234 Mineral Nutrition of Plants 



not be disregarded. The movement of inorganic solutes is so intimately 

 related to both permeability and energy intensity that the relative im- 

 portance of the latter two factors is difficult to evaluate (p). 



The postulate of some sort of inorganic solute-protoplasmic interac- 

 tion has been accepted by many investigators. Some consider that 

 adsorption may be an initial step in metabolic accumulation. Some 

 time curves of accumulation and the effects of various concentrations 

 of applied salt suggest such a possibility. The shapes of the adsorption 

 exchange curves with time vary with the particular ion concerned. 

 There are, for example, quite different rates of adsorption exchange 

 for rubidium and strontium, for phosphate and iodine, and between 

 these cations and anions (see chapter by Overstreet and Dean). Marked 

 difference also exists between the rates of this process, with a particular 

 ion, for living and for dead tissues. While adsorption is recognized as 

 one mode of salt-protoplasmic interaction, the possibility of specific 

 compounds in protoplasm, characteristic of the species, acting as car- 

 riers has been suggested. Such an interaction may be more of the 

 nature of chemical bonding. The salt-protoplasmic interaction may be 

 a bonding of variable strength. Possibly the configuration of the car- 

 rier may be labile, the bonding being variably strong or weak and 

 intimately related to metabolism. Perhaps this transition may parallel 

 the oxidative level. Thus, a polarized transport of salt in connection 

 with a specific labile complex could be a mechanism for the movement 

 across protoplasmic layers (/, 21, 35, 92). Bipolar protoplasmic com- 

 plexes could account for simultaneous or separate movement of the ions 

 of a salt. Proteins and amino acids have received a prominent place 

 as suggested ampholyte carriers (7). Chelation has been considered by 

 some as a possible means of bonding (8, 55). The possible role of 

 cytochrome as a carrier has been indicated earlier (43, 44). 



TRANSLOCATION AND ACCUMULATION 



Detailed discussions of the paths and mechanism of transport of 

 inorganic solute and their distribution with time, in plants, can be 

 found in the chapters by Burstrom and Biddulph. Certain limited 

 aspects of these processes, however, will be presented here. 



Early studies with unicellular algae showed the presence of high 



