NO. I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL. 33 



frost was recognized and verified in the Chisos Mountains, 320 miles 

 south of Lubbock. Later, other methods supplemented and extended 

 the possibilities of absolute dating. It should be emphasized that this 

 type of dating contrasts sharply with those methods dependent upon 

 counting or upon the presence of one checkpoint in many years. By 

 means of natural frost, artificial frost, or measured tip growth, the 

 growth layers of each branch used in this work have been dated 

 exactly. 



Absolute dating gives a great advantage in this type of work and 

 permits a breadth of application not heretofore realized. By this 

 means we should readily be able to detect multiplicity of growth 

 layers in a rather marked lower forest-border region as represented 

 by the Lubbock area. However, before the rather involved topic of 

 multiplicity is considered, it will be well to examine all types of growth 

 layers and to determine if they can be classified structurally or ge- 

 netically. This can lead us to a rational definition of a growth layer 

 — rational from the physiological point of view. 



V. CLASSIFICATION OF GROWTH LAYERS 



INTRODUCTION 



Classification is a necessary, initial step in an intensive investigation 

 of growth layers. Early studies indicated that certain growth layers 

 may be disposed over the plant body in a somewhat complex fashion. 

 In attempting to describe the variety of growth layers encountered, 

 we adopted a terminology that seemed suitable and suggestive. 



A consideration of growth layers from an anatomical point of view 

 and an appreciation of cambial activity from the physiological point 

 of view depend to a great extent upon the mental picture of a growth 

 layer as it exists within the body of a tree. In addition, this picture 

 possesses ecologic implications. The older idea, and the one largely 

 current today, is that of rings which are concentric circles as seen on 

 the end of a log or the top of a stump. Hence, the popular term 

 "tree rings" is used rather than "growth layers," and the simplicity 

 of a two-dimensional concept contrasts with the anatomical correct- 

 ness of the three-dimensional concept. The principles of physiology, 

 anatomy, and ecology leave no alternative but to consider tree growth 

 a complex process resulting in a three-dimensional growth layer more 

 or less intricately disposed over the body of a tree. 



In the construction of a terminology, the terms must be highly 

 descriptive ; they must conform to the three-dimensional picture even 

 though they are derived from the cross-sectional appearance of the 



