T. C. Broyer 233 



gators. The latter would rather consider oxidative metabolism to be 

 essential to accumulation of both anions and cations. Some theoretical 

 considerations and experimental data still seem to warrant the latter 

 viewpoint. 



From a priori considerations, it seems that concomitant expenditure 

 of energy would be required wherever inorganic solute accumulates, 

 irrespective of the mechanism. The test of whether certain solute 

 movement is a spontaneous process or requires work depends on the 

 sign of the energy intensity, or free energy difference, of the substance 

 within the system. The value of such energy intensity differences may 

 be computed from a summation of the number of mole ions of each 

 and all constituents which have moved in a given volume of solution 

 from one concentration (or activity) of the ions to another, again irre- 

 spective of mechanism. Such computations include the numbers and 

 concentrations of both cations and anions (//, go, 96). Under some con- 

 ditions more anions than cations appear to migrate per unit time from 

 external solutions into roots, the excess of the former being partially 

 balanced by cations from other sources (presumably metabolically pro- 

 duced), where inorganic solute is accumulated. Under other circum- 

 stances cations appear to migrate at the greater rate or often at nearly 

 equal rates. In any case of solute accumulation, energy is expended — 

 the immediate energy source is usually oxidative metabolism of some 

 sort. The energy requirements for the accumulation of inorganic solute 

 may be estimated from accompanying changes in salt concentrations 

 and may be compared with the concurrent oxygen involved or the 

 carbon dioxide evolved. With certain salts and storage tissues, a close 

 correspondence between the computed energy expenditure and inor- 

 ganic solute accumulation has been reported (68). 



Under favorable external conditions of aeration and temperature, 

 little inorganic solute will usually migrate from roots into distilled 

 water (13). Two situations may obtain — namely, either a low per- 

 meability to such solute (especially at the vacuolar surface) or a dy- 

 namic equivalent movement of inorganic solute inward and outward — 

 the latter requiring a continuous expenditure of energy to maintain 

 solute levels. Although a dynamic system is recognized, the importance 

 of a possible low permeability of protoplasm to polar compounds can- 



