NO. I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL. 269 



(11) The outermost two to four rows of cells in the annual dense- 

 wood band lack typical densewood characteristics. They may be nar- 

 row but at the same time thin-walled and slightly lignified in contrast 

 to the densewood cells immediately to the interior. As a matter of 

 fact, the entire growth layer may be atypical in contrast to neighboring 

 growth layers, as illustrated by 1939-1941 of TTP 23-4-c (pi. 17, fig. 

 2). If these growth layers are annuals, it is obvious that methods of 

 absolute dating are necessary for their identity. Thus it is clear that 

 the outer borders of annual increments are not all of the same degree 

 of sharpness; they may be less sharp and less well defined than the 

 border of an intra-annual within the same annual increment. An 

 intra-annual, furthermore, may have all gradations from the obviously 

 intra-annual, or "double," to one indistinguishable from an annual. 



An "outer thin" growth layer is an expression which has been used 

 in reference to a growth layer extremely thin in contrast to the 

 relatively thick growth layer which lies to the inside and constitutes 

 the bulk of the annual increment. The densewood ^^ of the outer thin 

 is commonly weaker than the densewood of the growth layer to the 

 inside, although where the inner densewood becomes weaker, the outer 

 becomes stronger on the same radius, and vice versa. The outer dense- 

 wood may even become indefinite. Absolute dating proves that the 

 outer thin growth layer is intra-annual ; in fact, not one case has been 

 proved to be annual. Among the trees used, examples have been found 

 in TTAp, TTC, XSC, TTJ, TTP, and XSP, chiefly in the years 1939 

 and 1940. This is a typical notation from an analysis of TTP 20-25-a : 

 "Densewood of inner growth layer is heavy and well-developed, thus 

 contrasting with the densewood of the outer growth layer which is 

 thin, weak, and poorly developed ; hence the densewood of the intra- 

 annual is typical of what is expected in an annual, whereas the dense- 

 wood of the actual annual is weak and atypical in the expected sense." 

 Examples of the outer thin are shown in plates 4, figure i ; 14, figures 



^^ Douglass (1928, p. 32) referring only to densewood says : "In the process of 

 counting and dating rings in Arizona pines, two sharp red [i.e., densewood] rings 

 sometimes occur close together, giving the appearance of a double and leaving 

 one in doubt as to whether one year or two is involved. ... If the two red rings 

 [densewood] are unequal in size and the smaller one is inside, that is, nearer the 

 center, it is likely to be a real double formed by the spring drought. If the 

 smaller one is outside the larger, it is probably a separate year. If the two rings 

 are equal and either one shows a further doubling, the two rings in question are 

 separate years." According to this, a "large" (thick) densewood must terminate 

 an annual increment ; a "small" (thin) densewood may or may not terminate an 

 annual increment. Absolute dating would appear to weaken the accuracy of the 

 above statements. 



