TERMINOLOGY 95 



enough pressure on the solution (by means of a leak-proof piston, for ex- 

 ample) to prevent any increase in its volume due to the entrance of water. 

 The osmotic pressure of the solution is quantitatively equal to the imposed 

 pressure. If the latter is 15 atmos., then the osmotic pressure of the solution 

 is 15 atmos., etc.^ 



It is a common practice to speak of solutions as possessing an osmotic 

 pressure whether they are under such conditions that a pressure can develop 

 within them or not. A weight molar solution of sucrose contained in a 

 bottle may therefore be spoken of as possessing an osmotic pressure of 27 atmos. 

 at 25° C. (Table 13). In other words the term osmotic pressure is com- 

 monly used to denote the potential maximum pressure which would develop 

 in a solution were it placed under the necessary conditions. Its use will be 

 confined to this sense in this book.'^ 



The term osmotic pressure is also often used as a designation for the 

 actual pressures developed as a result of osmosis, a procedure which leads 

 to confusion. Actual pressures developed during osmosis are seldom equal 

 to the osmotic pressure as defined above. If a solution having an osmotic 

 pressure of 12 atmos. be enclosed in a stoppered membrane permeable only 

 to water which is in turn immersed in a solution having an osmotic pressure 

 of 8 atmos., water will diffuse inwards until at equilibrium the actual pres- 

 sure developed in the internal solution will be (disregarding any minor 

 effect due to dilution) 4 atmos. (see later). Its osmotic pressure will still 

 be nearly 12 atmos., however. Even if the external liquid is pure water, 

 the actual pressure developed in the internal solution would not be equal 

 to its original osmotic pressure unless the membrane is completely inelastic. 

 Inward diffusion of water results in a dilution of the solution which, as we 

 will see later, always results in a decrease in its osmotic pressure. To avoid 

 the inevitable confusion caused by use of the same term in a dual sense we 

 might distinguish between the potential {i.e. maximum) osmotic pressure and 

 the actual osmotic pressure of a solution. In this discussion however, we shall 

 employ the term turgor pressure to refer to the actual pressure developed as a 

 result of osmosis. This term has long been employed as a name for the actual 



1 Osmotic pressure is sometimes defined as the pressure which must be imposed 

 on a solution to prevent any increase in its volume under the conditions specified 

 above, but this usage is particularly unsatisfactory in the interpretation of biological 

 phenomena. While the magnitudes of such an imposed pressure and the osmotic 

 pressure are mathematically equal, physically they are two entirely different quan- 

 tities. 



2 The osmotic pressure of a solution has also been variously called the osmotic 

 totential, osmotic power, osmotic value, and osmotic concentration of a solution. 

 Such terms are frequently encountered in the literature of plant physiology. 



