PITH-EAY FLECKS IX WOOD. 13 



reactions.^ This color serves to differentiate sharply the wound 

 tissue from the normal cells surroim.ding it and renders the closed 

 mines conspicuous in transverse sections (PL III). This is time 

 particularly of light-colored woods, such as bu'ches and maples. In 

 cherry, however, owing to the similar color of the wood, the mines 

 are quite inconspicuous, provided other pathological changes 

 descnbed herein, do not occur. 



In maple (Pi. I. fig. 2) and birch the mines stand out in tangential 

 or radial sections as bro^^Ti spots or lines of varying magnitude, 

 which nm in the direction of the fibers. This renders the wood 

 inferior in quality and useless for the finer grades of cabiQetwork. 

 The defect is emphasized for woods finished in the natural color, 

 siQce filling and varnishing rendei's the spots still more distinct. 

 Pith flecks are, however, frequently seen in all the cheaper grades 

 of furniture. In veneers the injury is a still greater fad or. Badly 

 infested cherry is often useless for this pui^ose. 



Another cause of discoloration is the result of mterference with 

 the normal physiological activities of the tree - where the medullary 

 rays are broken by mining larv^. The wood adjoining the mine for 

 varying distances toward the center darkens prematurely and changes 

 into heartwood. These discolored areas rarely cover more than two 

 annual rings of growth, but in some of the poplar woods they were 

 observed to extend through as many as eight annual rings. In other 

 wood the discoloration, as a rule, is comparatively slight, servmg 

 mainly to render the occluded mines more conspicuous and rarely 

 extending beyond the growth of the season. 



Disintegration of woody tissue due to pith flecks was observed 

 only m the wood of cherry. In various species of this genus there is 

 a breaking down or disintegration of the tissue, accompanied by the 

 usual formation of " cherry gum. '' This process is technically known 

 as ''gummation. ''^ The correlation of pith flecks ^\ith the disinte- 

 gration or liquefaction of normal woody tissues was probably fii'st 

 noted by Wiegand in 1863. He found that in sweet cherry pith 



1 In willow and poplar wood the oceiuded mines are often quite inconspicuous, owing to a lack of coloring 

 matter in the wound tissue. 



2 The medullary rays of the wood continue out through the growing layer into the inner bark. Here 

 they are in direct communication with the sieve tubes of the bast, and serve to conduct food material 

 elaborated in the leaves to the growing layer of the wood. They consist of comparatively tliin-walled 

 cells that form, with the wood parenchyma, the living portion of the xylem. The other wood elements 

 lose their protoplasmic contents soon after their fonnation, after which they perform the function mainly 

 of water and air conduction. The wood parench\-ma , however, lives for a niunber of years intimately 

 associated with the physiological acti\niies of the tree. 



3 Many conflicting opinions are held regarding the origin of cherr^- gum. Mikosch considers it a patho- 

 logical product which follows the wounding of the cambium, and asserts that its formation takes place 

 within the inner portions of the cambial layer. Rant ascribes its formation to three factors: (o) new for- 

 mation of tissue, (b) liquefaction, (c) activity of the dead cells as a wound stunulus through necrosis. The 

 latter may resiilt through physiological influences or through the traumatic working of foreign organisms, 

 such as bacteria, fimgi, or animals. Ruhland further infers that o-xygen is necessary for the process, and 

 that ta wounds where air is excluded no gunoniaLion results. 



