NO. I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL, 23 



with the microtome. It was often necessary to determine whether the 

 cambium was ahve or active at the time of taking the specimen, as in 

 freezing experiments ; and in the determination of the time of begin- 

 ning and of cessation of cambial activity, it was important that the 

 cambium cells be left as nearly normal as fixing and staining tech- 

 niques would permit. For these reasons, sections were cut either 

 from freshly sawed blocks, or else the latter were fixed in formalin- 

 acetic-alcohol and sectioned later with no additional treatment. 



All sections were examined under low-low (21X), low (60X), and 

 high (264X) powers. In those cases where it was possible, sections 

 were examined closely prior to staining, either under a wide-field 

 binocular (13.8X) or under a hand lens (i4^)- ^ comparison of the 

 results achieved with the unstained sections on the one hand and the 

 stained sections on the other, brought out vividly and emphatically 

 the great advantage of stained over unstained sections. Many cases of 

 uncertainty on unstained sections were resolved quickly and accurately 

 when the sections had been stained. 



IV. ABSOLUTE DATING— CRITERIA OF 

 MULTIPLICITY 



DISCOVERY OF PRECISE DATING 



In late 1938, Clock began the study of stained sections from 

 branches of trees growing on the campus of Texas Technological 

 College and noted a severe frost injury and recovery zone in the 

 inner portion of the outermost growth layer (Clock and Reed, 1940; 

 Clock, 1951). It was soon determined that the injury was due to a 

 severe frost early in April 1938, which almost universally damaged 

 the vegetation of the area. 



In February 1939 realization came that, with proper use as taught 

 and described by Bailey (1925), late spring frosts could be employed 

 as a method of precise dating of the annual increments of xylem be- 

 cause the time of occurrence of the frost was known with exactness. 

 This method of dating opened up the possibilities of extended investi- 

 gation which was, nevertheless, distinctly limited by dependence upon 

 the natural frosts which came with irregular frequency and intensity. 



While Clock was engaged in the task of dating annual increments 

 and their included growth layers by means of natural frost. Stud- 

 halter, in May 1939, conceived the idea of devising a method of pro- 

 ducing an injury in branches by means of frost artificially induced 

 by dry ice (Studhalter and Clock, 1942). He suggested to Clock that 

 artificial frost might supplement the efifects of natural frost and as a 



