144 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL, I40 



(i) TTP 20-23, among many others, gives absolute proof of two 

 sharp, complete, entire growth layers for one year, 1944, by exact 

 measurement of tip growth both before and after the growth oc- 

 curred. In TTP 20-22, one of the 1944 growth layers is a sharp lens. 

 If one used sharpness of margin as a criterion of annual character, 

 or if one "counted" growth layers for one reason or another, he would 

 have three "annual" growth layers formed in two years. 



(2) TTP 20-23 demonstrates that a growth layer does not neces- 

 sarily have to stop its growth at a certain time and mark that cessa- 

 tion by cells with a certain standard set of features such as narrow, 

 thick-walled, and heavily lignified. Had this branch been cut off the 

 tree two years later than it actually was, would the outer margin of 



1944 increment have been identifiable? 



(3) So-called "postseasonal growth" varies in amount all the way 

 from an isolated cell here and there to an entire but incomplete growth 

 layer and even to an entire, complete growth layer. Any one of the 

 variations leads to confusion and actual uncertainty as to the exact 

 position of the outer margin of the annual increment. In TTC 12-14-a, 

 at 178 cm., the outer border of 1945 (branch cut off November 17, 

 1945) is irregular because some of the radial rows of cells protrude 

 outward farther into the cambial region than the row or rows immedi- 

 ately adjacent. Such irregularity is common. The outer border of 



1945 in sections b (pis. 10, fig. 2; 11, fig. i), at 147 cm., is highly 

 irregular, partly sharp and partly diffuse by locally added large- 

 lumened, thick-walled cells. In sections /, at 27.5 cm., the outer mar- 

 gin is made strikingly indefinite by added large-lumened, thin-walled, 

 immature cells. At 16 cm., in sections g, the outer border is once 

 again only slightly irregular, as it is 178 cm. from the tip of the 

 branch. These different types and amounts of "postseasonal growth" 

 would have been identified as having been formed the following year 

 if the branch had been cut off a year or so later, or would have 

 masked the true position of the annual contact. Another point is 

 brought out by the immature postseasonal growth of TTC 12-14-f. 

 If the branch had not been cut off when it was, the immature cells 

 either would have been lignified the next season or would have re- 

 mained immature and thus resembled many observed cases of zones 

 of immature cells extending partially or completely around the circuit. 



Hidden annual contacts and misdated xylem add a factor of un- 

 certainty to any studies based upon exact measurement of xylem sup- 

 posedly formed in any one year. 



