NO. I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL. 287 



With the exception of methods of absolute dating, other methods 

 used were those more or less common to botanical techniques. 



DATING CRITERIA 



The discovery that growth layers could be dated with exactness by 

 the incidence of natural frost injury and recovery led directly to an 

 intensive inquiry into where and when growth occurs. Altogether 

 four methods were used to date growth layers : Natural frost effects, 

 artificial frost effects, periodic tip-growth measurements, and the re- 

 lation of tip growth to diameter growth. Materials used herein have 

 been dated absolutely unless otherwise noted. 



CLASSIFICATION 



The common idea of tree rings pictures them as concentric circles 

 of about equal width on the two-dimensional face of a cross section. 

 This picture is accurate for certain environments but cannot be ration- 

 alized for all environments. As a matter of fact, investigation reveals 

 that a ring is merely a cross-section view of a sheath, or layer, of wood 

 which more or less entirely covers the plant body, branches, trunk, and 

 roots; that growth layers may be entire or partial; that partial 

 growth layers may cover either minute fractions of the plant body 

 above ground or nearly all of it; and that partial growth layers may 

 include a variety of types which grade into each other longitudinally 

 and tangentially. 



Investigation also reveals that growth layers may vary radially; 

 that the outer border of densewood may terminate abruptly against 

 the succeeding lightwood or may grade outward through a complete 

 transition from densewood into lightwood ; that growth layers may be 

 either complete or incomplete radially by the presence or absence of 

 densewood; and that growth may cease without the formation of 

 densewood. 



Finally, investigation indicates that the growth processes of cambial 

 activity, differentiation, and maturation may be highly localized on the 

 plant body; that these growth processes are rhythmic and are not of 

 necessity visibly recorded ; and that physiological activity modified by 

 environmental influences leaves a more or less detailed record in the 

 xylem. 



An interval of physiological activity has been termed a growth flush. 

 If such activity produces xylem, it is called a growth layer. Specifi- 

 cally, a growth layer includes the xylem laid down during a single in- 

 terval of activity, regardless of where within the plant body the activ- 

 ity occurs, or when. 



