ECONOMIC "WOODS OF THE UNITED STATES 



33 



bordered in the vessel or tracheid, and simple in the adjacent 

 parenchyma cells (Fig. 11, F). Pits in typical wood fibres are 

 simple and slit-like, and usually in oblique position (Figs. 11, K; 

 2, B) . In many cases, however, where the fibres resemble tracheids 

 their pits are more or less bordered. The fibres of the bast have 

 only simple pits. 



The shape of the border is commonly circular, but may be 

 oval, lenticular, oblong, or, in the case of dense aggregation, polyg- 

 onal. Scalariform markings found on the vessel walls in certain 



p.c.w. 



P.c.w. p.c.w. 



c.w. °- w - 



Fig. 11. — Schematic representation of pits, greatly enlarged. A, section of 

 bordered pit showing cell walls (c. w.), primary cell wall (p. c. w.), pit canal (c), 

 torus (0 ; A', the same with torus (t) shoved to one side and lying lid-like against 

 the aperture of the pit canal; B, surface view of bordered pit shown in A or A', 

 showing aperture (a) and border (6) ; C, surface view of bordered pit with lentic- 

 ular aperture (a) , the crossed appearance being due to the fact that the apertures 

 on opposite sides of the pit are shown; D, surface view of a bordered pit with 

 slit-like aperture (a) , common in thick-walled tracheids of late wood in gymnosper- 

 mous woods; E, surface view of scalariform bordered pit with narrow, elongated 

 aperture (a) and border (6) ; F, section of a semi-bordered pit showing border on 

 one side only; G, simple pit with funnel-formed canal and appearing slightly 

 bordered in surface view; H, ordinary simple pit with canal (c) uniform or narrow- 

 ing outward (i.e., toward primary cell wall) ; K, surface view of slit-like pit com- 

 mon in wood fibres. 



woods (i.e., Magnolia [Plate VI, Fig. 3], Hamamelis and Liquid- 

 ambar in part) are merely much-elongated bordered pits which 

 appear as horizontal clefts with only narrow portions of the wall 

 between them (Fig. 11, E). 



The pit cavities of two adjacent pits are separated by the 

 primary wall which persists as a limiting membrane (Fig. 11, p.m.). 



