SCULPTURE OF CELL- WALLS. 



93 



broad, then the meshes appear as pits, as is clearly seen in the so-called pitted 

 vessels. ' 



In these cases of sculpture the thicker portions of the cell-wall necessarily 

 project inwards, when the cells are tightly closed in on all sides by other 

 cells. In isolated cells, on the other hand, as the pollen-grains of the Phane- 

 rogams and the spores of the Cryptogams, the thicker parts of the cell-wall project 

 towards the exterior, and present the most various forms of knobs, cones, prickles, 

 combs, ledges united into net-works, and so forth. Generally the whole outer surface 

 of such cell-walls is cuticularised, and the sculptured parts mentioned consist also 

 of cuticular substance. 



