24 
BULLETIN 1064, U. S. DEPARTMENT OF AGRICULTURE. 
more, to be the best for wood formation, as was shown by the com- 
parative observations made on material showing the 1915, 1916, and 
1917 growth rings. The maximum number of resin passages per 
unit area found in 1917 was not present even at the end of the 1917 
season (fig. 7). 
Wood formation under narrow chipping. — The width of the an- 
nual ring, the time when wood formation begins, and the amount 
and density of the summer wood, appear to serve as very good cri- 
teria of the extent to which the tree is affected by turpentining. 
A very severe chipping was generally found to be followed by a 
delay in wood formation and by a very marked reduction in both 
ring width and percentage of summer wood. 
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Fig. 7. — Number of resin centers, 1917, based on averages of five trees per crop, as observed 
monthly. S, standard ; X, narrow (top light line is the average of four normal narrow 
trees with one dying tree left out) ; D, double. 
The wood structure observed in the specimens from the narrow- 
chipped tract at Columbia, Miss., as shown in Tables 2, 3, and 1, ap- 
proached most closely the wood formation of the round or untur- 
pentined timber from that section. The width and character of the 
summer wood were also much nearer normal than in the specimens 
cut from the standard and double tracts. Hence it might be said that 
wood formation in general suffered little, or sometimes not at all, 
from this method of turpentining. In other words, the vitality of 
the timber and its capacity to respond were less reduced by narrow 
chipping than by either the standard or the double method. The 
narrow-chipped trees in many cases (PI. V, figs. 3 and 4) showed more 
wood formation in 1917 than in 1916, in spite of the fact that 1917, 
