290 



MANUAL OF BOTANY 



cells or fibres which form part of the soft portions of the 

 vascular bundles of the higher plants [fig. 626). 



It generally happens that when cell-walls are being thick- 

 ened, the new deposits of cellulose are not laid down uniformly 

 over the surface, but instead form ridges or prominences of 

 various patterns, with unthickened portions between them. The 

 thickening may take the form of a spiral band, winding 

 obliquely round the interior of the cell ; it may be deposited in 

 the form of rings placed regularly along its greater axis ; or it 

 may be irregularly laid down, giving rise to particular patterns 

 upon the wall. These forms are known as spiral, annular, and re- 

 ticulated thickenings respectively [figs. 632-634). In other cases 

 the thickening may extend over the greater surface of the wall, 

 leaving only small imthickened spots. Such a cell- wall is said 



Fig. 637. 



Fig. 637. Cells whose walls are in px'ocess of tliiekening. The thick and 

 thin plaees on the separating wall correspond in position, p. Pits or thin 

 places, a. Thickening deposit. 



to be. pitted. "When two cells side by side are undergoing 

 thickening simultaneously, the thick and thin places on the 

 separating wall correspond in position, and the pits appear as 

 delicate canals which in some cases serve as means of communi- 

 cation from cell to cell {figs. 625 and 637). 



In the cell-walls of the wood-cells of certain trees we lind, 

 in addition to the ordinary pits, large circular discs which 

 encircle them so that each pit looks as if it had a ring surround- 

 ing it (fig. 638) ; hence such cells have been termed cells with 

 bordered pits or disc-bearing wood-cells. This appearance is 

 produced by circular patches of the cell- wall remaining thin after 

 the general thickening has commenced and the rim growing 

 obliquely inwards, leaving ultimately only a narrow orifice in 

 the centre ; or, in other words, the opening of the pit into the 



