THe Errect oF MANGANESE COMPOUNDS ON SOILS AND PLANTS 393 
MnO:) and its use in the Weldon recovery process for the preparation of 
chlorine. The mixture of milk of lime and manganese chloride is termed 
Weldon, or manganese, mud. 
Oxidation by plant roots 
That roots of plants have an extracellular oxidizing power “ may be 
demonstrated by the use of suitable chromogens,”’ according to Schreiner 
and Reed (1909). In regard to their work on root oxidation in culture 
solutions containing alpha-naphthylamine, these investigators state (page 
17 of reference cited) that ‘‘ when the oxidation is performed by the grow- 
ing roots of a plant, the oxynaphthylamine is deposited upon the surface 
of the roots in characteristic zones. .... . The zone of primary meris- 
tematic celis immediately back of the root cap is marked by a distinct 
narrow band of color.”’ The browning of the roots of wheat in solutions 
of manganese salts resembles the staining caused by the oxidation of alpha- 
naphthylamine. 
The reports of investigators indicate that such browning is character- 
istic of plants other than wheat, when grown in manganese solutions. 
This browning has been reported as consisting of a deposit of manganese 
dioxide. As far as can be ascertained by the writer, no proof has been 
offered for this statement. That the dioxide is formed, however, is indi- 
cated by the following: The black deposit is insoluble in water but 
dissolves in hydrochloric acid. When this solution is evaporated and the 
residue is fused with an alkali carbonate on platinum foil, the character- 
istic green color of the alkali manganate is developed. Furthermore, 
the blackened roots are capable of liberating chlorine from a solution of 
a chloride and sulfuric acid. If the plants thrive long enough in the man- 
ganese solution, the whole root system becomes blackened. 
In writing of the deposit of manganese dioxide, Miss Houtermans (1912) 
states that the blackening is probably the result of enzymotic processes. 
The browning is the result of the oxidation which occurs on the surface of 
the root. The fixed alkali hydroxides precipitate from solutions of man- 
ganese salts manganous hydroxide, white, which readily turns to brown 
manganic hydroxide in the air or in contact with other oxidizing agents. 
Since manganous hydroxide is formed in the solution of a manganese 
salt by hydrolysis, it seems that it is deposited on the roots, as such, and 
