THE CELL-WALL. 



493 



ultimately at particular points, as at the contiguous end-surfaces 

 of those cells which become fused together to form vessels or 

 ducts ; and in the case of the layer closing the outer ends of the 

 canals of the pits or wood-cells, a similar destruction of the 

 primary membrane seems to occur. A phenomenon of this kind is 

 distinctly presented in the large spiral-fibrous cells of Sphagnum 

 (fig. 544), where the walls of old cells are found perforated by 

 large round orifices, produced by the separation of circular pieces 

 of the cell-wall, and in the cells of the leaves of Leucobryum 

 glaucum (fig. 545). In the cells of the Confervoids producing zoo- 



Fig. 545. 



Fig. 544. Cell of the leaf of Sphagnum cymbifoUum,mth annular fibres and orifices in the 



wall. Magn. 400 diam. 

 Pig. 545. Porous cells of the leaf of Leucobryum glaucum ; vertical section. Magn. 



400 diam. 



spores, the wall breaks open at definite places to allow these to 

 escape, exhibiting small lateral or terminal orifices in Conferva 

 (fig. 512, C, d) &c., or breaking quite across by a circular slit in 

 (Edogonium. In this last genus the cell-wall breaks across in the 

 same way in cell-division, to allow the new cells to expand ; and in 

 one of the Palmellece {Schizochlamys) the wall of the parent cell 

 splits off in segments every time a new generation of cells is 

 formed. In the case of the clathrate cells and sieve disks pre- 

 viously mentioned, the bounding membrane ultimately becomes 

 obliterated so as to allow of the passage of the protoplasmic con- 

 tents of one cell into the cavity of the adjoining cell. If a radial 

 section be made of a latticed cell in the stem of a Vegetable Marrow 



