622 
Journal of Agricultural Research 
Vol. XXIV, No. r 
due to lack of assimilable iron for the plant, may be induced either by 
an excess alkalinity or by the presence of large quantities of soluble 
phosphates in the nutrient. It is not improbable, however, that chlorosis 
that is due to precipitation of iron as phosphate could be obviated by 
proper adjustment of the reaction of the medium, though the degree of 
acidity necessary to accomplish this would be greater than it would be 
were the iron precipitated as hyroxid. 
EXCRETION OP SOLVENTS PROM PLANT ROOTS 
The reaction of the mass of a medium is not, however, necessarily the 
critical condition governing the availability of iron. Maz^, Ruot, and 
Lemoigne (j 6) attributed the resistance to chlorosis of some of their 
culture plants that were grown in alkaline media to the ability of the 
plants to excrete organic acids from their roots. These acids formed 
complex molecular compounds of iron that were soluble in the presence 
of calcium carbonate, and thus made available sufficient iron for the 
needs of the plant. 
The solvent action of root excretion on insoluble matter which was in 
contact with roots was recognized by Takeuchi (2j), who recommended 
the agitation of the precipitate in solution cultures to prevent chlorosis 
resulting from lack of iron. Maz^ (15), too, stated that a marked in¬ 
fluence was exerted on the plants in his solution when the roots reached 
the bottoms of the flasks or when the solutions were agitated and the 
precipitate settled on the plant roots. These observations indicate, as 
one significant difference between the reactions of plants grown in solu¬ 
tion cultures and of those grown in soils, that, in the first instance, con¬ 
tact between the insoluble nutrients is not maintained and the solvent 
action of the root excretions is dissipated, and that, in the second in¬ 
stance, the influence of such material, being localized, exerts its maximum 
effect. 
The excretion from plant roots of organic acids other than carbonic 
acid has, however, never been satisfactorily demonstrated. The results 
obtained by Kossowitsch {14) show that the elimination of carbon dioxid 
is apparently a normal function of the roots of many, if not of all, species 
of plants. It does not, therefore, seem probable that the variation in 
the susceptibility to chlorosis of different species of plants can be due 
under normal conditions to the excretion of a specific solvent. 
It may be that variations in the quantities of carbon dioxid which are 
evolved from the roots of plants of different species determine the sus¬ 
ceptibility of each plant to chlorosis when the plant is grown in less acid 
media; and as the source of this compound must be the carbon that is 
assimilated through the agency of chlorophyll, the activity of which 
in turn is dependent upon an adequate supply of iron, it is possible that 
in chlorotic plants the intensity of chlorosis depends, to a large extent, 
upon the balance existing between the potency of the agencies depressing 
the solubility of iron in the soil and the solvent power of the carbon 
dioxid that is eliminated from the roots. 
EEFEcn' OF UNASSIMILATED RESIDUES OF FERTILIZER SALTS 
It was shown by Kossowitsch (jj) and by Maz6 (75) that there might 
be a modification of the reaction of a culture as a result of plant growth 
and that, when ammonium sulphate was used as the source of nitrogen, 
the utilization of the basic radicle resulted in the release of the acidic 
radicle which, rejected by the plant at its roots, tended to produce an 
