138 



ECONOMIC WOODS OF THE UNITED STATES 



TABLE IX 



INDIGENOUS WOODS WITH SPIRAL MARKINGS IN PART OR IN ALL OP 



THE VESSELS 



The vessels of secondary wood are always pitted. (See PITS, 

 p. 31.) This feature is seen to best advantage in macerated 

 material, especially where the vessels are so large that most of the 

 wall is cut away in sectioning. The nature of the pitting is de- 

 termined by the contiguous elements. The number, form, and 

 arrangement of the pits on a given area of wall depends upon the 

 particular kind of cell in contact there and the breadth of the sur- 

 face of contact. The character of the pitting between adjacent 

 vessels and between vessels and ray parenchyma is the most im- 

 portant for diagnostic purposes. 



Pits between vessels are invariably bordered. The features 

 worthy of special notice are the arrangement of the pits, the size 

 and contour of the border, and the nature of the pit mouths. It 

 is very common to find vessels in groups so compressed that the 

 walls of mutual contact are flattened out broadly. In walls thus 

 flattened it is not uncommon to find pits that are greatly elongated 



