NO. I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL. I99 



nual increment, and the evidence of injury strongly suggest that this 

 "intra-annual" was the result of the cessation of growth and the death 

 of the growing tip prior to the start of the offset second flush. 



The whole matter of growth slowdown and dual tip flushes, both 

 as concerns gymnosperms and angiosperms, may be summarized in 

 outline form as follows : 



1. Two tip flushes for one diameter flush. 



a. Tip flushes separated by terminal bud scale scars. 



b. Tip flushes represented by sets of bare-needle zones. 



2. One tip flush for two diameter flushes. 



a. One tip flush for two sharp diameter flushes. 



b. One tip flush for two diameter flushes, one of which is wholly or 



partly diffuse, the other sharp. 



3. Two tip flushes for two diameter flushes. 



a. Both diameter flushes sharp. 



b. One diameter flush diffuse, giving a type of transition between i 



and 2. 



It is apparent that growth slowdown or actual cessation in gymno- 

 sperms is recorded initially in tip growth by a second set of bare- 

 needle zones, and in angiosperms by closely set "undeveloped leaves." 

 Subsequently it is also recorded in diameter growth, if the cessation is 

 sufficiently prolonged or intense. 



Secondary xyleni in terminal buds. — The existence of secondary 

 xylem in the bud deserves attention because of its characteristics and 

 because of the error it might introduce in the calculation of the annual 

 amount of xylem found for a particular year. Note of this phenome- 

 non as it affects analyses has been made on page 152. 



Sections TTP 20-3-c came from the inner part of the terminal bud 

 and, because they were cut December 15, 1939, they can show only 

 the concluding growth of 1939. From pith outward the sections gave 

 the following succession : ( i ) Primary xylem, four to six cells thick ; 

 (2) secondary xylem, 16 to 25 cells thick; (3) cambium; (4) 

 phloem; and (5) outer bark. Under high power the outer four or 

 five cells of the xylem, as seen in cross section, show up as somewhat 

 narrower and more lignified than the inner cells. This is not true of 

 all radial rows. Were the zone of four or five cells within a complete, 

 entire growth layer (as it would have been a year later had the branch 

 not been cut off) it would appear as a faint, highly diffuse intra-an- 

 nual, if noticed at all. Thus, xylem formed in 1939 would be an in- 

 tegral, but indistinguishable, part of that formed in 1940. The outer- 

 most two cells, as well as those only partially differentiated at the time 

 of cutting, were filled with protoplasm at the time the wood samples 

 were placed in preserving fluid. The possible relationships of xylem 



