The Great Florida Outdoors: Galls

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Dr. Robert Norman, Clinical Professor, Dermatology, Nova Southeastern University

What do these pictures show?
a. Callus
a. Tumor
b. Burl
c. Gall
d. All of the above

Answer = All of the above

These pictures show galls—the wood and bark that grow over a wound in the trunk or branch of a tree, also called burl, callus, or tumor. A gall is an outgrowth of greatly modified woody tissue that appears on tree branches or stems in response to irritation by an alien organism, often bacteria, fungi, or insects. A gall ranges from spindle-shaped to globose with a rough surface, either vertically or horizontally ridged and covered with small knobs of tissue. On large tree trunks, galls can reach a diameter two to three times that of the tree at the point of occurrence. A gall is a product of excessive division and enlargement of cells from abnormal cambial activity stimulated by bacteria or fungi and the wood is characterized by wildly contorted grain. Galls often contain small knots with pith centers, ingrown bark, and concentrations of stain.

In 2012, Kevin Smith, a plant physiologist at the U.S. Forest Service’s Northern Research Station, noted in an article that, “The crown gall bacterium is responsible for many burls. That common bacterium is especially interesting because it carries within it a little extra DNA, called a plasmid, which infiltrates the tree’s genetics. The plasmid prompts the tree to make special amino acids and growth regulators to produce the burl, which apparently is the preferred habitat for the bacterium. The bacterium that started the process can be long gone by the time the burl is of any size.”

“Burls are occasionally associated with dormant buds, but even that does not explain why they get ‘turned on’ here and not there. Burls don’t seem to do much harm to the tree or shorten its life,” said Smith. “The xylem, twisted and contorted though it is, still seems to do its job of transporting water and nutrients. The vessels are still working and there’s still starch storage in the healthy outer parts of the burl, so it’s capable of normal function, though I’m sure that function is diminished.”

According to the website for the Florida School of Woodwork, the burl grain variation is twisted and interlocked, causing it to chip easily. This “wild grain” makes burl wood extremely dense and resistant to splitting. These properties make it valued for bowls, mallets, mauls, and handles of hammering chisels and driving wooden pegs and each one is unique in its shape, pattern, color, and texture. It is the job of the turner to bring this natural phenomenon to the full potential of bringing out the beauty in a unique and challenging piece of burl by turning it and texturing, carving, and adding to the natural surface with color enhancements.

Where can you find these changes on the trees?

Get out and look for them in the Great Florida Outdoors!

Dr. Norman is an advanced master naturalist graduate of the FMNP program from UF and a board-certified dermatologist based in Tampa and Riverview. He can be reached at 813-880-7546.

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