92 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I40 



Many of the same features made up of densewood cells exist in 

 gymnosperms, in normal lightwood, and in compression wood. Their 

 common presence in compression wood seems to give a highly sensi- 

 tive indication of fluctuating growth conditions. Furthermore, rhyth- 

 mical alternations by a multiple succession of arcs and complete cir- 

 cles of narrow cells appear to characterize the xylem in certain trees. 

 Hence, all degrees of growth slowdown or even of cessation appear to 

 exist on one radius, on several radii, or even entirely around the cir- 

 cuit. The 1938 increment of TTC 30-i-a contains arcs and lenses of 

 densewood and compression wood plus several zones of narrow cells, 

 so that the growth layer appears to have had rhythmic growth through- 

 out the season. Also, the growth layer contains an alternation of 

 compression wood and lightwood which is not necessarily coincident 

 with the bands of narrow cells. 



The term curtain, or densewood curtain, refers to a zone of dense- 

 wood, or of highly lignified cells, at the start of a growth layer which, 

 therefore, does not have the simple, normal sequence of lightwood and 

 densewood (text figs. 32, 33; pis. i, fig. 2; 9). The lightwood of the 

 growth layer follows the curtain outward radially. In constitution, 

 the curtain may be either a band of uniform densewood cells within 

 the range of uniformity applied to densewood, or it may have varia- 

 tions in thickness of wall and in width of lumen. These variations may 

 be radial or tangential in the same curtain. Followed tangentially, the 

 typical definite outer margin of a curtain may become either a sharp or 

 diffuse band of densewood or the curtain may either disappear or 

 change into a genuine growth layer with a threadlike densewood. A 

 curtain may disappear by changing laterally into normal lightwood. 

 Again, it may disappear by its outer portion becoming typical dense- 

 wood with a sharp margin and its inner portion becoming lightwood ; 

 thus the curtain changes laterally into a sharp-margined intra-annual 

 growth layer. On the same circuit and within the same annual incre- 

 ment, a curtain may show all phases. Growth-layer interpretations, 

 based upon individual radii from several places on a section, would be 

 hard pressed indeed for an explanation of the variation among the 

 radii in absence of the complete section. 



The curtains we have encountered occur almost exclusively in the 

 Arizona cypresses from the grounds of the Texas Agricultural Ex- 

 periment Substation. In XSC 3-1-b the increment for 1935 contains 

 five complete, entire growth layers, the fourth of which has, at the 

 start, a curtain of cells considerably narrower and more heavily lig- 

 nified than those immediately to the exterior (text fig. 32). The 

 complete growth layer, in fact, appears to possess a two-way radial 



