2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I40 



sheath not only over all of a plant body during a single growth flush 

 but also as a partial sheath over a large or very small portion of the 

 plant; and (5) partial growth layers may be highly irregular in their 

 distribution among different trees or over a single tree. 



The present work has also shown clearly that : ( i ) cambial activity 

 may either be general over a tree during the so-called growing season, 

 or it may be highly localized in time as well as in space; (2) the rate 

 and intensity of cambial activity may vary in time and in space; (3) 

 the formation of densewood may be intermittent radially and tangen- 

 tially ; (4) the rate and intensity of densewood formation may vary 

 in time and in space; and (5) the contact relations between dense- 

 wood and lightwood may vary from abrupt alternation to complete 

 transition. 



After Clock had been using tree rings as a basis for chronologic 

 and climatic interpretations for some years, it became increasingly 

 apparent to him that the fundamentals of tree growth offered a more 

 fruitful field of investigation. Therefore, the dynamic and biologic 

 picture of tree growth to a large extent displaced in his mind the 

 static picture of tree rings as purely mathematical entities. 



Emphasis on the physiology and anatomy of tree growth quickly 

 expanded the scope of investigation beyond the resources of one man. 

 Soon after intensive work began, R. A. Studhalter was asked to take 

 part in the expanded project because of his interest in the botanical 

 aspects of growth-layer formation and because of his suggestions re- 

 garding artificial freezing of plant tissues, 



ACKNOWLEDGMENTS 



The work upon which the present report is based has received 

 financial support by way of grants-in-aid from the Society of Sigma 

 Xi, the Smithsonian Institution, and Texas Technological College. In 

 January 1954 the National Science Foundation made a grant (Re- 

 search Grant NSF-G610) with which to expedite and complete the 

 project, and in August 1959 it made a further grant (NSF-G9539) 

 toward publication of this paper. 



D. L. Jones, superintendent of Substation No. 8, Texas Agricul- 

 tural Experiment Station, at Lubbock, permitted us to experiment at 

 length on many of the trees at the station. C. W. Van Horn, super- 

 intendent of Yuma Farms, University of Arizona Agricultural Ex- 

 periment Station, sent us cuttings from citrus trees grown under ir- 

 rigation. 



To these organizations and persons we owe grateful acknowledg- 

 ment for assistance and courtesies. Likewise, we are greatly indebted 



