PITH-RAY FLECKS IN WOOD. 



PREVIOUS INVESTIGATIONS. 



Pith-ray flecks, or medullarY spots, often render lumber unfit for 

 certain uses. They are cpnimon in many of our native woods (PL I, 

 fig. 1), appearing in transverse sections of logs or lumber as crescent- 

 shaped, discolored areas, and in tangential, or bastard cut. boards as 

 brown streaks running usually in a vertical direction (PL I, fig. 2). 

 The pith rays, passing outward toward the cortex, end bhndly in the 

 discolored areas, and it is comparatively easy ^viih a microscope to 

 detect the connection of the two. 



The observance of pith flecks appears to be coincident -s^-ith the 

 early systematic study of wood. Tlieodore Hartig, noting them as 

 earh' as 1840 in Corylus, Betula, and Alnus, designated them '^Zell- 

 gange," or cellular passages. He described them as similar to the 

 passages of barkbeetles, extending down into the roots for 5 feet or 

 more and up into the stem. The cells, he says, are rich in starch, and 

 contain brown masses of an unknown substance insoluble in alcohol 

 or water. 



Ratzburg found similar markings in wood which he received from 

 Russia in 1852. He suspected their pathologic origin and named them 

 ^^Braunketten:" that is, '^ brown chains of cells." Xordhnger also 

 called attention to them in his ' ' Querschnitt von hundert Holzarten,'^ 

 but applied to them the name of "Markfleckchen." In spite of the 

 generally accepted behef in the pathologic origin of these markings, 

 attempts were made by some ^\Titers to use them to identify species. 

 Kienitz pubhshed a key of the most important German woods, in 

 which he endeavored to separate some species of Prunus and Salix 

 by the presence of pith flecks. 



Kienitz, in 1883, submitted the fu'st convincing proofs of the 

 pathologic origin of pith flecks. He found that they were due to the 

 work of the larvse of dipterous insects ^ h^4ng in the growing cambium 

 and eating passages in this layer while in search of food. According 

 to his observations the insect passes the ^^'inter in the pupal state in 

 the ground. The adult forms appear in the spring and deposit their 



1 These insects consist probably of one or more species which as yet have not been classified and 

 named. 



74312°— Cir. 215—13 n 



