NO. I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL. 121 



tion that a growth layer is a sheath of xylem laid down under the bark 

 over the entire plant body (except in stem tips and root tips) refers 

 to one type of growth layer only. Or, the definition that a growth 

 layer (or tree ring) is a circle of wood made up of springwood and 

 summerwood refers only to appearances on a cross section. 



It seems reasonable to ask for a definition based not only upon de- 

 scription or structural features but also upon those physiological proc- 

 esses which create the growth layer. Thus a certain type of physiolog- 

 ical activity may be called a growth flush if it produces a growth layer. 

 A growth flush is an interval of physiological activity which gives 

 elongation at the apical meristem and increase in diameter at the cam- 

 bium. This does not mean to deny the possibility of a period of 

 accelerated physiological activity without the production of wood or 

 the possibility of including the processes of maturation as part or all 

 of a growth flush. On the whole, a growth flush may be slight in 

 intensity and short in duration, or severe and prolonged. 



A growth flush commonly produces a growth layer. More specifi- 

 cally, a growth layer includes the xylem laid down by the cambium 

 during a single period of activity regardless of entirety around the 

 circuit, of completeness radially, of location within the plant body, or 

 of the particular time of the activity. 



VI. MULTIPLICITY OF GROWTH LAYERS 



INCIDENCE OF THE SINGLE ANNUAL 



Previous discussion contains many references to multiplicity — 

 multiplicity within annual increments from trees grown at the lower 

 forest border. By means of absolute dating the existence of such 

 multiplicity is established. Also the use of absolute dating permits the 

 collection of statistical data on the ratio of single to multiple annual 

 increments. 



Table 49 contains information on dated growth layers. Con T may 

 be used as an illustration. Within 41 branches (from 3 trees) there 

 are 49 years with a single growth layer per annual increment in all 

 sections of the particular branches for the particular years ; there are 

 7 years with both single and multiple growth layers along the same 

 branch ; and 44 years with multiple growth layers per annual incre- 

 ment in all sections of the particular branches for the particular years. 



The chief concern here is the relative incidence of single and mul- 

 tiple growth layers in the total picture. With 336 years bearing single 

 growth layers and 583 bearing multiples, the respective percentages 

 are 37 single and 63 multiple. The value of 63 percent is a minimum 

 because in 158 years some of the annual increments have multiple 



