62 BULLETIN 1380, U. S. DEPARTMENT OF AGRICULTURE 
The structure of these callous nodules is very irregular. When 
they are sectioned numerous cavities are exposed, representing the 
overlapping of successive layers of wound wood (PI. XIV, C). In 
many cases the callus never successfully closes the wound, but con- 
tinues to develop in an irregular manner. Its wood may become ex- 
posed at the edges through the sloughing away of the bark, so that 
growth ceases in a. tangential direction. In such cases the wood of 
the cut may be permanently exposed (PL XIV, C). 
The formation of these callous nodules in combination with those 
arising from other causes later described results in such a gnarled 
and irregular surface that the tree is ruined. The condition is ap- 
parent in the accompanying illustrations (PL XXXI, A and B). 
Although practically every tree that had been tapped for any length 
of time had become a mass of woody warts, occasionally a tree which 
had undergone the same treatment was unusually smooth. The 
cuts had healed smoothly and evenly with the surface. If there 
is a probability that there are strains of Hevea that do not respond 
in the usual way to this form of tapping, they should be considered 
in selection for propagation purposes. 
The overhanging dead bark on the upper side of the cuts may 
sometimes greatly interfere with the formation of the callus, for 
the reason that the bark is pushed off with difficulty, and irregular 
developments result. Ordinarily the occlusion callus should proceed 
more rapidly from the ends of the cut than from above or below, 
owing to the narrowness of the cut. This is actually what takes place 
where a series of cuts has been made one above another. There is 
a very obvious reason for this, which is explained when the reac- 
tion that takes place in the last cut above is described. Here the 
downward flow of elaborated food substances is uninterrupted by 
intervening cuts and is conveyed directly to the upper edge of the 
wound. The reaction is very similar to that when a wire is bound 
tightly around a tree or when the cortex % removed in a strip 
around the trunk. In the case in hand the influx of food substances 
frequently causes the development of flat wood plates that project 
downward from the tipper edge of the cut. These plates are more 
or less circular in outline and seldom more than an inch in thickness 
and are slightly appressed to the wound. These plates are attached 
only at the upper side and consequently do not heal the wound 
(PL XXX, A). Since the bark may be loosened from the wood for 
an inch or more above the cut, the callus originates at the point 
where the wood and bark are in union. This results in long thin 
pendent plates. Callous formations of this type have also been ob- 
served in Sapium. 
Like many abnormalities, when once initiated the growth of these 
callous warts is not arrested with the cessation of the original cause. 
Many old trees which had not been tapped for 20 years or more 
were still gnarled and knotted, with an abnormal development of 
the tapped area quite out of proportion to the untapped part of the 
tree. A return to normal surface conditions, however, would depend 
on the age of the tree. If tapping is discontinued on ypung trees 
soon after they are of tappable size, the wounds, if not infected by 
fungi, heal very quickly, and all trace of the injury is buried under 
a new growth. This condition has not only been pointed out by 
