NO. I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL. II5 



growth factors have any influence at all on the physiological processes, 

 then those processes must respond in nature and, to a certain degree, 

 to the fluctuations of the factors as they exist in the lower forest bor- 

 der. To expect otherwise, is to assume that no factor becomes limiting 

 after growth has begun in the spring until growth stops finally at the 

 end of the growing season. 



Classification in its simplest form permits the descriptive designa- 

 tion of growth layers in two dimensions, on cross section or on 

 longitudinal section. In its less simple form, classification emphasizes 

 in three dimensions the rather complex nature of differential cambial 

 activity. Three situations will be summarized. 



(i) All types and classes of growth layers appear to merge into 

 each other in three dimensions. This elementary circumstance cannot 

 be too strongly emphasized. Longitudinally, either inward or outward 

 on a branch, divided densewood may become a lens and the lens may 

 become an entire growth layer. An arc may become an entire growth 

 layer. A half-lens may become a lens. A lens may be a "patch" of 

 xylem; or a concurrent lens on a certain cross section may be the 

 "fingers" of an entire growth layer on a different cross section (text 

 figs. 34, 35). Such transitions take place within rather short distances. 

 Therefore, on the basis of one or several cross sections it is unwise 

 to state that a growth layer exists as a lens covering but a tiny per- 

 centage of the possible area of the plant body. Growth layers, also, 

 possess all manner of transition between lightwood and densewood, 

 both tangentially and longitudinally. The outer margin of a diameter 

 flush, whether entire growth layer, lens, half-lens, or arc, may grade 

 from sharp through definite, indefinite, to diffuse, and this last may 

 become so diffuse as to be indistinguishable from lightwood. In truth, 

 these gradations apply to all manner of growth layers, entire, par- 

 tial, annual, or intra-annual. 



(2) The above transitions may be thought of as applying to the 

 grosser forms of growth layers — to the more obvious results of inter- 

 mittent cambial activity. Beyond these, but of course joined to them 

 by transitional forms, there are the indications of slight, more or less 

 incipient, changes in cambial activity and growth processes. Here be- 

 long interrupted densewood, interrupted lightwood, and even some 

 cases of divided densewood. A single tangential row of narrow cells 

 (at places so dense as to merit the term "stringer") may be immersed 

 in lightwood, or immersed in other but less decided densewood. On 

 occasion these rows are repeated rhythmically. The conclusion seems 

 warranted that cambial activity and growth processes under lower 

 forest-border conditions do not remain constant in rate over long in- 



