14 
BULLETIN 1064, U. S.. DEPARTMENT OF AGRICULTURE. 
less than that observed at lower levels earlier in the year. The num- 
ber of resin passages per unit area 23 also was often smaller at the 
end of the season, but was throughout larger than that in the round 
timber. In 1916 this material was taken wherever a good chip 
chanced to be cut along the streak. 
In 1917 care was taken to cut the specimens midway between the 
peak and the corner in all cases. By comparing figures 1 and 2, 
it is apparent that marked variations occurred, nevertheless, in the 
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Mar. Apr Moy June Ju/y Auq 5epr. Oct Nov. Mar Apr May Jvn<3 July Auq. 5epr Oct Nov. 
Fig. 2. — Standard trees, 1917. 
Number of tracheids, observed March to 
November ; in 1917, growth, ring. ° Sum- 
ner wood present. 
Number of resin centers per unit area 
(an arbitrary tangential extent; diameter 
of microscopic field by the width of the 
annual ring observed). Observed March to 
November, 1917 ; in 1915, 1916, and 1917, 
growth rings. 
1917 material. Consequently these variations may be considered 
as characteristic of the annual ring, whether studied in the same 
or in different parts of its circumference and at different heights 
in the tree. During 1917, furthermore, many more resin passages 
than were normally found were produced, but at the levels where 
3 An arbitrary unit made up of the portion of the annual ring in question which could 
be included in a microscopic field of a given magnification, tbe diameter of which was 
moved so as to include the approximate rectangle bounded by the beginning and end of 
the annual ring. 
