234 PLANT PHYSIOLOGY 



Ions Act Like Molecules.— This difficulty was explained by 

 Arrhenius, and, in fact, the osmotic pressures developed by salts 

 are considered among the strong supports of the ionic theory of 

 solution connected with the name of this great Swedish chemist. 

 According to Arrhenius, salts (which are composed of a base 

 united with an acid) partly dissociate in solution to form the basic 

 and acid ions (Chap. XI). NaCl, e. g., partly dissociates to form 

 sodium and chlorine ions. In a solution there are present not 

 only molecules of sodium chloride but also ions of sodium and 

 chlorine. In the solutions used by de Vries about one-half of the 

 salt thus dissociates, so that the number of osmotically active 

 units would be increased 50% over what it would be if no ions 

 were present but only molecules. This then explains very beauti- 

 fully why the isosmotic coefficient for KN0 3 and NaCl is 3 in- 

 stead of 2. Likewise in the case of potassium sulphate if half the 

 molecules are dissociated, instead of two molecules of potassium 

 sulphate there are one of the salt, two ions of potassium (since 

 the salt is divalent), and one ion of the sulphate. The isosmotic 

 coefficient is thus twice that of materials like sugar which do not 

 dissociate (nonelectrolytes) or 4. The degree of dissociation 

 varies with the concentration, and consequently the figures em- 

 ployed here cannot be used for higher concentrations. In general, 

 the more dilute the solution the higher the percentage of dis- 

 sociation. 



Osmosis and Diffusion. — This explains in part some of the 

 results obtained in the experiments with osmometers, but there 

 are still many questions which here can be touched upon only 

 briefly. First, why does the water pass into the solution? One 

 answer has been that the surface tension of the solution is different 

 from that of the water outside and, in order to equilibrate the two 

 surface tensions, water must pass in according to the laws of 

 thermodynamics. Others have suggested that the hydrostatic 

 pressure which develops when the solution is separated from water 

 by a semipermeable membrane, is the difference between the 

 normal osmotic pressure inside the osmometer and the atmos- 

 pheric pressure on the surface of the liquid outside. The water goes 

 in until this difference is turned into hydrostatic pressure and 

 the two forces are balanced. However, the most popular explana- 

 tion of the phenomenon and the one easiest to comprehend, after 

 what has been said in the previous chapter, is that based upon 



