NO. I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL. 7I 



layer can begin with densewood and end with hghtwood. The dense- 

 wood of Ai is equivalent tangentially to Hghtwood in the region Ag. 

 Conditions at one place gave densewood, at another lightwood. Was 

 the pause in cambial activity at the densewood of Ai brief or was the 

 formation of lightwood at A2 a very slow process ? 



Fig. 12. — Suggested transitions from lens to half-lens, and disappearance of 

 the latter. A study in variation of cambial activity and maturation processes. 



The whole system of lenses in 1939 is a mixture of compound, over- 

 lapping, and, in a certain sense, transgressive lenses and half -lenses. 

 Lenses C and D (text fig. 14) are not quite centered on the same axis. 

 They are compound in a certain sense, notwithstanding the fact that 

 D overlaps C slightly. The densewood bands at the inner and outer 

 margin of lens C unite to the left with an outer band of densewood at 

 Y and Yi ; to the right they unite at X and Xi with an inner band of 

 densewood, that of the half-lens, Ai. If we consider the bands of 

 densewood alone, the bands Y-X and Yi-Xi transgress outward across 

 the lightwood, B-C-D, from the densewood of the half-lens to the 

 densewood at D which constitutes the outermost part of the annual 

 increment. Sections XSC i-i-a, 13 cm. inward, show the same lens 

 system save that margins are somewhat less definite. 



