RELATION 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, 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. pie, 59 Apu VecrtaRLE 
Though the conditions of the first Sache x 500. (After 
experiment are approximated to, they ;, cenwall;-p, protoplasm; 
are not altogether realised. The ee ya ae nu- 
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 
