108 THE CELL 



always quite continuous over the whole surface. At 

 certain spots the wall may not be thickened, so that 

 channels or pits are left, leading from the middle lamella 

 to the cavity of the cell. The pits of adjacent cells are 

 practically always formed opposite to one another. This 

 arrangement greatly facilitates the passage of water 

 and solutes from cell to cell. 



Osmotic Pressure and Turgor. What is the force 

 which causes the distension of the cell during its growth 

 from the embryonic to the adult condition ? It is the 

 pressure (often called " osmotic pressure ") caused by 

 the " attraction " of osmotic substances within the cell 

 for water, which is drawn into the cell in the effort 

 to establish equilibrium (cf. p. 57), and develops a liquid 

 pressure against the wall. We must suppose that the 

 vacuoles are initiated in the protoplasm by the local 

 concentration of an osmotic substance, e.g. .sugar, in 

 certain spots, perhaps within plastids. 1 Water is 

 attracted to these spots and more water enters the cell 

 from without to replace what is thus removed from the 

 general cytoplasm. As the pressure in the vacuoles 

 increases the cytoplasm is pressed against the cell 

 wall, which is itself progressively distended, just as the 

 inner tube of an inflated bicycle tyre is pressed against 

 the outer cover. If the cell were freely suspended in 

 water this process would continue till the wall burst 

 or till the elastic reaction of the wall counterbalanced 

 the distending force due to the attraction of the osmotic 

 substance for water. But when the cell is surrounded 

 by other cells similarly distended they all press against 

 one another and give rigidity to the whole tissue. 

 This condition of a tissue is called turgor or turgidity, 



1 In any case a semi-permeable membrane is formed round each 

 vacuole, whether derived from a plastid or formed in the endoplasm. 



