54 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I40 



DENSEWOOD STRINGERS 



It is not at all uncommon to find the densewood of growth layers, 

 either annual or intra-annual, consisting of as little as a single row, 

 or stringer, of cells. In fact, Arizona cypress is prone to have the 

 densewood of any one growth layer varying from a band many cells 

 thick on one radius to a thin band of one-cell thickness on another, or 

 the opposite radius. The lightwood may likewise vary in thickness 

 from many cells to one cell. Hence, a complete growth layer may 

 consist of one lightwood cell plus one densewood cell, or one dense- 

 wood cell only, which, in the nature of the case, lies immediately on 

 the outer margin of the densewood of the growth layer next interior. 

 (Many examples of very thin densewoods are to be seen on the plates. 

 See especially pis. 20, 21, 29, 30, and 35, fig. 2.) 



Instances are at hand where several such stringers are separated 

 radially from one another by one to three or more cells, one stringer at 

 least being the outer densewood of an annual growth layer. Stringers 

 of the type described pose several problems: recognition, dating, 

 cambial activity, and factors responsible. Only the first two are of 

 concern now. In order to "see" the stringers and recognize them as 

 such, the higher powers of magnification are necessary. Under low 

 powers some of them tend to merge radially into a hazy band whereas 

 others remain nearly, if not quite, invisible. Dating or the differentia- 

 tion of one annual increment from the other, is a problem of difficulty 

 equal to that of recognition. Some workers unconsciously consider a 

 growth layer to be annual if it has a robust development of lightwood 

 and densewood and they become perhaps unduly suspicious of an 

 atypical growth layer, at times so much so that they refuse to use the 

 sequence bearing it. Thus, a discarded specimen may carry ecologic 

 information of far greater importance than so-called typical speci- 

 mens. 



Many examples could be cited, in addition to the plates already 

 mentioned. The growth layer for 1938 in TTC 36-7-a has an irreg- 

 ular border and, in addition, its densewood varies from a thick to a 

 very thin band. In TRP 2-1-a, 23 cm. back from the tip, all dense- 

 woods are mere stringers of narrow cells which are difficult to resolve 

 under any power. Ten cm. outward, in TRP 2-1 -b, the densewood 

 zones are thicker and hence the contacts sharper. In XSC 6-3-a, 1939 

 is described in our notes as a sharp to diffuse, complete, entire growth 

 layer. Much of its densewood is an irregular stringer, a single cell 

 thick, which lies one to two cells out from the 1938 densewood. In 

 fact, the multiple densewood of 1938 plus the indefiniteness of 1939 



