RELATION OF WATER TO THE PROTOPLASM 638 
exert a fairly high osmotic pressure. Water consequently 
passes into the cell, at first only in such quantities as to 
distend it somewhat. As the process goes on, more liquid 
is taken up than can be stored in the molecular interstices 
of the protoplasm. 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 
protoplasm (fig. 51). These soon run together as the 
amount of water still increases, while the gradually in- 
creasing hydrostatic pressure stretches the extensible cell- 
wall and so enlarges the cavity. The 
growth of the protoplasm does not 
keep pace with this extension of the 
wall, and therefore 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. 
Not only does the protoplasm 
regulate the entry of substances into 
the cell, but it prevents their escape ia. 52—Aputr Vzarr- 
by an exosmotic flow. We may (ater goey — 
think of it as a semipermeable mem- A, cell-wall; p, protoplasm ; 
3 k k’, nucleus, with nu- 
brane as far as the organic contents cleoli ; ¢ s’, vacuoles. 
of the cell are concerned, though it is 
certainly much more complex than the term suggests. A 
simple experiment will illustrate this point. Take a cell 
of the coloured cortex of the root of the beet and put 
it into contact with a solution of higher osmotic pressure 
than that which is contained in its own vacuole; for 
instance, a solution of common salt of about 10 per cent. 
concentration. Watch its action on a slide under the 
microscope. As the salt solution reaches the cell, the 
protoplasm of the latter gradually retreats from the walls 
(fig. 58), at first at the corners and then all round the sides, 
