NO. I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL. II3 



II. Partial — Continued II. Partial — Continued 



6. Divided lightwood 10. Postseasonal growth 



7. Interrupted densewood Scattered cells 



8. Interrupted lightwood Lens 



9. Curtains Concurrent lenses 



Entire Incomplete, entire growth 



Partial layer 



Factors adding complexities to the above outline will be considered 

 next. 



TYPE RELATIONSHIPS 



Two points should be mentioned relative to the extension of the 

 simple classification portrayed above. One has to do with so-called in- 

 visible margins and the other with the form of growth layers in three 

 dimensions. 



The first point considers half-lenses and arcs as actually forming 

 the visible part of closed systems. In the case of apposed half- 

 lenses, the densewood, midway between the cusps, fails, lightwood ap- 

 pears to be continuous radially, and the densewoods of the half-lenses 

 appear to "dangle" or "float" free in the lightwood. In the case of a 

 single half -lens, densewood exists at one cusp only, whereas none is 

 visible either at the median portion of the growth layer or at the ap- 

 posed cusp. Half-lenses, therefore, represent lenses whose densewood 

 is present only at one or both of the cusps. Hypothetically, densewood 

 may fail at both cusps as well as in the median region. From this we 

 can tentatively conclude that cambial activity and growth processes are 

 cyclic and intermittent whether or not the evidence is anatomically 

 visible; this corroborates similar conclusions by plant physiologists. 



Further evidence in the same matter is yielded by divided dense- 

 wood of certain types, interrupted densewood, and interrupted light- 

 wood. 



Arcs may be thought of as belonging to one of three types : (i) A 

 portion of a lens, (2) a portion of an entire growth layer, or (3) 

 possibly an entity whose densewood "floats" free around the entire 

 margin, (i) Here the arc simply is the median portion of a lens or 

 of two apposed half -lenses. This, then, constitutes the complement of 

 the half -lens, that is, the visible median portion compared with the 

 visible cuspate portions. (2) In the second type, the arc is the only 

 portion of the densewood present — at least visibly so — around the 

 entire circuit. Thus the arc reveals an episode of cambial activity 

 otherwise unsuspected. The longer the arc is, the more plausible the 

 revelation becomes. (3) In cross section, arcs certainly are entities. 

 But can they retain their characteristics in three dimensions? This 

 leads directly to our next topic. 



