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



8i 



interpretations (text figs. 26 and 28), show increasing amounts of 

 complexity all due to intermittent cambial activity. 



A study of arcs, and particularly an attempt to delimit and classify 

 them, focuses attention on the intermittency in time and space of 

 cambial activity, and underlines the importance of a thorough study 

 of growth fundamentals. Arcs whose densewood is entirely free 

 floating in lightwood are theoretically possible, but complete proof 

 would depend upon serial sections. We think that "free" arcs do exist, 



Fig. 24. — XSC 10-3-a. Increment for 1939. Complexity in one annual in- 

 crement. Sequence: arc; msce; compound psL; msce; sL. 



their densewoods in the shape of an open collar. Arcs whose dense- 

 wood is not entirely free floating apparently are longitudinal projec- 

 tions from a cyhnder of densewood, part of which is closed. From 

 the evidence at hand, arcs may change into half -lenses, lenses, and 

 entire growth layers. 



A point to be emphasized is the wholly fallacious record given by 

 one radius, and only one, taken anywhere on one of these sections 

 which came from a tree grown under extreme lower forest-border 

 conditions. Not only would the various radii disagree among them- 

 selves but also none would represent the section as a whole. 



