﻿Medullary spots and their cause 



J. G. Grossenbacher 



Introduction 



It is practically an established fact now that the common types 

 of medullary spots or "pith flecks" so frequently found in the 

 wood of trees and shrubs, are due to the mining of insect larvae 

 in the cambium. The reliability of such marks as aids in deter- 

 mining the wood of forest trees therefore depends upon the 

 relative distribution of the different species of cambium miners and 

 the plants they infest. 



The injurious effect of the mining on the health of the host 

 usually depends upon the relation of the mined or eaten area of 

 cambium to the area of the portion that is uninjured. In the case 

 of large trees the effect on the host is usually slight while on a small 

 tree or shrub a very heavy infestation may result in marked stunt- 

 ing of the plant's growth. Some woods may become worthless 

 for finishing purposes owing to the presence of numerous occluded 

 mines. Perhaps the chief pathological interest in this matter lies 

 in following the response of the affected regions of the bark which 

 results in proliferations to occlude the channels left by the foraging 

 miners, and also in the development of new cambium over the 

 outside of the mines. 



This article is an addition to and a continuation of a short 

 paper* written a few years ago, in that more of the details of the 

 host's response as well as the complete life history of the miner 

 discussed in the latter part of the earlier paper (pp. 63-65) are 

 given. It is based on notes and specimens accumulated during 

 the years 1910-12. 



Some additional and more recent literature 



All of the important earlier literature on the botanical side of 



the subject of medullary spots was reviewed in some detail in the 



