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This investigation has as its object the elucidation of the mode of action of dithiocarbamic acid derivatives as fungicides, with die help of the theory of variability, heavy metal salts being dealt with separately.

The experiments were carried out using Macrospcrium sarcinaeforme as the test organism; the chemical materials used are listed.

Deviations from the experimental and mathematical techniques described in previous papers of this series are explained.

From the results it is inferred that: (a) dithiocarbamates are decomposed by the spores to a greater or lesser degree into amine and carbon disulphide, both being toxic; (b) the amines can be absorbed only in a combined form, which may be the dithiccarbamate itself but is more probably a derivative, possibly an ester; (c) the carbon disulphide probably acts through the mediation of a thiocarbonate; (d) thiuramsulphides act through being decomposed to dithiocarbamates (or their derivatives mentioned above). The monosulphides are largely, and the disulphides slightly, decomposed; in the latter case with reduction.

It is suggested that the greater toxicity of the methyl-derivatives over all others tested is due to the shortness of the hydrocarbon chain.

It is concluded that the role of the carbon disulphide is chiefly that of a chemical carrier of the amino group, but that, owing to the negative index of variation of the spores to the latter, the carbon disulphide must play an essential part in completing the toxicity.

It is suggested that heavy-metal dimemyldithiocarbamates may show promise against M. sarcinaeforme, but that others may be effective against other fungi.