142 THE MECHANISM OF ABSORPTION AND TRANSLOCATION 



molecules, or of the osmotic pressure, will be caused there through such 

 absorption and passive secretion l . The fact that the turgor rapidly sinks, 

 as the glucose is removed from the bulb scales of Allium cefla, indicates 

 the absence of any such mode of storage in this plant. 



As a general rule, the osmotic pressure of a mixture will be the sum 

 of the partial pressures exerted by its individual constituents 2 . So long 

 as the number of molecules, or free ions, remains the same, the osmotic 

 pressure will be unaltered, whatever interchanges may take place. By 

 means of the osmotic coefficients given in the table on p. 146, it may 

 be shown that the same pressure is generated in a mixture of potassium 

 chloride and magnesium sulphate, no matter whether the chlorine is com- 

 bined with magnesium or with potassium 3 . These approximate coefficients, 

 though not precisely accurate, are sufficiently so to determine the osmotic 

 value of a mixture from a physiological point of view, if the amounts 

 present of sugars, organic or inorganic acids, alkalies, or alkaline earths 

 are known ; indeed de Vries has estimated the hydrostatic pressure exerted 

 by the different groups of substances present in the cell-sap *. 



From the table it may also be seen that the isosmatic coefficient of 

 an acid increases by unity for every atom of an alkali it combines with, 

 but remains unchanged when united with an alkaline earth, and hence 

 the replacement of potassium by magnesium causes the osmotic pressure 

 to fall. 



In order to understand the causes which regulate turgidity, all the 

 factors at work must be known, for in the service of the plant, turgidity is 

 employed and regulated in the most various ways, as will be seen when the 

 individual vital functions are dealt with in detail 5 . The regulation of turgor 

 which accompanies growth, and which also occurs when plants are trans- 

 ferred to more concentrated or more dilute solutions, has already been 

 incidentally mentioned. This is attained by an absorption or a metabolic 

 production of osmotic substances, or by both taking place together, until 



1 Pfeffer, Unters. a. d. Hot. Inst. z. Tubingen, 1886, Bd. II, p. 309. Compensatory adjustments 

 are always possible. 



4 This is not in all cases precisely true, owing to the influences (combination, dissociation, &c.) 

 which the constituents of a solution may exert upon one another. See for example Tamman, 

 Zeitschr. f. physik. Chemie, 1892, Bd. ix, p. 108. Nevertheless, in most cases the first state- 

 ment holds good, as is shown by the rates of diffusion of the individual components remaining 

 the same (see Ostwald, Allgem. Chemie, 1892, 2. Aufl., Bd. I, p. 692), and also by osmotic 

 determinations (Pfeffer, Osmot. Unters., 1877, p. 67; de Vries, Jahrb. f. wiss. Bot., 1884, Bd. XIV, 

 p. 480). 



3 This is the case with all mixtures of salts, provided that no precipitate is formed, nor any of 

 the new products removed by diosmosis from the cell. Deherain, by estimating the freezing-point 

 of expressed sap, has found that in young seedlings the osmotic pressure lies between 4-8 and 9-8 

 atmospheres (Compt. rend., 1896, T. XXIII, p. 898). 



4 De Vries, 1. c., p. 541. 



* For a general account see Pfeffer, Druck u. Arbeitsleistungen, 1893, pp. 296, 428 ; Studien z. 

 Energetik, 1892, pp. 237, 248. 



