EELATION OF WATEK TO THE PBOTOPLASM 63 



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 

 by an exosmotic flow. We may 

 think of it as a semipermeable mem- 

 brane as far as the organic contents 

 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. 53), at first at the corners and then all round the sides, 



Fia. 52. ADULT VEGET- 

 ABLE CELLS. x 500. 

 (After Sach*.) 



h, cell- wall ; p, protoplasm ; 

 k k', nucleus, with nu- 

 cleoli ; s s', vacuoles. 



