NO. I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL. 259 



What postseasonal growth may look like, in some cases, after 

 another season has passed is probably illustrated by TTC 34-4-a, cut 

 off November 29, 1941. The entire outer contact of 1937 is irregular 

 and ragged. Collapsed green cells are followed by one or two rows, in 

 most cases, of rather narrow, heavily lignified, thick-walled cells, 

 under the succeeding typical lightwood. It seems probable that some 

 growth was taking place after the regular growth of 1937 had been 

 finished and before low temperature affected the cambium. This type 

 of end to a season is not at all rare. In TTC 34-4-c, 12 cm. outward 

 from a and 50 cm. from the tip, the densewood of 1937 is separated 

 into two bands by one to two rows of wider cells which are immature 

 and crushed. Under low-low power this appears to be an extra growth 

 layer. 



The end of January and the first part of February 1942 experienced 

 two weeks of exceptionally warm weather with effective rains, fol- 

 lowed by several days of freezing weather. Because of the high 

 temperatures seven branches were sectioned February 21, 1942, to 

 ascertain whether or not the cambium had been stimulated into ac- 

 tivity. TTC 33-8, sectioned 33 and 17 cm. from the tip, was definitely 

 not growing in the sense that new cells were being set off. However, 

 the cambium is two to three cells in thickness and full of cytoplasm ; 

 in sections h especially, the cambial initials are prominently nucleated. 



Sections TTL 3-5-a were taken 5.5 cm. from the tip. The outer 

 margins of the xylem are wavy and irregular in part. The cambium, 

 six cells thick, seems to have been active at the time of cutting ; the 

 swollen cells are full of cytoplasm and prominently nucleated. 



In TTM 1-5-a, taken 14 cm. from the tip, the above situation is re- 

 peated. 



In TTP 20-17-a, 13.7 cm. from the tip, the outer margin of 1941, 

 irregular on the whole, contains many immature cell walls. The cam- 

 bium, three to four cells thick, was in excellent condition, its large cells 

 full of cytoplasm. At 2.5 cm. from the tip, in TTP 20-17-c, the situa- 

 tion is the same. 



The cambium in TTCw 1-4-a, 10.5 cm. from the tip, appears to have 

 become active just before cutting. The band of cambial initials is 

 three to four cells thick, each cell rich in cytoplasmic contents. 



On TTP 24-9 the terminal bud had just expanded and lengthened. 

 TTP 24-9-a, 8 cm. from the tip, shows no grov/th added to 1941. 

 Cambial cells are narrow, and conspicuously nucleated only locally. In 

 TTP 24-9-b, 1.5 cm. from the tip, over an arc of about 45°, translu- 

 cent, immature cells of moderate size had been added to the mature 

 xylem of 1941. Here the cambium was in better condition than in a. 



