268 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I4O 



have been interpreted as one growth layer. The maples, TTM 2 and 3, 

 possess densewood zones so indefinite or so intermittent around the 

 circuit of cross sections that one would hesitate to call the growth 

 layers annual increments were it not for the fact that the branches 

 had been measured periodically. 



(7) Growth is gradually resumed after densewood has formed. 

 This commonly occurs only over a part of the circuit. For instance, 

 six annual increments in TTC 5-7 have short arcs of indefiniteness 

 except in sections b where 1938 is indefinite for more than one-half its 

 circuit. The contact for 1938 is diffuse over those arcs radially op- 

 posite to the sharp portions of adjacent annual increments. This habit 

 of sharpness opposite diffuseness characterizes intra-annual, as well as 

 annual, growth layers — it is widespread throughout all materials 

 studied. 



(8) The outer part of the densewood is complicated by immature 

 crushed or cupped cells mingled with, or followed by, fairly typical 

 densewood cells — as if an autumn freeze had caught postseasonal 

 growth during formation. 



(9) The densewood of an annual increment may be a heavy band 

 many cells thick on one radius and a thin stringer one cell thick on the 

 opposite. The Arizona cypress has given many examples of such. In 

 fact, such variation characterizes not only the densewood but also the 

 lightwood. The complete growth layer may consist of one row of 

 Hghtwood cells and one of densewood cells ; or, as described hereto- 

 fore, it may consist merely of one row of densewood cells. In XSC 

 6-3-a, cut February 22, 1941, 55 cm. from tip of branch, the following 

 sequence was charted : 1938, multiple densewood ; 1939, one to two 

 rows of lightwood cells and one of densewood. This causes an in- 

 definiteness and uncertainty which would make dating impossible 

 were it not for the 1938 frost injury and the known cutting date. As 

 a feature defining the radial margin of a growth layer, a thin 

 "stringer" of densewood is particularly unconvincing as the outer 

 border of an annual increment. This is especially true when the 

 growth layer is viewed under low magnification. Thin "weak" dense- 

 wood is restricted neither to annuals nor to intra-annuals. If it were 

 not for the methods of absolute dating, most of our specimens would 

 have to be discarded. 



(10) A sharply bordered densewood band may be followed out- 

 ward by lightwood cells which are atypical in size and wall thickness. 

 Illustrations exist among Arizona cypresses and ponderosa pines of 

 Lubbock and New Mexico. 



