286 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I40 



VIII. SUMMARY 



DISCOVERY OF ABSOLUTE DATING 



The discovery of absolute dating by means of natural frost effects 

 opened the way to a study of tree growth, especially in reference to 

 the place and time of cambial activity. The formation of partial 

 growth layers is linked directly to the locality on the tree body where 

 cambial activity occurs ; this belongs properly to the subject "Classi- 

 fication of Growth Layers." Subdivisions within the annual incre- 

 ment are linked directly to the time of cambial activity ; this belongs 

 to the subject "Multiplicity of Growth Layers." 



The possibilities opened up by the discovery that natural frost 

 effects could be used to date annual increments with exactness led 

 directly to the observations, measurements, and experiments reported 

 upon in the present work. 



Soon after natural frost effects were put to chronologic use, artificial 

 freezing was introduced as a second potent method of absolute dating. 



TREES, LOCALITIES, AND ENVIRONMENTS 



Some 36 different species, including both gymnosperms and angio- 

 sperms in nearly equal numbers, were brought under study at one time 

 or another. However, work was concentrated on members of the two 

 genera, Cupressus and Pinus. 



Although most of the work was done on trees on the extreme 

 lower forest border in and near Lubbock, Tex., trees from a wide 

 range of moisture, temperature, and length of growing season were 

 measured and sampled at different times. Types of environment 

 varied from that of Yuma, Ariz., to that of the upper timber line in 

 New Mexico and the Appalachian Piedmont of Maryland. 



METHODS 



The results obtained from observation, measurement, and experi- 

 ment furnished the bases for the present report. Fieldwork preceded 

 preparation of specimens and microscopic analysis of stained thin 

 sections, out of which emerged a picture of growth layers described 

 under "Classification" and a resolution of the complexities of the an- 

 nual increment as described under "Multiplicity." 



Attention was concentrated upon the branches of trees except for 

 eight trunks which were dissected in whole or in part in order to de- 

 termine whether or not features typical of branches also characterized 

 the trunks. 



