6 BULLETIN 909, IT. S. DEPARTMENT OF AGRICULTURE. 



• STRUCTURE. 



Black walnut is classed as a diffuse-porous wood, the pores be- 

 ing scattered throughout the annual-growth ring. These pores are 

 comparatively large in size and easily visible to the naked eye, espe- 

 cially in the spring wood, and they gradually decrease in size toward 

 the outer portion of each annual ring. Tyloses are present but do 

 not completely fill the pores. The large pores of the spring wood 

 sometimes give the appearance of a ring-porous arrangement. 

 Plates I, II, and III show the structure of the wood in cross section 

 at different magnifications. The pith rays are narrow and can not 

 be seen without the aid of a hand lens. The structure of butternut 

 wood closely resembles that of black walnut, but the wood of the 

 former may be distinguished from the wood of the latter by its light 

 chestnut-brown color. 



INSECT AND FUNGUS ATTACK. 



Black-walnut timber is largely free from insect and fungus attack 

 in the tree, log, lumber, and finished product. Thrifty trees are 

 generally sound. If the trees are very limby and are found on poor 

 soils where growth is less vigorous, decay often gains access, through 

 knots, to the interior of the tree trunk. A heart rot sometimes at- 

 tacks the central portion of the trunk at the butt. This is called 

 red butt rot and is most often found in the northeastern section of 

 the area of distribution. It is especially prevalent in trees originat- 

 ing from sprouts, the fungus making entrance from the stump of the 

 original tree as it rots away. Large white grubs also often gain 

 access to the butt, and generally burrow out a small portion only. 

 Injury from these agencies does not usually cause a loss of more than 

 1 to 3 feet of the merchantable length at the butt, although occa- 

 sionally it may extend the entire length of the log. A more serious 

 fungus attack than the red butt rot is a white top rot which is found 

 mostly on the walnut of the southwestern area. This rot often ex- 

 tends over the greater part of the entire merchantable length of the 

 tree, and often renders a log unsuitable for manufacture into either 

 lumber or veneer, on account of the discoloration of the wood caused 

 by the fungus attack. 



Defective trees and logs are often attacked by small larvse which 

 leave what are called u pin holes." Such an attack often causes 

 black streaks in the wood, and this damage unfits it for use on the 

 face of veneered panels. In general, western timber suffers more 

 than eastern timber from such attacks. 



Walnut suffers little injury from fungi or insects either in the 

 lumber or in the finished product. Green lumber or lumber partly 

 dried is subject to fungus attack, but there is little liability of injury 

 if ordinary care is taken. 



