1920] 



LANGDON—DIOON SPINULOSUM 



"3 





first pitted tracheids 5-6.5 mm., tracheitis in the vicinity of the 

 cambium 7-9.8 mm. 



The protoxylem elements are of the reticulate and scalariform 

 types, and in passing from the metaxylem to the first formed 

 elements of the secondary wood all transitional stages occur in the 

 reduction of the scalariform structure into imperfectly formed, 

 multiseriate, bordered pits. 



While the majority of the tracheids of the secondary xylem 

 exhibit on their radial walls the multiseriate type of pitting 

 so characteristic of this 

 wood, many of the 

 tracheidal elements, 

 especially those consti- 

 tuting the secondarily 

 formed wood in the 

 upper trunk, have their 

 radial walls covered with 

 small bordered pits of a 

 very irregular arrange- 

 ment. 



In the wood of the 

 lower trunk tertiary 

 spiral thickenings of the 

 tracheid walls were ob- 

 served occurring in the 



first few rings of growth FlG x . — ^Radial longitudinal section of trachc ids 



and also in the older from lower portion of adult trunk: s, tertiary 



wood (fig. 



These spiral thickenings of tracheid walls; p, wood paren- 



common 



chyma; X225. 



to all the tracheids, but 



are generally sporadic in their appearance and may be quitr 

 inconspicuous. In some cases, however, they are characterized by 

 considerable prominence, and are so compact as to suggest a reticu- 

 late rather than a spiral formation. 



lignified elements of 



wood 



narrow elongated cells with transverse walls, the longitudinal stor- 

 age parenchyma. These cells, like those of the radially disposed 



