6 BULLETIN 909, U. S. DEPABTMEZST OF AGBICULTTTBE, 
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 town*! 
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 tree- 
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 c. 
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 great t. 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 larva? which 
leave what are called u pin holes/* Such an attack often ; 
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. 
