44 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I40 



In TTL 5-1-a the outer margin of the xylem is partially smooth and 

 sharp and partially irregular, the irregularity being caused by prom- 

 ontories of mature xylem which protrude outward into the phloem 

 region. The cambium loops out around the promontories and perhaps 

 could still have been active because the sections were cut July 25, 1944. 

 However, many examples exist where the sections were cut during 

 the winter (see pi. 7, fig. i). Sections TTP 24-2-a, for instance, were 

 cut November 29, 1941, and show a highly irregular outer margin of 

 promontories and reentrants on the xylem (pi. 18, fig. 2). The cambial 

 zone has variable width, being smooth on its outer face and filling 

 the reentrants in the xylem with nucleated cells. All cambial cells are 

 large, nucleated, and full of cytoplasm; indeed, it scarcely seems they 

 can be in a resting condition. In the case of TTC 33-io-b (pi. 12, fig. 

 i), cut January i, 1943, and many other specimens of Arizona cy- 

 press, the irregularity of the outer surface of the growth layer is 

 much finer in detail. Single radial columns of xylem cells protrude 

 outward beyond the general surface. With a unified row of narrow 

 cells as a tangential baseline, contiguous radial columns support from 

 zero to four extra cells thus making the outer surface highly irregular 

 in detail. If the sections had not been cut but had been left to grow 

 another year, what would have been the appearance of the contact on 

 the outside of the 1942 increment? This would depend upon the na- 

 ture of the lightwood cells laid down upon and between the columns 

 of "postseasonal" cells; the lightwood cells could be normal, giving 

 a diffuse contact, or thick walled and heavily Hgnified, giving a "cur- 

 tain effect" or "reverse sequence" (pi. 23, fig. i). Examples of both 

 are common. If TTC 33-io-b had been cut off during the growing 

 season, we would unhesitatingly say that the cambium was dividing 

 actively. Such irregularities found definitely on the outer surfaces 

 of annual increments constitute simply one type of feature which can 

 mask or destroy the sharpness of an annual increment. 



If the xylem responsible for irregularities is formed after the close 

 of the normal growing season, it is called postseasonal growth. It may 

 be represented by an immature cell here and there, by a few widely 

 scattered mature cells, by local patches of cells, or by a layer of cells 

 entire tangentially but incomplete radially as a growth layer. Post- 

 seasonal growth, so far as we have observed, may vary from the 

 merest hint up to a nearly complete growth layer. In TTJ i-i-a and 

 i-i-b, cut January 11, 1940, scattered immature cells lie just under the 

 cambium. Outward on the branch, in TTJ i-i-d, the postseasonal 

 growth has been reduced to the merest hint of a cell or two. The sec- 

 tions of TTC 12-14 (pis. 10, fig. 2; II, fig. i), cut November 17, 



