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, 

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Mar Apr Moy Jtsna Jufy Aua 5epr. Oct Nov. Mar Apr MoyJunaJuly Auq. Sept Oct Nov. 



Number of tracheids, observed March to 

 November ; in 1917, growth, ring. ° Sum- 

 mer wood present. 



Fig. 2. — Standard trees, 1917. 



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 



B 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. 



