138 THE MECHANISM OF ABSORPTION AND TRANSLOCATION 



Osmotic pressure is a function of the number of molecules present 

 in a unit of volume. The reason for the absence of a precise correspon- 

 dence in certain cases between the ratio of volume to gaseous pressure and 

 that of osmotic pressure to molecular weight, is that the number of dis- 

 solved molecules present in a unit of volume may be more than the 

 molecular weight of the dissolved substance would indicate, owing to 

 dissociation having taken place in the process of solution. In colloid 

 substances, the relationship of molecular weight to unit of volume is such 

 as to generate only a relatively trifling osmotic pressure, and this in the 

 case of a one per cent, solution of gum-arabic amounts to 0-085 of an 

 atmosphere, which is not a fortieth part of the pressure generated by a 

 solution of KNO 3 of the same concentration. 



The earlier researches with bladders, parchment, &c., necessarily 

 gave relatively high osmotic pressures for colloid substances, since these 

 diosmose but slowly or not at all \ and hence yield nearly the maximal 

 possible osmotic pressure for such substances. With membranes of this 

 character the maximal pressure can never be reached, and it falls more 

 and more below its real value as diosmosis is increasingly active. 



Just as is the case with gases, the osmotic pressure increases with 

 the number of molecules present in a unit of volume. With highly 

 concentrated solutions certain deviations occur, but nevertheless from a 

 physiological point of view, and under the conditions usually existing, the 

 osmotic pressure may be assumed to be proportionate to the degree of 

 concentration of the given substance. 



Similarly, the osmotic pressure alters with the temperature, according 

 to the same laws that influence gaseous pressure. Since by a rise of tem- 

 perature of i5C. the pressure is only raised from 100 to 105-5, it is evident 

 that temperature can never exercise any marked direct effect upon turgor 

 in plants. A plasmolytic condition of equilibrium is for example not 

 perceptibly modified by a change of temperature 2 . 



From what has been said, it follows that, provided the active substances 

 do not diosmose, the thickness or quality of the film of plasma, and its 

 permeability to water, influence the time necessary for the restoration of 

 equilibrium, but have no effect upon the pressure ultimately produced. 



1 Pfeffer, Osmot. Unters., 1877, p. 73. 



2 See Ostwald, Allgem. Chem., 1891, 2. Aufl., Bd. I, pp. 659, 669 ; Pfeffer, Plasmahaut n. 

 Vacuolen, 1890, p. 307. The reactive power of the organism is such, that a change of temperature 

 may act as a stimulus and give rise to a different result, an alteration taking place in either the 

 osmotically active substances, or in the diosmotic properties of the protoplast. Nevertheless, the 

 physical fundamentals mentioned above hold good here also, as well as in those cases in which 

 the cell develops an active pumping (or sucking) force (Sect. 46), and thus decreases (or increases) 

 the osmotic pressure, according to the amount of water driven out (or sucked in). In every case, 

 it is only possible to determine the work done by osmotic pressure, and made use of by the plant, 

 by direct observation. No further remarks can be made here, since we are concerned only with the 

 exposition of certain fundamental principles (Krabbe, Jahrb. f. wiss. Bot., 1696, Bd. XXIX, p. 447). 



