CELLULAR TISSUE. 25 



20. Although these pits and chinks are not real pores or 

 openings in the cell-wall, yet they often become so with age, 

 by the breaking away of the thin primitive membrane after 

 the cell has lost its vitality. 



21. By what power the sedimentary matter left on the sides 

 of such tissue as this is prevented from choking up the pits 

 and' chinks is at present unknown. The sedimentary matter 

 left by the evaporation of the 'fluid contents of the cells is 

 certainly not deposited mechanically, as in the ordinary cases 

 of evaporation ; but it appears to be controlled in its dispo- 

 sition by the higher vital and physiological action in the cell- 

 walls. 



22. Sometimes, however, the secondary deposit itself is 

 restricted to a delicate thread, which either occurs in rings or 

 fragments of rings, or winds about the interior of the cell in a 

 spiral (Fig. 5). The delicate membranous walls of such cells 



Fig. 5. 



are sometimes ruptured at maturity, by the elastic expansion 

 of the spiral fibre within them. This takes place in the 

 fructification of the hepatic mosses, which consists of spindle- 

 shaped cells inclosing spiral threads called elaters, which, by 

 their elastic expansion, burst the cell-walls and scatter the 

 3 



