NO. I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL. 79 



arc. The contact elsewhere between growth layers i and 2 is invisible 

 because densewood is absent. If it were not for the arc, we would 

 have no means of knowing that the cambium experienced two episodes 

 of activity during 1933 as represented on the cross section of SA 

 I -a. The methods used in the present work give us no clue to this in- 

 terruption in cambial activity at the so-called invisible contact. In the 

 future it is possible that either microchemical tests or examination 

 under high power of cell walls on specially prepared sections will yield 

 evidence for otherwise unknown interruptions in cambial activity. 

 Growth layer 2, in continuation, becomes an exterior lens; growth 

 layer 3 becomes a "complete," entire growth layer whose outer con- 

 tact is partly sharp, partly diffuse, and partly invisible ; growth layer 

 4 becomes a lens as indicated by the two facing half-lenses ; growth 

 layer 5 is a lens which partially overlaps growth layer 4 ; and growth 

 layer 6 is a lens and, together with 4 and 5, makes a compound lens. 

 In naming the features on text figure 20 (see also pi. 3, fig. i), one 

 becomes involved in what could perhaps be called the complexities of 

 oversimplicity. For instance, exactly what constitutes the arc, the 

 half-lens? These difficulties, although not made elementary by any 

 means, largely disappear when the cross section is viewed as a product 

 of cambial activity as set out in text figure 21. 



Text figure 22 shows parts of the annual increments for 1938 and 

 1939, the frost spot at the left indicating the start of 1939. The fig- 

 ure, as well as the section itself, appears complicated at first sight, al- 

 most hopelessly so, especially on the actual specimen. However, if we 

 set the above figure to a pattern, text figure 23, the complexities seem 

 to fade to a great extent. The whole section includes for the most part 

 entire growth layers and lenses, some simple and some compound, 

 some sharp bordered and some diffuse. Growth layer i is a sharp 

 lens, short and thick; growth layer 2 is complete and entire, partly 

 sharp, partly diffuse; growth layer 3 is a long, sharp lens; growth 

 layer 4 is a short, thin lens, partly sharp, partly diffuse; growth 

 layer 5, a husky sharp-bordered lens, completely overlaps growth layer 

 4, and with growth layers 3 and 4 forms a compound overlapping and 

 doubly overlapping lens system ; growth layer 6 is a complete, entire 

 growth layer, so far as the figure extends, whose presence is revealed 

 solely by the arc of densewood ; growth layer 7 is a sharp, complete, 

 entire growth layer ; growth layers 8 and 9 are a system of compound 

 lenses. It is of interest to note here in passing that the visible dense- 

 wood of growth layer 6 lies in the same circuit as the frost injury and 

 undoubtedly is genetically related to low temperature. 



Text figure 24, and text figures 25 and 27 with their accompanying 



