NO. I GROWTH LAYERS IN TREE BRANCHES CLOCK ET AL. 69 



appears to be simply a lens whose one cusp has come free from the 

 neighboring densewood. Structural character has been altered at 

 once. The half-lens becomes a feature of the main growth layer (text 

 fig. 10) whereas the lens is a patch of xylem added onto the main 

 growth layer — a great difference. If the cusp drops inward, then the 

 half-lens is an exterior one ; it is a feature of the inner main growth 

 layer and is the result of an irregularity in the main episode of cambial 

 activity and growth processes producing the inner main growth layer. 

 If the cusp rises outward, then the half-lens is an interior one; it is 

 a feature of the outer main growth layer and is the result of an ir- 



FiG. 10. — XSC i-2-a. Part of increment for 1939. Lensing : ^L, ; see ; sL. 



regularity in the main episode of cambial activity producing the outer 

 main growth layer. This linking of the half-lens to a lens leads to 

 some confusion as to the exact identity of the half -lens. 



The problem can perhaps be clarified by reference to text figure ii. 

 Initial attempts at classification, it now appears, considered merely the 

 band of densewood, h (text fig. ii). But densewood alone is only 

 part of a radially normal growth layer. If densewood, h, at some place 

 longitudinally moves out to join d at both tangential extremities, then 

 c would constitute a proper lens. Is c-d in the figure the half -lens ? Or 

 is it a-h ? The lightwood c is visibly continuous with lightwood E and 

 yet the cambial activity which resulted in the segment a-h-c-d has had 

 a different history than that at E or F. From the standpoint of cam- 

 bial activity solely, a-h is the half-lens, set off as a feature by localized 

 cessation of that activity. The history of the cambium has not been 

 uniform as regards rate and place of activity or change of rate and 

 change of place. If a-h be taken as the half-lens, cambial activity 

 slows or ceases locally for a half -lens and, in contrast, remains active 



