NO. I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL. 55 



gives the sequence 1938 to 1940 an uncertainty which would make 

 dating impossible were it not for the 1938 frost injury, a knowledge of 

 the cutting date, and crossdating with previous specimens. The an- 

 nual increments do not have sharp borders and are otherwise atypical 

 but do not have to be discarded. 



COMPLETENESS 



A growth layer may be considered complete if densewood has been 

 formed. Ideally, the only complete growth layer should be the sharply 

 bounded one, the one which supposedly records complete cessation of 

 cambial activity. However, the presence of sharply bounded dense- 

 wood cannot be the only criterion of completeness because then the 

 difficulties of description and understandable nomenclature would be 

 almost hopelessly great. It is the margins which are not sharp that 

 give the complexities inherent in the problem of completeness. (For 

 incomplete growth layers, see especially pis. 31, fig. 2, and 35, fag. i.) 



SUMMARY AND CONCLUSION ON CONTACTS 



The unaided eye and low magnifications encounter a minimum of 

 trouble with growth-layer contacts. Perhaps this is a boon for certain 

 types of work. Even if work based on low magnification were ul- 

 timately proved fairly accurate, it would still touch only the high 

 spots, not necessarily the critical, in the life history of a tree. 



High powers reveal many features not otherwise visible, as well as 

 the great amount of variation along growth-layer contacts — the one 

 carrying a more complete history, the other a more accurate history of 

 cambial activity than that obtainable otherwise. Complexity, not 

 simplicity, characterizes many growth-layer margins. Hence, it fol- 

 lows that cambial activity is at least equally complex and, indeed, more 

 so if the significance to the classification of growth layers is used as 

 a criterion. 



Contacts may be sharp, definite, indefinite, or diffuse, and may 

 possess graduations between any two. On any one cross section the 

 outer margin may be entirely sharp, or entirely diffuse, or the one cir- 

 cuit may contain all types of contacts. Similar unity or variation ex- 

 tends in the longitudinal direction. As a matter of fact nearly any 

 growth layer, no matter how sharp appearing under low power, yields 

 its sharpness under high power to a greater or lesser extent some- 

 where on the circuit by reason of one or more of the features dis- 

 cussed under causes of diffuseness. In contrast there are examples, 



