5i ELEVENTH REPORT. 



ORIGIN OF OAK. BURLS, 



In October; 1908, an observation was made near Ann Arbor that seemed 

 to show at least one wa}' in which the socalled oak burls or knots may 

 originate. These knots are common on the white oak, and may be found 

 up to a foot or more in diameter, standing out prominently on the trunk 

 or branch, in a form often approximately hemispherical. The form of knot 

 meant is not the one which shows an open wound or ''canker," nor such a 

 knot as sometimes forms around the base of a dead branch or stub, but such 

 as show a generally smooth unbroken surface, produced by an hypertrophy 

 of the cambium, long continued on a limited area on the trunk. 



The observation here recorded indicated that the primary cause of such 

 hypertrophy may l^e an injury produced by birds pecking holes into the tree 

 ,trunk deep enough to penetrate the cambium. The wound callus formed 

 in a series of such holes, instead of covering the wound smoothly, began to 

 bulge outward on the trunk and a large number of these holes near together 

 had become the starting point for one of the knots or burls. The old bark 

 still covering the newly formed callus showed that each hole pecked by a 

 bird became the center of a disturbance of growth. The proximity of these 

 to each other and their large numl^er formed the basis of a knot which would 

 be of a large size from the beginning, on a tree 16 to 18 inches in diameter. 



The question arises whether the birds were not boring for insects, and 

 whether the insects may not have been the cause of the growth disturbance. 

 The regularity in the arrangement of the holes seemed to be against this 

 view\ Again, it might be asked does the stimulus of the single mechanical 

 injury ]5roduce a disturbance of growth which continues for an indefinite 

 numl)er of years, or does the injury produced b}' the bird offer an opportunity 

 for the entrance of some plant or animal parasite whose presence acts as a 

 continual stimulus to hypertroj^hy? The observations do not give an answer 

 to this, but the latter alternative seems the more j^robable. Bacteria are 

 known to induce the formation of knots liy hyi^ertrophy of the camlDium 

 in several kinds of trees, but even if that is the case here it is the bird that 

 gives the primary injury. 



University of Michigan, April 15, 1909. 



