90 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL, I40 



transition outward, is separated into two parts by the insertion of one 

 or more tangential rows of densewood cells narrower than those im- 

 mediately to the interior and exterior (pis. i6, fig. 2; 18, fig. i). In 

 other words, the normal outward transition from typical lightwood to 

 the growth-layer boundary is interrupted by the narrower cells as 

 shown in text figure 31a and plate 19, figure 3. 



What interrupted densewood means physiologically is not clear. 

 Cell enlargement stops suddenly and maturation appears to have a 

 concentrated effect. In any event, the factors permitting growth were 

 given a sudden but temporary check. 



Inward some 12 cells from the outside of the 1940 growth layer in 

 TTP 24-17-a, there is an arc of narrow cells coming as an interrup- 

 tion to the normal succession of densewood cells. The arc has become 

 an entire circle in sections h. In the 1940 increment of TTP 24-16-a, 

 47 cm. from the tip of the branch, several arcs of narrow cells exist ; 

 in sections h, 35 cm. from tip, and in sections c, 26.5 cm. from tip, 

 the arc of narrow cells has lengthened into a band extending com- 

 pletely around the circuit as interrupted densewood. These examples 

 could, of course, be multiplied. 



The densewood of angiosperms appears to be marked by intermit- 

 tency; that is to say, the densewood exists as patches, the gaps being 

 filled with tracheids typical of lightwood. Whether the densewood be- 

 longs to an annual or to an intra-annual makes no difference. Silver 

 maple and Siberian elm, among the angiosperms used in our work, 

 show these gaps as a common feature. 



This sudden insertion of cells which failed to enlarge but are "over- 

 matured" relative to their immediate surroundings may, as a matter of 

 fact, occur anywhere throughout a growth layer and, if within the 

 compass of the lightwood, they are termed interrupted lightwood (text 

 fig. 31b; pis. 2, fig. 3; 3, fig. 2; 10; II, fig. 2). These cells may be 

 disposed as circles, arcs, patches, and isolated cases of abnormally 

 narrow, thick-w^alled cells in one or two rows. Certain angiosperms are 

 especially prone to such. Cross sections of the Siberian elm habitually 

 possess arcs, stringers, bands, and patches of narrow cells. All growth 

 layers of TTE i-i-a, for instance, have stringers of narrow rectangu- 

 lar cells which contrast strikingly with the irregular polygonal tra- 

 cheids elsewhere in the growth layers. These cells cannot be distin- 

 guished from ordinary narrow cells of densewood. The sections of 

 TTE i-2-a give many hints of lenses that cannot be sharply delimited. 

 High magnifications obscure general relationships, and very low 

 magnifications mask cell contrasts. 



