254 XIX. STRUCTURE OF THE THALLOPHYTA. 



therefore recognisable as colourless balls in the general pale red 

 of the cell. Under the pressure of the dead outer plasmic mass 

 the vacuole is often constricted, or the dead plasm may peel off, 

 and in part lay the vacuolar membrane bare. This resisting 

 inner layer may often remain living for days ; de Vries has 

 named it tonoplast, as it is by it that the pressure of the cell-sap 

 is controlled. By a too rapid action of the saltpetre solution the 

 outer plasm may be killed without contraction, only the wall 

 of the vacuole remaining living ; and this may withdraw from the 

 dead plasmic masses and may contract itself into balls. Ordi- 

 nary plasmolysis may also be effected with 5 per cent, or weaker 

 solution of table salt. Protoplasmic threads not infrequently 

 connect the contracted protoplasm with the end-walls of the 

 cells. 



