190 Mineral Nutrition of Plants 



ity. The solution theory which has been modified in numerous ways, 

 is now combined with another concept, that of ultrafiltration through 

 a sievelike nature of the surface. In the latter hypothesis, the rate of 

 migration of a substance into a cell is related entirely to the size of its 

 existing particles or aggregates, the limiting surface serving as an ultra- 

 filter. Early studies of Ruhland (jo) suggested a close correlation be- 

 tween the rate of movement of nonelectrolytes into cells and their 

 molecular diameter. However, this relation was later reinvestigated by 

 Collander and Barlund (ig). Little correlation was found between per- 

 meability and molecular size. Lipoid solubility is recognized as a prime 

 factor by Collander; however, he considers it incomplete in so far as 

 it merely indicates what substances will enter cells easily under certain 

 conditions. Although probably important, at least in some cases, ultra- 

 filtration likewise cannot be accepted as a complete theory of cell per- 

 meability. 



Making use of the colloidal character of protoplasm, Clowes (75) has 

 proposed an effect of electrolytes through phase inversion, on cell per- 

 meability. Here, it is conceivable that since protoplasm can be regarded 

 as an emulsoid colloidal system, there will be a continuous phase of one 

 composition (dispersion medium) through which is dispersed at least 

 one other phase, the particles of which are discontinuous. It is supposed 

 that changes in permeability are brought about by the continuous and 

 discontinuous phases changing places, that is, by phase inversion. 

 Penetration would take place only through the continuous phase. The 

 hypothesis would be equally applicable to a solution or a chemical 

 combination theory of permeability. This hypothesis in part may be 

 considered suggestive of others included under theories of chemical 

 combination or adsorption. Some investigators ($8, ^], 83) propose that 

 the substance combines chemically with, or is adsorbed on, a constituent 

 of the protoplasm at its limiting external surface. Absorption would be 

 effected through disturbance of the equilibrium between different pro- 

 toplasmic constituents, with the result that the chemical or adsorption 

 union is subsequently broken again and the substance is released at 

 other places within, or from other parts of, the cytoplasm. Extension 

 and modification of these concepts have been made by Briggs (5), 

 Brooks (7, 8), and others (2, 42, 43, 45. 68). Some of these investigators 



