NO. I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL. I43 



The series of branches TTP 20-22 to 40 (tables 72-84) was cut 

 off the tree November 11, 1944, and the amount of tip growth for 

 1944 was measured November 14, 1944. On all branches, tip growth 

 had been measured January 30, 1943, February 5, 1944, and Novem- 

 ber 14, 1944. Sections were cut, in all cases, from the measured tip 

 flushes. 



In table 72 the entire growth layer and the lens are represented by 

 one tip flush only. 



Table 72.— TTP 20-22 



5.3 on. 1.8 cm. TF 



1944 I see I see i 



I sL 



Table 73.— TTP 20-23 



2.6 cm. 0.8 cm. TF 



1943 I see I 



1944 2 see 2 see i 



ine 



A wide-field binocular revealed two, possibly three, growth layers 

 on unstained sections of TTP 20-23. Sections at 2.6 cm. (table 73) 

 were cut from the middle of the 1943 tip growth and those at 0.8 

 from the middle of 1944. In 1943, one diameter flush corresponds to 

 one tip flush, whereas in 1944 two diameter flushes plus (at 2.6 cm.) 

 an incomplete growth layer correspond to a single tip flush. The 

 densewood of the outer sharp, complete, entire growth layer is fol- 

 lowed by two to three rows of large-lumened, rather heavily lignified 

 cells which make the outer margin of the xylem indefinite under the 

 cambium and indicate that another growth layer was in process of 

 formation, and was not complete, when growth ceased for the year. 

 Without doubt this "added," or postseasonal, growth would be con- 

 sidered a part of the 1945 increment had the branch been cut off a 

 year or more later. The significance of such mistaken dating of xylem 

 formation is immediately apparent when, for any interpretative pur- 

 pose whatsoever, the amount of xylem formed in a certain year is 

 measured with a high degree of accuracy. Whether the postseasonal 

 growth increases or decreases inward on this branch is unknown ; the 

 significance of the fact is that incomplete growth layers (i.e., lacking 

 densewood) can be formed as the final growth of a season. 



Because of the significance to studies of tree growth, three points 

 will be summarized here, points which are illustrated by a great many 

 of our specimens. 



