260 



A TEXT-BOOK OF BOTANY. 



Thickening or Marking of Walls. ^In the formation of 

 the wall each cell appears to work in unison with its neighbors 

 for the building up of the plant. The thickening of the walls 

 of the cell is primarily for the purpose of strengthening the walls, 

 but if the walls were uniformly thickened, osmosis, or the trans- 

 ferral of cell-sap from one cell to another, would be hindered. 



FIG. 131 . Phytomelane, an intercellular carbon-like substance occurring on the outer layers 

 of the stone cells in Brauneria pallida (Echinacea angustifolia) . 



Thus we find that the contiguous walls of the cells are thickened 

 at definite places opposite each other, leaving pores or canals 

 which permit rapid osmosis. The pores thus formed are known 

 as simple pores, and when seen in surface view are somewhat 

 elliptical or circular in outline, and may be mistaken for some of 



