12 PITH-KAY FLECKS IN" WOOD. 



is at first very small and rarefy over 5 mm. even in the roots. The 

 majority of mines are well within the margin of annual rings of 

 growth; they rarely border on the spring or autumn wood of other 

 rings. The larva begins its wanderings some time after growth 

 begins and stops before the end of the growing season. 



Nordlinger states that in certain species the mines are always in 

 close proximity to the pith. This doubtless was correlated with bark 

 characteristics because certain hosts were attacked only during their 

 early existence. The present investigation, however, indicates that 

 pith flecks in the eastern American hosts at least occur throughout 

 the whole transverse section of the basal log. A large specimen of 

 river birch was examined on July 22. The age could not be accu- 

 rately determined, but a conservative estimate would place it at 

 about 110 years. Many mines were observed at the base of the tree 

 in the last season's growth. Other trees ranging from 10 to 80 years 

 were found to be infested with the larvae. In at least one species, 

 river birch, both the young and the old trees are frequently attacked. 



TAXONOMIC VALUE OF PITH FLECKS. 



Early writers on wood ascribed a taxonomic value to pith flecks 

 and efforts were made to use them in separating genera and species. 

 If the distribution in certain genera were constant, the importance of 

 these markings would be readily apparent. Record has pointed out, 

 however, that they are of no value in this connection. The present 

 investigations confirm his observations and considerably enlarge the 

 list of infested woods. Pith flecks are clearly of pathologic origin, 

 and their distribution in genera, species, and individuals is extremely 

 irregular and uncertain. Their presence or absence, therefore, can 

 not be used to identify woods. ^ 



DETERIORATIVE EFFECT ON QUALITY OF WOOD. 



Among the important quality factors of wood are strength, elas- 

 ticity, texture, and freedom from knots and discolorations. Any 

 agent that adversely affects one of these qualities may decrease the 

 commercial value of the wood. Pith flecks may mar the natural 

 beauty of woods by causing discolorations. In some woods pith 

 flecks cause definite areas of disintegration in which normal tissues 

 become involved. Intercellular spaces or cavities may thus arise. 



The discoloration due to the work of dipterous larvae arises from two 

 causes. In all native species (except the willows and poplars) marked 

 with pith flecks (pp. 9 and 10) the cavities of the newly formed wound 

 cells become filled with a dark-brown substance responding to tannin 



1 Logically, of comse, no pathologic condition or other accident could have any taxonomic value. 



