NO. I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL. 2$ 



based upon extensive sampling at the end of the 1938 growth period, 

 upon sampling of trees growing under conditions of constantly avail- 

 able soil moisture, and upon a combination of meteorological and 

 phenological records and observations. A combination of all of them 

 left no doubt as to the identity, characteristics, and intensity of the 

 typical frosts enumerated above. Moreover, the fact that the tem- 

 perature of 1935 did not drop below freezing in the Lubbock area 

 after the middle of February eliminates all possibility of the existence 

 of frost injury in the xylem grown in 1935. A knowledge of this ab- 

 sence serves as a check interior to 1936 whereas the time of first 

 sampling, the winter of 1938- 1939, placed a limit exterior to the 1938 

 growth layer. 



In many of the sections studied, the following typical sequence 

 served as an anchor for extension into other parts of the same 

 branch, other branches, other trees, and other species : 



1934 annual increment, with moderately intense frost effects. 



1935 annual increment, in which frost effects are impossible. 



1936 annual increment, with moderately intense frost effects. 



1937 annual increment, with light frost effects showing commonly as spots. 



1938 annual increment, with intense frost effects. 



Early work in dating depended upon the presence of the character- 

 istic 1938 frost effects in the outer part of our sections, whereas later 

 work attempted to reach back to include 1938 frost effects in the inner 

 part of the sections and to reach forward to tie in with the 1945 

 frost effects, which will permit us to extend the work into the future, 

 with precise dating assured. Recent fieldwork indicates that the frost 

 injury of 1945 will be as much of a key to absolute dating as was that 

 of 1938. 



The entire sequence, as well as each unit of the sequence, served as 

 an absolute check in precise dating. These checks controlled the dating 

 and established multiplicity wherever it occurred. In some cases, 

 where there was a question as to the date of different frost effects be- 

 cause of certain atypical occurrences among them, the fact that the 

 1935 increment could not possibly contain a frost acted as a control 

 in the dating of the increments both preceding and following it. The 

 correct combination of frost effects in different years had to include a 

 1935 increment which was free of those effects. 



The presence of frost effects depends, of course, upon the intensity 

 of the climatic factor, upon the species concerned, and upon the condi- 

 tion of growth at the time of the frost. If the frost was intense, as for 

 instance in 1938, and if the meristematic tissues were active, very few 

 species in the Lubbock area escaped the effects. The silver maple was 



