120 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I40 



Table 48. — TTP 20-4 



At 34 cm. in TTP 20-4 (table 48), the cambium had a few good 

 cells and many completely plasmolyzed. Outer xylem cells were partly 

 immature ; densewood was poorly developed at places. At 19 cm. the 

 cambium had a few good cells and many plasmolyzed cells. Outer 

 xylem cells were partly immature ; densewood graded from normal to 

 weakly developed to absent. 



The increment for 1939 in TTP 20-5, cut February 29, 1940, con- 

 sists of a short, very thin lens on the short radius whose outer margin 

 varies from sharp to diffuse. Actually, the densewood is so poorly 

 developed that, if the section were to have been cut several years later, 

 1939 would have been a diffuse lens were it identifiable at all. The 

 cells of the cambium are for the most part empty or plasmolyzed 

 around the entire circuit, the exception being the cells immediately 

 over the 1939 growth layer. As a summary of TTP 20-1, 2, 3, 4, and 

 5 : the cambium was in better condition in those sections where the 

 growth layer is entire; and where it is a lens, the cambium was in 

 better condition over the lens than elsewhere. The first conclusion to 

 come to mind is that there is an explanation of lenses — dead cambium. 

 Such, however, would entail a great many complex physiological 

 processes in the repeated local death and regeneration of the cambium, 

 necessitated by many of the sections hitherto used as illustrations. 



No attempt will be made here to explain reaction or compression 

 wood. So far as classification and multiplicity are concerned, the ef- 

 fects of compression wood were taken into consideration or else the 

 sections were not used. Compression wood seems to be a much more 

 sensitive recorder of small-amplitude fluctuations in cambial activity 

 than ordinary xylem. In many instances the presence of the abnormal 

 cells at the start of a growth layer masked the nature of the contact 

 unless high power was used. Instances exist where the complete 

 growth layer was composed of compression wood except for the 

 outer two or three rows of cells which were normal. 



DEFINITION OF A GROWTH LAYER 



A consideration of growth-layer types, of their intergradations in 

 three dimensions, and of their relationships to physiological processes 

 reveals that it is no simple matter to define a growth layer. The defini- 



