern image.
There are many possible ways the burls are processed by the tree, as there are many reasons that can make a tree develop a burl, but those that have had the opportunity to slice a full size burl, they will see this curl effect and very identical on both sides of the burl...!
It can get very technical, believe me but, I'm only trying to give a simple to understand scenario and burl formation/growth in very uncomplicated terms, I hope that I have some how succeeded...!
:wink:
Cheers
George
