EELATION OF WATEE TO THE PEOTOPLASM 57 



than can be stored in the molecular interstices of the proto- 

 plasm. Drops consequently appear, and these gradually 

 run together until a distinct though small vacuole, and 

 later a number of such vacuoles, are apparent in the proto- 

 plasm (fig. 51). These soon run together as the amount of 

 water still increases, while the gradually increasing hydro- 

 static pressure stretches the extensible cell wall and so en- 

 larges the cavity. The growth of the 

 protoplasm does not keep pace with 

 this extension of the wall and there- 

 fore after a time the protoplasm 

 forms a layer round the cell-wall, 

 enclosing a single large cavity in 

 which the surplus liquid is held 

 (fig. 52), the hydrostatic pressure of 

 the latter pressing the living sub- 

 tance against the wall. 



But, as has been said already, the 

 process is not a simple physical one. FlG- 52 ._A DU L T VEGETABLE 

 Though the conditions of the first g: x 50 - < After 

 experiment are approximated to, they ^ J^. ^ protoplasm; 

 are not altogether realised. The * *. nucleus, with nu- 



~ _ _ . cleoh ; s s , vacuoles. 



syrup in the bladder finds its repre- 

 sentative in the osmotic substances formed by the proto- 

 plasm and dissolved in the water in its substance ; the 

 water outside the cell is much the same as the water in 

 the outer vessel. But there is a great difference in the 

 membrane. The bladder of the experiment is replaced by 

 a film of cell-wall substance, which we may speak of in 

 general terms as cellulose, and this is lined by a delicate 

 coating of protoplasm. This again is not homogeneous, 

 but has on its surface, which is adpressed to the cell- 

 wall, a very thin dense layer which forms a kind of 

 membrane known as the ectoplasm. As soon as the 

 vacuole is recognisable its cavity becomes lined by another 

 similar membrane, and between the two lies the nearly 

 homogeneous protoplasm. These plasmatic layers are 



