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A large burl on Jeremy's Run Trail in Shenandoah National Park
Common Name
: Burl, Gall, Tumor, Crown Gall, Burr (UK) – From the Latin burra meaning a shaggy cloth; a burl is also defined as a knot in fabric threads. Burly, an adjective for strong and stout, has a separate etymology from the Old English beran, to carry. A burl is nonetheless burly.


Scientific Name: None
The lack of a scientific name for burls is appropriate as their provenance is uncertain. Burl is a generic term for any irregular edema on the trunk or branches of a tree, and is occasionally and not necessarily incorrectly referred to as a gall (a general swelling of plant tissue) or a tumor (a general swelling of animal tissue). Among the many suggested causal factors of a burl, the most common are fungi, bacteria, insects, environmental stressors such as mechanical impact or frost damage, and, as it is quite evidently a large abnormal growth, a carcinogenic genetic mutation. The dearth of arboreal research concerning the nature of burls is a matter of the prioritization of limited forestry funding. As tree burls retain an outer layer of the same bark sheath that protects the rest of the trunk, trees with burls are not prematurely senescent; there is therefore no impetus to prevent them.

Burls are not only benign – they are in some cases, quite desirable; burl growth can result in wood patterns known as ‘figure’
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Tree with two burls on Limberlost Trail
that are quite aesthetic and are hence valued for use in the creation of unique, decorative wood products. The demand for the intrinsic art of nature manifest in wood figure produced by burls has risen dramatically over the last few years; a large burl cross section can be sold for thousands of dollars. According to the New York Times, in April of 2014, the latest of 18 known cases of “burl poaching” occurred in the Redwood National and State Park in northern California; a 1,000 year old redwood was butchered for its burl. The two “midnight burlers” were apprehended and charged with felony grand theft after they had sold their pelf to a wood dealer in Del Norte County several weeks later. Park Rangers reported that thieves in a previous incident had cut down a 400-year old redwood tree to reach its 500 pound burl that was 60 feet above the ground. Trees can have multiple burls that can many feet above the ground.


The interest in tree burls is not solely of North American provenance. Burls are sought after on a global scale for their unique properties; due to their scarcity, there have been numerous attempts to induce trees to produce burls artificially using a variety of creative, though largely specious techniques: burning, grafting and binding. Starting in the 17th Century, wood workers in Algeria induced excrescences on the boles of thuya trees (Tetraclinis articulata), a type of cypress indigenous to the Mediterranean region, by burning one side of the tree; tree swellings occurred on the opposite side due presumably to the burn damage. Thuya “burlwood” is still marketed for laminated flooring. Grafting, the mainstay of arboriculture, and the basis for the citrus industry, was an obvious choice - to simply graft additional branches on burl trees to produce wood stock. This was tried with dubious result on walnut trees in Rockport, Indiana in 1929, and more recently on birch trees in Poznan, Poland and Hørsholm, Denmark. Mechanical inducement of burl by binding was what one might call the direct approach – the syllogistic logic: If you can make a tree bulge, then surely it will be a burl bulge. While these contrivances did create something superficially resembled a burl, there was no appreciable wood figure in the result. It is apparent in the foregoing that the science of tree growth and disease has not yet been applied to burls. Since the growth of a burl is like a tree trunk tumor, Agrobacterium tumefaciens would be a good candidate for burl inducement; this very peculiar and historically important bacterium creates tumors in plants called crown galls.

A crown gall is similar to a burl in that it is a swelling of plant tissue; the two are sometimes confused in the scant literature on the subject. However, unlike the gnarly bark-covered burl, crown galls form either on the roots or on the stem near the ground, a region of the plant that is known by horticulturalists and nurserymen as the crown (hence the name). They are relatively small, soft, cream-colored rounded nodules that turn brown with age. Crown galls can be malignant on many of the approximately 150 species that they infest - a doppelgänger to the beneficent burls. Among the more notable species affected, many are economically important, including trees with pome fruits (apple and pear), trees with stone fruits (cherry and apricot), trees with nuts (walnut and almond), and horticultural ornamentals such as roses and chrysanthemums. And, perhaps most importantly on grape vines. Crown galls are more insidious than they are outright deadly; they kill by debilitation over time with deleterious physical effects including stunting and chlorosis (leaf yellowing). The host plant is weakened and therefore more susceptible to environmental extremes and insect parasitism. Due to these adverse agricultural results, there has been extensive research into the cause of crown galls directed at prevention.

