42 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I40 



5-2-a, its growth layer for 1938 bears a row of parenchyma cells along 

 its outer face where it is succeeded outward by an exterior lens. In 

 the sections of XSC 8-1 -a, parenchyma cells are associated with the 

 effects of natural frost. The frost and recovery of 1938 are followed 

 immediately by a band of densewood whose outer margin is set with 

 dark parenchyma cells. In the case of the 1938 increment of CMJ 

 2- 1 -a, the frost and recovery are in patches separated by cells of 

 densewood and of parenchyma. Such cells give a more or less false 

 diffuseness to otherwise sharp contacts. 



A touch of indefiniteness is added to contacts in species normally 

 considered to be lacking in terminal parenchyma cells by the existence 

 of green-stained cells along or just within the contacts. Some of these 

 green cells, which were alive at the time of preparation, are mis- 

 shapen, some crushed, some collapsed, and some cupped inward. In 

 TTC 34-1, the outer margin of 1938 gives an excellent example of 

 green crushed cells. Furthermore, the outer margin of the densewood 

 is irregular and gives the impression that many of its cells failed to 

 mature when 1938 growth stopped or that extra cells were added 

 here and there after the normal densewood was completed at the end 

 of the season. Studies of densewood immediately interior to the 

 cambium show that both of these alternatives do occur. The phe- 

 nomena are more decided in 1937 of TTC 34-4-a. Here the outer 

 margin consists of green collapsed cells followed by one to two rows of 

 rather narrow, heavily lignified, thick-walled cells whose outer border 

 is decidedly irregular. It may be that some growth was occurring 

 after the regular growth layer had been finished; in other words, 

 these features appear to indicate certain conditions at the season's 

 end. The above described features, in part probably frost effects on 

 immature (nonlignified) cells, mask or destroy the characteristics 

 normal to densewood. Sharp contacts are replaced by diffuse con- 

 tacts, and high power serves merely to reveal the cause of the dif- 

 fuseness. 



Features that are interior to the margin and that give diffuse con- 

 tacts have chiefly to do with the failure of one or more of the char- 

 acteristics of densewood, such as failure of the cells to remain narrow, 

 failure of normal lignification, or failure of wall thickening. Matura- 

 tion is held in abeyance or takes place irregularly. In TTC i-ii-d (pi. 

 6, fig. 2), for instance, the densewood of the outer growth layer of 

 1943 is feeble and intermittent — intermittent in the sense that radial 

 columns of cells which show no indication of increasing narrowness 

 outward — alternate with radial columns which do show normal in- 

 crease in narrowness. 



