3^6 
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 rungus 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 Hy 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 seme nine month, 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. ” 
