NO. I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL, 187 



The inner portion of a bare-needle zone bears no needles whereas 

 the outer portion does. One ordinarily assumes that a bare-needle 

 zone constitutes a tip flush, set off inward and outward by terminal 

 bud scale scars. This, however, does not always follow because two 

 sets of bare-needle zones between terminal bud scale scars are rather 

 common. 



TTP 20-15 was observed almost daily and was measured weekly 

 from October ii, 1941, to December 31, 1942. By April 4, 1942, the 

 terminal bud had increased in length to 4.0 cm. ; by April 1 1 the new 

 growth had lengthened to 4.3 cm. ; and by April 19 to 6.3 cm., which, 

 excluding the terminal bud, comprised the season's tip growth. Little 

 growth was noted between April 4 and 11 whereas a 50-percent in- 

 crease in length occurred between April 11 and 19. This great increase 

 formed the second set of bare-needle zones for the season. The dia- 

 gram, text figure 42, shows the relationship when the branch was cut 



L6 

 T B^ N^ /b"- n"- IB 



2.lc/n 1. 7cm 0.9cm K5 



Cm. 



Fig. 42. — Tip growth for 1942 on TTP 20-15. T, terminal bud scale scars at 

 end of 1941. Bi and B^, bare spaces; N^ and N^, spaces bearing needles. TB, 

 terminal bud ; and LB, lateral bud. 



off December 31, 1942. No terminal bud scale scars separate N^ and 

 B- ; however, B^ had a slightly constricted appearance, especially at the 

 start, and could easily be mistaken for the beginning of a year's 

 growth. In spite of appearances, the growth for 1942 contains but 

 one tip flush. 



Cross sections in TTP 20-15 were taken from 1941 growth, from 

 zone B^ (text fig. 42), from zone N^, and from N-. The two sec- 

 tions from 1941 and those from B^ and N^ showed i see for 1942. 

 Scattered narrow cells within the body of the 1942 increment could 

 in no sense be interpreted as continuous. It is not surprising to find a 

 lack of distinction in the xylem between a bare zone and its following 

 needle zone, but it is somewhat surprising that the two sets of bare- 

 needle zones are not distinct in the xylem. This could mean, of course, 

 that tip growth did cease for a very brief time, so brief, in fact, that 

 densewood failed to form in the xylem. 



In contrast with the preceding, TTP 20-20 was cut from the tree on 

 April 25, 1942, by which time two sets of bare-needle zones had been 

 formed, the first (y."] cm. long and the second 3.3 cm. No terminal bud 

 scale scars separated the two sets. Sections a from the inner bare zone 



