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



a half-lens attached to a simple arc. Over total range, arcs vary from 

 those with heavy densewood of sharp outer contact, to those of dif- 

 fuse contact, and finally to those with densewood so feeble as to be 

 almost invisible. 



The arc as described is a feature seen in cross section. What occurs 

 in longitudinal section? Two things are possible: first, the arc, as 

 outlined by its densewood, actually "floats" free with no part of its 

 densewood joining any other band of densewood; and second, the 

 arc longitudinally changes into other growth-layer forms, partial and 

 then perhaps entire. 



An abundance of evidence supports the transition of arcs into half- 

 lenses, lenses, and entire growth layers. Examples are plentiful but 

 a few will be cited. In the 1940 increment of TTC 33-11, a sharp 

 arc on sections a becomes a sharp, complete, entire growth layer on 

 sections h, 12 cm, outward on the branch. In the 1937 increment of 

 TTC 5-7, a sharp arc on sections a becomes a sharp lens on sections 

 h, 33 cm. outward. In the 1936 increment of SA 6-1, a partly sharp, 

 complete, entire growth layer on sections a becomes a sharp arc on 

 sections h, 26.5 cm. outward. In the 1942 increment of Con T 2-7, a 

 diffuse arc on sections a becomes a sharp, complete, entire growth 

 layer on sections h, y.y cm. outward, and a diffuse, complete, entire 

 growth layer on sections c, 1.4 cm. farther outward. In the 1940 in- 

 crement of TTC 34-2, a diffuse arc on sections a becomes a sharp 

 half -lens on sections b, lo cm. outward, and back to a diffuse arc on 

 sections c, 10 cm. farther outward. In the 1944 increment of TTP 

 20-40, a diffuse lens on sections a becomes a diffuse, complete, entire 

 growth layer on sections h, 2 cm. outward, and a diffuse arc on sec- 

 tions c, 6 cm. farther outward ; on sections d, 12.5 cm. outward from 

 c, the arc has disappeared. Such examples could be multiplied, ex- 

 amples of arcs changing outward on the branch to half -lenses, lenses, 

 or entire growth layers, or any of these changing outward to arcs. 



Any treatment, any description of arcs seems so intimately bound 

 up with the areal extent of the growth layer and with cambial activity 

 that neither one can be discussed alone. Text figure 20 is a drawing 

 of a cross section of a rather simple-appearing shrubalthea stem (see 

 also pi. 3, fig, I ) , As soon as we begin to visualize the stem in terms 

 of cambial activity, text figure 21 seems to give the only logical result. 

 With this interpretation, difficulties of identity disappear — the whole 

 problem becomes a matter of cambial activity and the development of 

 densewood. Thus, growth layer i (text fig. 21) assumes an identity 

 as a "complete," entire growth layer whose densewood exists only 

 over a portion of its circuit — in the vicinity of the branches and at the 



