John R. Porter porter at SHRSYS.HSLC.ORG
Thu Oct 3 21:09:53 EST 1996

On Thu, 3 Oct 1996, John R. Porter wrote:

I have looked at several sources to answer the question about the origin 
of burls.  As a botanist and woodworker, I have more than a casual 
interest in the structures.  Relatively few botany texts, whether for 
theoretical or practical approaches, even broach the subject, but one 
Saigo & Saigo, 1983, Botany, Principles and Applications, Prentice-Hall, 
Englewood Cliffs, NJ, p 155.
   Occasionally growth abnormalities occur, resulting in the formation of 
bulbous growths called burls on the main trunk or branches (ref to Fig on 
pg 156).  Burl formation may be stimulated by injury, disease, or 
invasion by such parasitic plants as mistletoes.  Within a burl, the 
cambium proliferates adventitious buds and abnormally large amounts of 
xylem tissue, which become rippled and convoluted because it has no room 
to grow longitudinally.  Dense, swirled patterns form in the wood, making 
burls popular for decorative woodwork, such as bowls, veneer panels, 
furniture, and pipes."
 Another reference, of a woodworking nature, is Constantine, Albert J., 
Jr. (rev by Harry J. Hobbs), 1975, Know Your Woods, Charles Scribner's 
Sons, NY, p 64:

"Burls are responsible for the formation of a number of interesting 
natural designs.  A burl is an abnormal, wartlike excrescence on the 
trunk or branches of a tree.  Examined closely, it may appear to consist 
of a great mass of 'eyes' or dormant buds.  The surface of such a bulge 
may be smooth or rough.  In either case the alignment of the fibers is 
very irregular and the burl is thus gnarled.  Because of this many 
bizarre figures are derived from veneers cut from burls.  Causes of burls 
are imperfectly understood and the following reasons have been advanced 
to explain this unusual growth:  injury from frost, fire or mechanical 
contact; and irritation of the cambium by bacteria, fungi and possibly 
viruses.  A legend states that wounds inflicted by woodpeckers cause 
burls.  However, this is easily discredited in that trees produce burls 
in regions where woodpeckers do not exist.  Burls can and do grow on all 
species of trees; however, they are found more commonly on certain trees, 
as for example, the redwood (_Sequoia sempervirens_)."
 John R. Porter
 porter at shrsys.hslc.org

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