139 
They originate as small structures'for the most part, raising the 
outer surface of the bark in lumps, and in this condition they have 
earned for themselves the name of yie “pea disease.” Arising in 
the cortex they have at first no connexion with the main wood of the 
plant. By their continual growth and fusion with each other large 
structures are produced of irregular shapes and sometimes measure- 
able by feet, the outer surface of the trunk becominggnarled and 
warked. At first they are superficial to the latex layer; by further 
growth, however, they fuse up with the main wood and the inter- 
vening latex and cambium layers are squeezed out of existence. 
Eventually, however, a latex layer is developed superficially to the 
nodules. The fusion of the nodules with the main wood takes place 
at first by small points which grow inwards to the main wood. 
Each nodule possesses a central woody core composed of vessels 
which are for the most part irregularly arranged. Outside the core 
is a cambium, by virtue of which the nodule can increase in size 
independently of the surrounding tissues. Their rate of growth is 
slow, the slow growth resulting in the production of a central mass- 
of wood which is excessively hard. 
The development of large cores resulting from the fusion of a 
large number of separate cores frequently renders tapping impos- 
sible and is, therefore, of some considerable economic importance. ^ 
Such structures are not uncommonly met with on several 
different kinds of trees. The commonest cause of their production 
in forest trees is the wounding of the cortical tissues, such as by the 
grazing of a cart wheel. They frequently develop on the boles of 
forest trees which are exposed to increased illumination b}~ the 
felling of their companions, and in such cases the increased illumi- 
nation of the lower parts is regarded as the cause of their develop- 
ment. In these cases they are essentially “ dormant buds ” which 
commence to develop owing to some artificial stimulus ; but, having 
no connexion with the transpiration current, they are robbed of the 
food necessary for the production of shoots, and the result is the 
accumulation of a slowly growing mass of wood. 
The burrs on Hevea are similar in all respects to these above- 
mentioned structures. They are in their nature and mode of origin 
buds which have failed to develop into shoots. The most convincing 
evidence in favour of this is the abnormal occurrence in which shoots 
can sometimes be produced from such burrs, there being a definite 
organic connexion between the shoot and the core of the burr. 
Repeated examination has failed to show that they are caused 
either by insect or by fungus injury, nor can they be attributed to 
increased illumination on the lower parts of the plant. That they 
are stimulated to growth by wounding appears to be clear enough 
from the fact that they occur far more commonly on the tapping 
area than on other parts of the plant ; and their excessive production 
on £uch older trees as have been at one time mercilessly hacked 
