THE INTERNAL STRUCTURES OF PLANTS. 199 



here and thin there. As the cell grows older its wall 

 thus increases in thickness and density, and often by 

 these changes opposes the passage of water through 

 it. 



Under the most powerful microscopes, no appear- 

 ance of porosity is seen in the primitive cell- wall. 

 But the breaks of its lining membrane make it thin 

 in some places and thick in others, and these broken 

 places appear as markings on the wall. When they 

 are minute and frequent, the cell-wall looks dotted, 

 and such cells are called dotted cells (Fig. 360). 



FIG. 360. FIG. 361. FIG. 362. 



"When the breaks are little slits, or bars (Fig. 361), 

 we have what are called, fenestrated cells. 



"When the internal membrane breaks up to a 

 considerable extent, the resulting fragments take on 

 various shapes of bands, rings, and spiral markings, 

 and so form what are called reticulated, annular, and 

 spiral cells (Fig. 362). 



In most cells that enter into t^ie permanent sub- 

 stance of a plant, the cell-wall continues to thicken 

 long after it has ceased to enlarge. Sometimes a 

 third and even a fourth layer is developed within the 

 cell. These layers are generally moulded exactly 

 upon the others, so that the thin places remain, and 

 the thick grow thicker. 



