84 



THE PLANT CELL WALL 



orders, but its most important forms are probably the Bacillario- 

 phyceae, the diatoms. These organisms ordinarily contain pectin 

 and silica but rarely cellulose. The wall consists of overlapping 

 valves that fit as do the halves of a petri dish. The silicified layer is 

 laid down as an areolated or striated sheet, with patterns character- 

 istic of the genus and species. 



Table 6 



In the Phaeophyta, the cell wall commonly consists of a firm 

 inner cellulosic region and a gelatinous outer layer of polyuronic 

 acid and non-cellulosic polysaccharide. Unlike the green algae, the 

 phaeophycean polyuronide is calcium alginate, poly-l,4-a-D-galac- 

 turonic acid, an isomer of pectic acid. The non-cellulosic poly- 