Crown gall damage has been a bane of European viticulture for well over a hundred years. The French entomologist Jean-Henri Fabre first noted plant tumors in 1853, naming them broussin, which is French for burr or burl, adding to the gall-burl synonymity. In the yet to be consolidated German states of the mid-19th Century, names for the condition varied according to state and included Ausschlag (which means rash), Krebs (cancer or tumor) and Kropf (goiter, an enlarged thyroid gland). In Italy, the grapevine gall disease was called tubercoli, Italian for tubercles which are small, rounded protuberances; it was in Italy that its provenance was first determined. In 1897, Fridano Cavara, the future Director of the Royal Botanical Gardens of Naples, discovered that tubercoli was caused by a flagellate rod-shaped bacterium. He published his results in Le Stazioni Sperimentale, Agrari Italiane, naming it Bacillus ampelopsorae. Subsequent research in the United States by Erwin Smith (who traveled to Italy to meet with Cavara in 1907) and his colleague C. O. Townsend isolated a similar bacterium that caused galls on chrysanthemums. After numerous iterations by subsequent researchers, the Cavara bacterium that caused grape vine galls was named Agrobacterium vitis (Latin for vine) and Smith and Townsend’s more ubiquitous bacterium was named Agrobacterium tumefaciens (Latin for swelling). The generic name Agrobacterium was appropriate in intimating a relationship between the domain Bacteria and the domain Eukarya (living things with nucleated cells like plants and animals) – a bacterium that interacted directly with plant cell nuclei, an agrobacterium. This was something quite new to botany.

The discovery of Agrobacteria elicited further research to establish the etiology of the crown gall disease – How did it work? As with all science, it was trial and error with several erroneous hypotheses – for example that A. tumefaciens produced a chemical irritant that induced tumescence. After a detailed assessment of the gall cellular structure revealed that it contained no bacteria, it was posited that a genetic transformation had taken place and that the bacterium had directly induced the mutation of the plant cells through the transfer of DNA. Proof of this hypothesis took years of rigorous laboratory investigation, culminating with the discovery that a small DNA containing molecule called a plasmid produced by A. tumefaciens had directly deposited its DNA into crown gall cells. The meticulous measure necessary is evident in the metrics: 0.0011 percent of DNA in the cells was found to be alien – about one hundred thousandth. The findings were published in 1980 in Proceedings of the National Academy of Sciences; Mary-Dell Chilton from the University of Washington in St. Louis was the lead author. What became obvious at this point was that Agrobacteria provided the mechanism to insert alien genes into extant plant nuclei – the fons et origo of genetically modified organisms, now more commonly referred to by the epithet GMOs.

According to Daniel Charles (the NPR science reporter) in his 2001 book Lords of the Harvest, the seminal event of GMO development took place at the Winter Symposium on Molecular Genetics of Plants and Animals in Miami, Florida in January of 1983. Mary-Dell Chilton presented her laboratory’s progress in modifying A. tumefaciens to replace the tumor-forming DNA with a DNA segment with favorable qualities; the ultimate goal was to induce genetic modifications in plants with the surrogate genes. After another competing group from Belgium presented similar results, Ron Horsch from Monsanto (also, incidentally, in St. Louis) took the podium to announce that their scientists had already successfully grown petunias that were genetically modified using A. tumefaciens as the insertion agent, a major coup. “The scientists had grasped a locked door and swung it open …Any plant that was susceptible to Agrobacterium - tomatoes, potatoes, cotton, squash, poplar trees - suddenly seemed within the reach of genetic engineering.” Although all three scientific groups represented at the conference had essentially found the same key to the penetration of a plant cell nucleus, it was Monsanto that was able to capitalize on the discovery, one of the first instances of the synergy of resources and science in an industrial laboratory. Having come prepared, Monsanto issued a press release to the Wall Street Journal that broadcast the major scientific breakthrough on its front page several days later. The long and winding road to Bt corn (named for Bacillus thuringiensis - a bacteria that is toxic to insect larvae like corn borers) and Roundup ready soybeans (which are genetically protected from Monsanto’s glyphosate herbicide named Roundup) began with nature’s genetic engineer – the crown gall producing Agrobacterium tumefaciens. The Gadarene development of genetically engineered plants that promised to solve world hunger has since run headlong into a brick wall of public backlash, particularly in Europe. The outcome is yet to be determined.

Burls look like tumors because they probably are - a genetically induced mutation that results in uncontrolled growth; a benign cancer. The physiology of tree growth supports this thesis. Only about 1 percent of a trees cells are actually alive – mostly in a thin layer one to two cells thick adjacent to the bark which is known as the cambium. As the cambial cells reproduce inward, they create the xylem (wood) that conducts water and minerals to the photosynthetic leaves; as the cambial cells reproduce outward, they create the phloem that conveys the food from the leaves to the rest of the tree. The random nature of the tumorous burls suggests a single cell provenance – such as a cambium cell infested with Agrobacterium tumefaciens growing ever outward. The hypothesis is then that trees grow in soil that contains the Agrobacteria (which are relatively ubiquitous, as are crown galls) that penetrate various plant cells with some serendipity according to proximity and probably a host of other variables. If the penetrated cell is a root cell, then a crown gall forms. If the penetrated cell is a cambium cell, it becomes a tumorous cambium cell, growing at an unusually rapid (cancerous) rate. Over time, the tumorous cambium cell moves from its initial position at the top of the soil (the so-called crown), the gall growing outward as the tree grows upward.