NO. I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL. 27 



beginning of their respective annual increments, especially where they 

 are multiple. 



A striking feature of a few branches was the occurrence of the 

 effects of two frosts in the 1938 increment. The initial effects, feeble, 

 inconspicuous, and commonly restricted to the two cusps of lenses, 

 involved the very first cells set off when diameter growth began in 

 the spring. The second or later frost produced the effects of the 

 typical 1938 frost. In a rare instance or two, there was a third injury 

 and recovery resembling those of frost ; however, we cannot be cer- 

 tain of such an identification. Where present, the double frost effects 

 can be used in dating. 



ARTIFICIAL FROST 



Artificial frost injury and recovery resemble their natural counter- 

 parts in all respects. Our experiments were confined to the branches 

 of Arizona cypresses, yellow pines, and junipers on the campus of 

 Texas Technological College and the grounds of the Texas Agricul- 

 tural Experiment Substation near Lubbock (pis. 2, figs, i and 2 ; 23, 

 fig. 2; 24). 



The original purpose of artificial freezing was to place an internal 

 tag in the nature of an injury and recovery within the xylem before it 

 matured. By a series of cahbration experiments, it was intended to 

 determine the time and intensity of application necessary to produce 

 identifiable results. If this could be done (and it was), it would give 

 immediately a method of placing an internal tag at any desired time in 

 relation to diameter growth and at any desired place in the woody 

 framework of a tree. Excellent results, as a matter of fact, were ob- 

 tained in those cases where the cambium and cambial derivatives were 

 in the proper state to receive and record the shock of freezing ; how- 

 ever, the proportion of successful internal tagging experiments was 

 low in relation to the total number carried out. 



Internal tagging was unsuccessful under two circumstances: the 

 freezing was applied too late in the growing season, or it was too in- 

 tense to permit subsequent recovery. In the climate of the Lubbock 

 region, it was found that artificial freezing should be applied before 

 the middle of April in order to strike the cambium in the condition 

 most susceptible to frost injury. Growth here begins anywhere be- 

 tween March i and April i, and may be well along in the first flush of 

 diameter growth by the middle of April. 



As a matter of fact, even though experiments in internal tagging 

 were not carried through in the numbers to be desired, the anatomical 



