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Walter Stiles 
permeable, a state of affairs rarely realisable in practice. When such 
a cell is immersed in a solution it is assumed that the latter per¬ 
meates the cell wall so that the concentration of the solution in the 
cell wall is the same as that in the external solution. If the cell wall 
were absent, or if it were indefinitely extensible but completely in¬ 
elastic, water would pass across the semi-permeable membrane until 
the difference in osmotic concentration on the two sides of the semi- 
permeable membrane is zero. With water or very dilute solutions 
forming the external medium, water will pass into the cell in some 
cases until the latter bursts owing to the attenuation of the proto¬ 
plasm, as we have already noted in the preceding chapter. 
But with the presence of the elastic cell wall another force comes 
into play. We have already noticed that as the cell swells with 
absorption of water, the stretched cell wall exerts an inwardly 
directed pressure increasing with increase in stretching of the wall, 
and opposing the passage of water into the cell resulting from the 
difference of osmotic concentration on the two sides of the proto¬ 
plasmic membrane. In the case of the turgid cell equilibrium will 
therefore be reached when the force sending water into the cell, that 
is, the difference in osmotic pressure of the internal and external 
solutions, is equal to this inwardly directed pressure exerted by the 
stretched cell wall. Before equilibrium is reached the pressure forcing 
water into the cell, that is, the difference in osmotic pressure of the 
internal and external solutions, is only partly compensated by the 
internally directed pressure of the stretched cell wall, so that the net 
pressure sending water into the cell is the difference in osmotic pres¬ 
sure of internal and external solutions less the inwardly directed 
pressure of the stretched cell wall. This net pressure obviously gets 
less and less as the difference in osmotic pressure inside and outside 
the cell is less and the pressure exerted by the wall greater. 
The terminology relating to these water relations of the turgid 
cell is unfortunately somewhat confused. It will be noted that we 
are concerned with the following quantities. There is firstly the 
osmotic pressure of the external solution. This may be expressed in 
terms of molecular solutions of sucrose or potassium nitrate, or in 
the corresponding atmospheric pressure. Secondly, there is the os¬ 
motic pressure of the liquid in the vacuole, the cell sap. For this 
quantity a number of terms have been employed such as osmotic 
pressure, concentration of the cell sap, potassium nitrate or saltpetre 
value, turgor, turgor pressure and others. The term used here will be 
osmotic pressure when it is the pressure sending water into the cell 
