EELATION OF WATER TO THE PROTOPLASM 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, and 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). 



But, as has already been said, the 

 process is not a simple physical one. 

 Though the conditions of the first 

 experiment are approximated to, they 

 are not altogether realised. The 

 syrup in the bladder finds its repre- 

 sentative in the osmotic substances 

 formed by the protoplasm and dis- 

 solved in the water in its substance ; 

 the water outside the cell is much 

 the same as the water in the outer 

 vessel. 



erice 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 plasmatic membrane. 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 mem- 

 branes are immediately formed as soon as the surface of 

 the protoplasm comes into contact with water. The mem- 

 brane of the cell, therefore, through which osmosis must 

 take place, is composed of four different layers. In the 

 experiment we have assumed that the outer liquid was pure 



FIG. 52. ADULT VEGETABLE 

 CELLS. x 500. (After 

 Sachs.) 



But there is a great differ- &>ce u.waU; P , protoplasm; 



cleu 



k k, 

 cleol 



nucleus, with nu- 

 s s', vacuoles. 



