NO. I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL. 6^ 



distinguished than in sections a. However, in TTP 24-3-c, 5.5 cm. 

 outward from 24-3-b, our analyses of 1939 say: "An excellent ex- 

 ample of the annual type of divided densewood.*^ The densewood of 

 1939 lies immediately against the densewood of 1938 over most of the 

 circuit; where it does not, the insertion of incipient (only partially 

 developed) light wood cells makes 1939 visible under low power for 

 half this latter extent, the other half of this small part of the circuit 

 being visible under high power only." The increment for 1939 pos- 

 sesses characteristics which make its identification quite impossible 

 except on stained sections under high power. All this would be in the 

 nature of assumption were it not for methods of absolute dating and a 

 coordinated series of slides. For instance, the visible "lenses" of 1939 

 bear injury by late spring frosts near their interiors. In TTP 24-3-d, 

 10 cm. outward from 24-3-c, 1939 has been reduced to such an extent 

 that only faint hints of its presence remain : "A slight touch of frost 

 in some sections and the merest hint of divided densewood." 



Second, reference is made to the transition toward which growth 

 layers of such reduced radial dimension appear to be pointing. It 

 seems obvious that complete transition exists from an entire growth 

 layer normal radially, to an entire growth layer with lenticular light- 

 wood, and to an entire growth layer whose lightwood has been so 

 much farther reduced that the only visual evidence of the growth 

 layer is what we call divided densewood. Perhaps the last vestige of 

 lightwood disappears, the combined densewoods appear as a unified 

 band, and the record of a distinct interval of cambial activity becomes 

 invisible. In the case of TTP 24-3-a, our notes of November 1946 

 state : "The intimate contacts of the densewoods of 1939 and 1938, 

 and of the interior concurrent lenses of 1941 with the densewood of 

 1940, resemble closely what we have been calling divided densewood 

 in the descriptions of previous sections. Heretofore, divided dense- 

 wood has been thought to be of little consequence. Now we find, how- 

 ever, that divided densewood and even undivided densewood may 

 represent two distinct intervals of growth or, further, two separate 

 seasons. Divided densewood where seen prior to this time is certainly 

 one form of multiplicity." Note especially plates 31, figure i ; 10, 

 figure i; 5, figure 2; 23, figure 2; 25; 13; and 19, figure 2, for 

 gradations in divided densewood. 



Half-lenses may be thought of structurally as open lenses; that is, 

 at one edge on a cross section the growth layer closes in a cusp with 

 lightwood and densewood wedging out against the densewood either 

 of the preceding or succeeding growth layer, whereas at the other edge 



6 Divided densewood is considered in some detail later in this section. 



