CLASSIFICATION AND MULTIPLICITY OF 



GROWTH LAYERS IN THE BRANCHES 



OF TREES 



AT THE EXTREME LOWER FOREST BORDER 



By 

 WALDO S. CLOCK 



Macalester College, Saint Paul, Minn. 



R. A. STUDHALTERi 

 Texas Technological College, Lubbock, Tex. 



AND 



SHARLENE R. AGERTER 

 Macalester College, Saint Paul, Minn. 



(With 36 Plates) 



I. INTRODUCTION 



OBJECTIVES AND RESULTS 



The work described in the following pages began as a desire to 

 answer the apparently simple questions: When does a tree grow? 

 How much does it grow ? Where, on the plant body, does growth take 

 place? 



The objectives may be stated more formally in terms of cambial 

 activity and maturation processes. These activities vary appreciably in 

 time and space, and it is therefore pertinent to trace them in as much 

 detail as possible with respect to diameter increase, formation of 

 densewood, and tip elongation. 



Results of observation, measurement, and experimentation have 

 shown clearly that : ( i ) growth neither begins nor ends consistently 

 at a certain time in all species, among all trees, among all branches, or 

 over a single branch; (2) growth flushes may occur not only as an- 

 nual but also as intra-annual cycles; (3) the amounts of xylem in- 

 cluded in annual increments fluctuate, in general, over a relatively 

 narrow range, whereas the amounts included in growth layers may 

 fluctuate to an astonishing degree; (4) xylem may be formed as a 



1 Dr. Studhalter died on March 28, 1958. 



SMITHSONIAN MISCELLANEOUS COLLECTIONS, VOL. 140, NO. 1 



