98 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I40 



postseasonal growth of 1939 throw hght on the conditions during the 

 formation of 1937 and 1938 growth layers. In sections & of TTC 

 30-2, 46,5 cm. from the tip, the evidence for postseasonal growth is 

 less accentuated. In sections c, 32.5 cm. from the tip, and sections d, 

 18.5 cm. from the tip, the 1939 increment is composed of xylem ma- 

 ture outward to its sharp margin. 



The third type of postseasonal growth does not show a failure to 

 complete a growth layer in the season to which it rightly belongs, but 

 rather a transition outward from the outer densewood into cells which 

 can only be classed as lightwood. These cells are, in the main, larger, 

 thinner walled, and less heavily lignified than the cells immediately to 

 their interior. If this thin zone of transition were examined some 

 years later, rather than when examined originally, the examiner 

 would be in a quandary not only as to the actual identity of the annual 

 increment but also as to the exact boundary between 1939 and 1940. 



Con T 1-18 (sections a, 4.6 cm. from tip, and sections h, 0.2 cm. 

 from tip) was sectioned January 16, 1943. For a short distance along 

 the outer margin of 1942, the outer three or four rows of cells become 

 larger and thinner walled, perhaps correlating in part with a second 

 tip flush. 



TTC 5-9-a, cut November 9, 1940, 69 cm. from the tip, bears a 

 slight hint of added immature cells ; sections h, 34 cm. from the tip, 

 have no added xylem. On February i, 1941, a neighboring branch, 

 TTC 5-10, was cut and sectioned. It shows no postseasonal growth. 

 Other branches, cut at 6-week intervals, did not permit identification 

 of the prior postseasonal growth. If once present, it had been in- 

 corporated into the next year's growth. TTP 20-1 reverses the se- 

 quence of TTC 5-9 as regards change along the branch. Sections a 

 of TTP 20-1, cut November 26, 1939, 37 cm. from the tip, have one 

 sharp, complete, entire growth layer for 1939, whereas sections h, 5 

 cm. from the tip, have one sharp, complete, entire growth layer plus 

 one diffuse lens and postseasonal growth whose cells were larger than 

 the normal densewood to the interior. 



Many specimens of XSC show excellent postseasonal growth but 

 they are not cited as examples because all of them were subjected to 

 artificial freezing and hence may have had their normal course of 

 growth interrupted. 



The increment for 1939, in TTC 30-1 (pi. 11, fig. 3), cut November 

 4, 1939, consists of one incomplete growth layer. Apparently growth 

 was in progress on the date of cutting. In the zone of compression 

 wood, no narrowing of cells is detectable ; away from the compression 

 wood, some cells have become narrower but they are followed outward 



