34 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I40 



growth layers. The terms "locally absent" (Douglass, 1935, p. 68) 

 and "locally present" (Glock, 1937, pp. 8-9), for instance, are loosely 

 descriptive because they lack clear-cut definition and originated in 

 an intensive study of rings on cross sections, whereas "lens" not only 

 gives a definite picture but also is inherently a three-dimensional 

 affair. In other words, a lens is at once visualized as a patch of xylem 

 on the body of a tree. 



Even greater difficulties are brought into focus by the use of such 

 negative terms as "occasionally absent," "commonly absent," or "miss- 

 ing ring" (Douglass, 1935, pp. 58-72), because these depend wholly 

 upon one radius or at most one cross section from each tree. Else- 

 where along the body of a tree the situation may be decidedly dif- 

 ferent. It may be illustrated as follows : A certain "ring" is entire 

 around the circuit on one cross section of a tree ; another cross section 

 shows the same "ring" to be locally present ; and a third section shows 

 it to be absent or missing. Of course, a ring is missing only in the 

 sense that it was never formed. Attention cannot thus be centered too 

 exclusively on a two-dimensional viewpoint, or be concentrated on a 

 single radius or cross section from each tree. The term "lens," on the 

 contrary, includes all three of the above concepts and necessarily pro- 

 hibits the concentration of attention on a single plane in the body of 

 a tree. 



A problem worthy of investigation arises from the designation 

 "missing ring." Can a growth layer, present entirely or partially in 

 one tree, be completely absent from the body of another tree? Eco- 

 logical physiology says "yes" if the growth layer is an intra-annual. 

 However, the problem becomes acute if the growth layer actually or 

 supposedly represents one full year of growth. Such absence would 

 signify physiological activity sufficient only to keep the meristematic 

 tissues alive, but insufficient, over an interval exceeding a year, to 

 permit cell division and maturation at any time during that interval. 

 The problem is too complicated and far-reaching to be settled here. A 

 first step would be to establish without doubt the annual character of 

 the growth layer which tends to be partially or entirely absent. The 

 present paper, it is hoped, is a step forward in the solution of this 

 problem. 



Of course, designating a growth layer as locally absent, commonly 

 absent, or missing on a single radius or section does not mean that 

 the worker considers it to be the same throughout the entire body of 

 the tree. The use of such terminology may eventually restrict his 

 viewpoint. Genuine harm, however, would come from the actual sub- 

 stitution of an unduly small linear dimension for a volume quantity, 



