16 



OF WOOD IN GENERAL. 



to almost obliterate the cavity or lumen, as it is termed. Their 

 walls are generally at least partially lignified and give a reddish 

 colour with Schulze's solution, and the thickening is absent from 

 some spots on their walls. These unthickened spots are known 

 as pits. Pits, which are important as occurring also on some of 

 the elements that make up wood, are of two main classes, simple 

 and bordered. A simple pit is a spot at which a cell-wall is left 

 unthickened, generally on both sides, each successive thickening- 

 layer leaving the same space uncovered. It appears accordingly 

 as a bright spot on the wall ; or, if in section, as a canal, the 

 length of which depends upon the thickness of the wall. A 



Ai 



Ill 



A2 



FIG. 9. Pits. A, Simple pit; A\, in tangential longitudinal section ; A'2, 

 in surface view. B, Bordered pit ; .81, in tangential longitudinal section ; 

 JJ'2, the same, with the middle lamella thrust to one side ; B3, in surface 

 view ; B4, in semi-profile. 



bordered pit is so-called because the bright spot appears surrounded 

 by, or crossed by, a second circle or ellipse. The structure will 

 be best understood from the diagrams (Fig. 9). In the thicken- 

 ing of the cell-wall the area of the outer circle is at first un- 

 thickened, but successive layers of thickening overlap this 

 unthickened area more and more so as to make a short canal 

 broad at the end near the original cell-wall and narrow at the end 

 towards the centre of the cell. Subsequently a slight thickening 

 termed the .torus forms in the centre of the unthickened area. 

 Pressure of liquid on one side of the pit-membrane often forces it 

 against the "border," in which case the torus does not completely 

 occupy the opening in the border or inner circle. The whole 

 mechanism has been compared to a laboratory filter, the border 

 being the funnel that acts as a support, the unthickened mem- 



