3 86 



the bark by an adventitious cambium which has no connection with 

 the main cambium of the stem. In the case of large growths, the 

 latex probably passes down the inner layer of bark, while the outer 

 bark dries up and thus acquires a dull claret colour. The layer of 

 latex between the bark and the main trunk may be due to an internal 

 rupture resulting from the tensions induced by unequal growth. Simi- 

 lar growths occur normally in Beech trees, as a result of wounds in 

 apple and pear, from insect attacks in oak, and from fungus attacks 

 in eucalyptus. In Hevea, they are the result of excessive irritation 

 caused by careless tapping. It has been suggested that the inser- 

 tion of the cup at the base of the V is responsible for this irritation, 

 but the position of the knobs negatives the idea. 



In a second instance the specimens submitted consisted of 

 lengths of the stem of Hevea, 3 to 4 years old, and about 4 inches 

 in diameter, grown at an elevation of 2,500 feet. The normally 

 smooth bark was interrupted by gnarled patches, circular or irregu- 

 larly shaped, varying from one-half to three inches in diameter, 

 with a margin formed by the slightly upturned edge of the normal 

 bark. The smaller patches resemble branch scars', but their num- 

 ber, as many as thirty-six on a length of one foot, showed that this 

 was not correct. On removing the bark, the wood immediately be- 

 neath was found to be swollen and sometimes raised in small lumps 

 and thorns, but there was no sign of any fungus. After chiselling 

 out the sound wood to a depth of half an inch, thin black patches, 

 about an inch in diameter, were disclosed, and a microscopic exam- 

 ination showed that these consisted of dead elements of wood 

 and bark thus forming a separation layer between the wood on 

 either face. It is evident that these patches represent injuries 

 incurred by the tree some nine months previously which have been 

 overgrown by a new layer of wood and bark. The important point, 

 however, is that the extra effort required to accomplish this has not 

 ceased when the gap was closed, but has continued to produce 

 more than the normal quantity of material, thus making the wood 

 at that point half as thick again as the wood produced in the same 

 time in places where the stem has not been injured. The pressure 

 exerted by this additional material enlarges the external "canker" 

 and prevents the formation of a smooth bark. 



Bark attacked by the true Hevea canker is, as a rule, unbroken 

 or only slightly cracked, dark on the exterior, a dirty claret colour 

 internally, friable, and full of moisture. Gnarled bark is usually 

 merely the expression of extreme irritability. 



In the case of "knots" forming knobs on the trunk, it may be 

 possible to secure a smoother surface by cutting them out together 

 with their enveloping cabium, but care must be taken not to remove 

 the inner bark, and the operation should be performed when the 

 knots are small. No method of treatment will remove the cause 

 of the irritation in the second case referred to ; this roughness, 

 however, will not be such an obstacle to tapping as the large swel- 

 lings produced by the " knots. " 



