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



at least as far back as 32 cm. from the tip. In TTM i-i and 1-2, 

 where tip growth was extensive, only slight lignification took place in 

 the xylem either in densewood or lightwood. In TTM 1-3, in contrast, 

 where tip growth was rather slight, lignification of the xylem was 

 much more pronounced, especially in the densewood. 



The 1941 increment in TTC 34-2-c (table 22) contained 2 see plus 

 2 sL plus I inc L. All these were formed prior to May 18 because on 

 that date the cambium was killed by artificial freezing. In relation to 

 their total thicknesses the growth layers possessed heavy bands of 

 densewood. As a matter of fact, these bands on some radii were even 

 heavier than the densewood of the 1940 increment which was a growth 

 layer six times as wide as any of the 1941 intra-annuals. The case of 

 TTC 34-2-c thus presents the problem as to the possible cause of 

 densewood variation, whether due to variation of climate, food supply, 

 or the part of the season when deposited. 



The entire subject of partial growth layers is intimately connected 

 with the place where cambial activity is initiated. Longitudinally there 

 is evidence that diameter growth begins at some point back of the tip 

 and spreads both outward and inward on the branch. It is clear, at 

 least, that growth does not necessarily begin at the tips of the branches 

 and spread uniformly inward and downward. In all likelihood there 

 are multiple foci of growth initiation, something to be expected in the 

 light of growth-layer types and of waves of cambial activity. 



Transversely, evidence shows that the cambium becomes active at 

 one or more foci from which the activity spreads. Two variations 

 stand out when sections are cut or killed so as to interrupt growth at 

 successive stages. At the start, new xylem appears as a few isolated 

 single cells around the circuit or as one or more tiny clusters of cells. 

 The isolated cells eventually are joined by others and thus become 

 lenses. All the evidence of the present study strongly suggests that 

 all growth layers begin as lenses. Therefore, partial growth layers 

 are entire growth layers whose development was arrested or prevented 

 before the entire cambium became active. 



In the sections of TTC 12-io-a, cut April 21, 1940, growth for 

 1940 consists radially of one to eight cells and tangentially of one long 

 and one very short lens (table 19). The inner half of the long lens 

 is strongly lignified. In branches which are eccentric, new growth 

 commonly, but not always, appears first on the long radius or on what 

 will be the long radius for that growth flush. 



Growth for 1940 in TTC i-io-a, cut April 21, 1940, covers the en- 

 tire circuit, 8 to 16 cells thick radially. On the long radius, and covering 

 most of the circuit, the inner three-quarters of the growth layer has 

 matured, whereas on the short radius all cells are immature. 



