84 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I40 



The term divided densewood is applied to that form of partial 

 growth layer wherein the densewood, in its normal transition outward, 

 is separated into two parts by the insertion of one or more tangential 

 rows of lightwood cells. Text figure 29a gives an idealized but none- 

 theless accurate drawing. In a way, divided densewood has greater 

 interest than the more clean-cut growth layers because it actually 

 holds a transitional position in the array of partial growth layers (pis. 

 5, fig. 2 ; 10, fig. I ; 13 ; 19, fig. 2 ; 23, fig. 2 ; 25 ; 31, fig. i). 



Perhaps a description of one of our early encounters with the type 

 will show how our attention was first attracted. In TTP 21-2-a, the 

 increment for 1939 contains an exterior lens immersed for over three- 

 quarters of its circuit in what would be called densewood under low 

 magnification. Passing outward the sequence is as follows: About 

 seven cells of decreasing width ; one to three cells whose radial widths 

 are greater than those immediately inward and outward; and last, 

 one to four narrow cells completing the proper densewood transition 

 and forming the outer part of the increment. Under low power the 

 so-called lens appears to have an indefinite inner margin. This in- 

 definiteness is not so much a matter of the feebleness of the outer 

 margin of the inner band of narrow cells whose outer margin is 

 actually very definite, as it is a lack of contrast between the narrow 

 cells of the inner band of densewood and the slightly wider cells of 

 the following lighter wood. This would bring physiological emphasis 

 upon the impact of growth factors initiating or accelerating cambial 

 activity, or maturation, after an interval of arrested or inhibited ac- 

 tivity. In the case of TTP 21-2-a, the separation of the densewood 

 into two parts may mean that growth-stimulating factors were actually 

 feeble although still fluctuating in amplitude, or that growth factors 

 were still operating on the tree but that the tree could not respond to 

 the degree that it had earlier in the season. 



Divided densewood undergoes various changes and transitions not 

 only from branch to branch but also along a single branch. In tree 

 TTC 33, and in other trees growing under much the same conditions, 

 the bands of densewood have a tendency to separate into two or more 

 bands, at some places the separation being barely apparent, at other 

 places becoming a discrete lens or entire intra-annual growth layer. 



The changes along a single branch are many and various. Com- 

 plexity within an annual increment increases either inward or out- 

 ward ; divided densewood expands inward or outward into lenses or 

 entire growth layers. Because of the nature of the xylem, our in- 

 formation is restricted to the gymnosperms. The following are a few 

 of the examples of appearance and expansion inward : In the 1937 



