NO, I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL. 257 



of Other evidence, that the formation of xyleni and the nature of the 

 growth layers depend upon the variations of growth factors not only 

 during the so-called growing season but also throughout the year. In 

 some cases, the Conservatory growth layer is separated from the chief 

 1939 growth layer by a transition zone which makes identification and 

 dating simple; in other cases, the separation is much more definite. 

 Hence the nature of the growth layer, whether annual or intra-annual, 

 depends upon the variations in the impact of growth factors, varia- 

 tions which may or may not be annual, and which may or may not be 

 sufficiently intense to make either annual or intra-annual growth layers 

 indistinguishable, one from the other. 



Tree TTP 20 had more constant observation over a longer interval 

 of time than any of the other trees. Late in the autumn of 1939 the 

 terminal buds expanded and elongated. The following is quoted from 

 notes written December 14, 1939, when sections were taken: "A 

 branch from the south-southwest side. Buds full and much elongated. 

 Taken after a warm autumn, with rain several weeks ago. (Has a lens 

 been made?)" The increment for 1939 in TTP 20-2-a, 44.5 cm. from 

 the tip, possesses a long exterior lens that appears sharp under low-low 

 power but indefinite under low. In TTP 20-2-b, 24. cm. from the tip, 

 the lens of a has become an entire growth layer whose inner margin 

 varies from sharp to dift'use under low-low power. The outer walls of 

 the outer cells of the added growth layer are incomplete and the outer 

 margin irregular. In TTP 20-2-c, 12 cm. from the tip, the densewood 

 underlying the "extra" growth layer of 1939 has become so diffuse 

 and weak that, although it has the position of the lens and "extra" 

 growth layer of the previous sections under low-low, under high power 

 it loses its identity as densewood. The outer walls of the outermost 

 cells of 1939, and in many cases the complete cells, are immature, and 

 the outer margin of the increment is highly irregular. In TTP 20- 

 2-d, 2.5 cm. from the tip, resin canals around the circuit seem to 

 occupy the radial position of the intra-annual densewood which exists 

 farther inward on the branch. The outermost margin in d has, in 

 places, typical densewood cells rather than the partially immature cells 

 and irregular margin farther inward. 



Postseasonal growth appears to indicate a persistence or return of 

 conditions which stimulate growth after the grand growth period has 

 been completed earlier in the season. The presence of postseasonal 

 growth depends, apparently, not upon the time of cutting of sections 

 so much as upon the presence of growth-stimulating conditions. 



