96 Mineral Nutrition of Plants 



conditions are given for a transport of anions. The change of valency 

 proceeds as an electron wave, if the enzyme system lies parallel to a 

 redox gradient. If the molecules of the enzyme are arranged in such 

 a way that they serve as a boundary between two media of different 

 redox potentials, owing to the wavelike proceeding oscillation of the 

 Fe-valency, they will transport anions from the medium with higher 

 oxydation power (Fe+++; ox-side) to the medium with lower oxyda- 

 tion power (Fe++; red-side). The anions are transported in opposite 

 direction to the electrons. . . . The transference of an electron between 

 two Fe-atoms moves one anion from the oxidized to the reduced stage." 

 On the basis of the character of the isotopic exchange curves, Jacob- 

 son and Overstreet (2]) were led to conclude that the plant complexes 

 R-K, RRb, RSr, R'H 2 P0 4 , R'l, etc., probably do not represent 

 combinations of the ordinary electrovalent type. They suggest that 

 cations may be bound in plants in the form of chelated complexes. In 

 this class of structure the metallic ion is attached at two or more points 

 in the same molecule and one of the bonds is frequently coordinate in 

 nature (see Yoe and Sarver, Organic Analytical Reagents, John Wiley 

 and Sons, 1941). Many plant substances such as proteins, amino acids, 

 and organic acids are known to form chelated compounds, particularly 

 with polyvalent cations. Chelated structures in general are rather stable 

 arrangements and metallic elements so bound do not readily undergo 

 isotopic exchange. An example of this class of compound is given in 

 the proposed structure for the calcium complex of ethylene-diamine- 

 disodium-tetraacetate : 



NaOOCCH 2 CH 2 -H 2 C CH 2 COONa 



PLC 



COO' 



The foregoing outline of hypotheses concerning the nature of the 

 plant substances RH and R'-OH is by no means exhaustive. Neverthe- 



