THE CELL-WALL. 99 



Markings. — Sometimes the cell-wall is of uniform thick- 

 ness throughout, but at other times its thickness varies 

 at different points, so that there result several markings. 

 In the case of isolated cells or of free cell- walls, the 

 markings are produced on the outside of the wall, but in 

 that of united cells they are formed on the inside. In the 

 latter, the markings are sometimes definite ; among them 

 some result from hollows made on the internal surface of 

 the cell-wall, and. others from prominences on the same 

 surface. 



_ _ ( Those resulting from hollowB. 



Markings-^ „, ... % 



{ Those resulting irom prominences. 



Among the markings resulting from hollows, some are 

 caused by true perforations, and others by canals opening 

 only at that end which is turned inwards. 



Markings resulting 

 from hollows 



Those resulting from true perforations. 

 Those resulting from canals opening 

 only at one end. 



Even the canals opening only at one end become often 

 completely perforated in the coarse of growth: 



Those resulting from true perfora- Pig. 111. 



iions are termed Sieve Markings (Fig. 

 111). There are some cells which 

 have markings much resembling the 

 above but not perforated; these are 

 called Latticed Cells. 



Among the markings resulting from Pia. ill.— Cell with 



, , -i sieve marking from the 



canals opening only at one end, some stem of the T6n a SU 

 consist of tubular canals and appear (GucurUtaPepo). 

 as bright spots when seen on the external surface ; some 

 of almost hemispherical canals widened at their base and 



