32 



ECONOMIC WOODS OF THE UNITED STATES 



Intermediate forms exist whose reference to either group is 

 . arbitrary. 



A simple pit is one in which the thickening about a spot on 

 the primary wall forms a canal which is equally wide throughout 

 its length, or narrowing outward (Fig. 11, H). The length of the 

 canal is determined by the thickness of the secondary wall. When 

 simple pits occur in very thick-walled cells, there is often a tend- 

 ency to a slight funnel-formed enlargement of the canal toward 





cocoon 



Fig. 10. — Cross section of a wound area in Tsuga canadensis (eastern hemlock) 

 showing five traumatic resin ducts (tr. r. d.), in tangential row. Note thick-walled 

 epithelial cells (e), and occasional resin cells (r. c), showing sieve-like end walls. 

 Magnified about 150 diameters. 



the primary wall. Often the canal widens sufficiently to present 

 the appearance of a narrow border (Fig. 11, G). Seen in profile, 

 as in section, the pit canal of such a pit is narrow at the end 

 toward the centre of the cell, but widens gradually outward. 



A bordered pit is one in which the canal widens suddenly, 

 that is, with a distinct angle, toward the primary wall (Fig. 11, A). 

 In surface view a bordered pit appears as a bright spot or slit 

 within a circle or ellipse (Fig. 11, B). This outer circle marks 

 the limit of the unthickened area; the bright spot is the inner 

 opening or aperture of the canal; the zone between the two is 

 called the border. 



Pits, especially bordered ones, usually are paired on opposite 

 sides of the primary-cell walls. Pits between vascular elements 

 are invariably bordered; between parenchymatous elements, 

 invariably simple; between vascular and parenchymatous, they 

 may be simple, but more frequently are semi-bordered, that is, 



