THE SPRUCE FIR 



243 



pits. Their construction is shown in Fig. 99. Seen 

 in surface view, i.e. in a radial section through the 

 wood, each pit appears as two concentric circles (Fig. 

 99 , A). The meaning of the two circles only becomes 

 clear when we examine a pit in sectional view, such 

 as we see in either a tangential () or a transverse 

 (0) section of the wood. We now find that the pit, 

 like all other pits, is essentially a thin place in the 

 cell-wall. The thin 



A j C 



part of the wall or 



dosing membrane is 



over-arched on both 



sides by a circular rim, 



which projects over it 



so as to leave only a 



narrow opening in the 



middle of the rim. 



The closing membrane 



itself is not of equal 



thickness all over, 



but has a thicker 



part in the middle, 



called a torus (t in 



Fig. 9 9 , .Z? and C). We now see that in the surface view 



(A) the outer circle corresponds to the external edge 



of the projecting border, while the little circle in the 



middle is the opening in the border leading to the 



closing membrane. The structure of all bordered pits 



is similar, but they are most easily studied in plants 



of the Fir kind on account of their large size. 



We must remember that the tracheides are closed 



FIG. 99. Bordered pits of a Fir. A, 

 Part of tracheide with pit in radial 

 view. , Pit in tangential section : 

 t, torus. C, Transverse section of trach- 

 eide, showing pits : t, torus ; m, middle 

 lamella ; i, inner layer of cell -wall. 

 Magnified about 400. (After Stras- 

 burger. ) 



