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 the " 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 becoming gnarled 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 by 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 slowiy growmg 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 piirts 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 th.^ plant ; and their excessive production 

 on such older trees as have bee n at one timv.' mercilessly hacked 



