CELL-MEMBRANES. 



57 



or on both. The prominences appear as knobs, spines, hairs, 

 ridges, etc., which may be quite irregular or may show definite 

 arrangement (Fig. 25). In the elongated cells of certain parts of 

 wood the prominences have the form of ridges, which project 

 from the inner face of the membrane as rings, spiral bands, or 

 reticular networks, as shown in Figs. 26, 27. A like diversity 

 exists in the form and arrangement of the depressions in the cell- 

 membrane. They often appear as rounded pits, which may extend 



FIG. 27. (After Sachs.) Cells 

 from a liverwort {Marchan- 

 tict), showing thickenings 

 on the inner face of the 

 membrane; s, s, rhizoids 

 (i.e , root-like structures) 

 with the thickenings in the 

 form of irregular spiral 

 bands; r, cell from the ul- 

 terior of the plant with ir- 

 regular net-like ("reticular") 

 thickenings; e, peculiar elon- 

 gated cells (elaters) from the 

 spore-cases, showing very 

 regular spiral thickenings. 



FIG. 28. (After Sachs.) Elongated 

 wood-cells ("pitted tracheids") from 

 ordinary pine-wood, showing round- 

 ed pits and holes in the membranes. 



completely through the cell-membrane so as to form actual per- 

 forations or pores (Fig. 28). In very thick-walled cells (see Fig. 

 4) such pores form branching canals running through the walls. 

 The pits or pores may be elongated so as to become oval or slit- 

 like, as shown in Fig. 29, and in this case may be either trans- 

 verse or oblique to the long axis of the cell. 



In short, there is scarcely a limit to the variations in number. 



