NO. I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL. I73 



1941, and was cut off July 4, 1941. In the 1939 increment, the intra- 

 annuals fade outward. In 1940 the arc was made by natural spring 

 frost. In 1941 the lenses are concurrent. 



XSC 10-3 (table 119) was frozen artificially at 36 cm. on May 9, 

 1941, and was cut off July 22, 1941. In the 1939 increment, complex- 

 ity decreases outward. 



XSC 1 1-2 (table 120) was frozen artificially at 38 cm. on May 9, 

 1941, and was cut off September 11, 1941. In the 1938 increment, the 

 lens is concurrent in each case and becomes thicker outward. In 1939 

 the densewood at 44 cm. resulted from frost action. As is so common 

 in 1939, the outer entire growth layer is very thin and weak, its dense- 

 wood one to two rows of cells thick. Here and there radial columns 

 of cells fail to narrow down, thus creating "gaps" in the densewood. 

 It should be noted that all these so-called outer thin growth layers, 

 weak and inconspicuous though they may be, constitute the outer por- 

 tions of the annual increments. In many cases the inner growth layer, 

 or intra-annual, is more distinct, more conspicuous, and more de- 

 veloped. 



XSC 1 1-3 (table 121) was frozen artificially at 38 cm. on May 9, 

 1941, and was cut off September 27, 1941. In the 1938 increment, one 

 of the sharp growth layers was caused by the 1938 natural frost with 

 which it is connected. At 38 cm. the only visible frost effects are a 

 circle of parenchyma cells and the band of densewood. The dense- 

 wood of the annual increment is divided over a major portion of its 

 circuit. At 29 cm. the divided densewood has actually become a gen- 

 uine, complete, entire growth layer. Within incipient compression 



