112 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL, I40 



(or complexity) may increase in both directions from a focal point. 

 The evidence also indicates that different species act very much alike 

 and that different branches of the same tree possess different se- 

 quences for the same year. 



Thus there can be little doubt that growth layers pass from one type 

 to another longitudinally; that the results of one episode of cambial 

 activity may be traced along a branch with more or less difficulty ; and 

 that text figures 34 and 35 give a first hint of the variations to be ex- 

 pected among partial growth layers and the reasons therefore. 



SUMMARY OF TYPES 



In the synoptic view, growth layers group themselves into a rela- 

 tively small number of distinct types. The simplest classification, 

 whether justified or not, depends upon densewood alone. For instance, 

 a half-lens is a band of densewood dangling at one end. Such is no 

 doubt all right if mere identification is the ultimate aim. It goes with- 

 out saying, however, that of far greater import is the physiological 

 activity of the cambium which gives rise to the various types of growth 

 layers, types that under a too simple classification would be entirely 

 overlooked. Physiological activity is, in a manner of speaking, a func- 

 tional bridge between variations in the complex of habitat factors and 

 the type, continuity, and definition of resultant xylem. As an example, 

 the invisible radial termination of a growth layer which extends be- 

 tween the two "open" ends of half -lenses facing each other is quite 

 understandable in relation to physiological activity but is meaningless 

 under a simple classification. 



Nevertheless, a descriptive classification in two dimensions is a 

 method of approach to the understanding of growth layers in three 

 dimensions, and their transitions from one to the other. The two- 

 dimensional picture, therefore, takes our attention at the moment. 



A condensed outline of growth-layer types follows: 



II. 



