32 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL, I40 



Certain problems and concepts having to do with tip flushes and 

 their transitional forms, as well as the relation to actual tip flushes 

 of multiple zones on a branch, each zone composed of a bare portion 

 and a needle-covered portion, will be considered later in the chapter 

 on multiplicity. 



In view of the generally accepted fact that one tip flush can rep- 

 resent no more than a single year of growth, the existence of two 

 growth layers in one tip flush is acceptable evidence for multiplicity. 



Further discussion of the relationships between diameter flushes 

 and tip flushes will be given later (pages 183-204). 



CORRELATION OF STRUCTURAL FEATURES 



Within the same branch, and even within the same tree, special 

 features such as width of grovv'th layer, similar sequence, character- 

 istic "doubles," circles of parenchyma cells, or injury other than 

 frost could be used occasionally to help identify growth layers from 

 section to section as an extension of absolute dating. No great reli- 

 ance was placed upon these features. 



For instance, an injury in the growth layer for 1936 in several of 

 the Arizona cypresses persisted farther out along the branches than 

 did the known natural frosts by which the growth layer was dated. 

 Another example of marked assistance was an annual increment con- 

 sisting of two sharply bounded growth layers, the inner one wide 

 and the outer very narrow. This "double" proved especially charac- 

 teristic and helpful for 1937, 1939, and 1940 among Arizona cypresses 

 and junipers. As in the case of all structural features, the "double" 

 was first dated on an absolute standard before it was used in a 

 secondary role. 



Growth-layer width by itself, as seen in cross section, was of little 

 correlative value. However, relative width of adjacent growth layers 

 did offer assistance when based initially on absolute dating. 



SUMMARY STATEMENT 



The problem of the unity or multiplicity of growth layers in one 

 year has been discussed more or less emphatically from time to time 

 by different workers. Evidence for unity has been cited from one 

 region, whereas evidence for multiplicity has been announced from 

 another. If precision methods, or absolute dating, could be applied 

 over a region, the uncertainty would be eliminated. 



Absolute dating was made possible by the recognition of the ef- 

 fects of the 1938 natural frost in the Lubbock region. This same 



