ECONOMIC WOODS OF THE UNITED STATES 137 



VESSEL MARKINGS: SPIRALS AND PITS 



The first-formed elements of the primary wood, those nearest 

 the pith, have walls characteristically marked with annular and 

 spiral thickenings. During the process of rapid elongation of the 

 stem these elements are stretched out, the spirals or rings sepa- 

 rated, and the thin, unpitted walls between the thickenings are 

 likely to be torn and broken down. Such elements comprise that 

 portion of the primary wood known as the protoxylem. The cells 

 of the primary wood subsequently formed make up what is known 

 as the metaxylem. The walls of the vascular elements of the 

 metaxylem are thickened in a scalariform (ladder-like), reticulate 

 (net-like), or pitted (dotted) manner. 



The vessels (and tracheids) of the secondary wood are pitted, 

 are without annular thickenings, and may or may not be spiral. 

 The presence of spirals is a valuable diagnostic feature, and the 

 vessels of smaller lumina exhibit them to best advantage. In a 

 given wood all of the vessels may bear spirals or, especially where 

 there is considerable variation in the size, only the smaller vessels 

 may be thus marked. Conspicuously large vessels are invariably 

 without spirals just as they are also without scalariform perfora- 

 tions. 



Spirals exhibit considerable variation in distinctness. In 

 some cases, as in Ulmus, they are very pronounced, in others, e.g., 

 Tilia, they are fine but distinct, and again they may be very fine 

 and indistinct, as in Magnolia. In some instances, as previously 

 stated, only the overlapping tips of the segments are spiral and 

 these are often indistinct. 



Tracheids which closely resemble vessel segments except in the 

 absence of perforations, have the same markings as the vessels. 

 Fibre-tracheids may also be spiral. This is normally the case 

 in Ilex and occasionally in certain Rosaceae, Ericaceae, and others. 

 The author has noted them in Arbutus and Arctostaphylos. Their 

 presence provides a valuable diagnostic feature. 



