Plate 13] In t r o d u c t io n The increase in knowledge of the virus diseases of plants has led to the possibility of attempting an investigation of the nature of the virus agent itself. It has been proved that various plants, notably N. glutinosa, react to inoculation with the virus of tobacco mosaic by the formation of necrotic lesions on the rubbed leaves. It has been shown, further, that the number of lesions formed increases with the concentration of the virus up to a definite value,

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... h varies with the experimental conditions (Caldwell, 1933) . At high concentrations of the virus, there is apparently not a sufficient number of susceptible areas on the leaves to allow of reaction to all the virus units present, and the number of susceptible areas becomes the limiting factor in lesion formation. At lower con centrations the number of virus units is the main factor involved. Youden, Beale, and Guthrie (1935) have recently published a paper in which they show that all the available data on the formation of lesions by different dilutions of virus, obtained by different workers, can be fitted to a curve with the formula y -N (1 -It has been shown that many substances, when added to the virus, have the effect of reducing the number of lesions on inoculation, or of completely suppressing lesion-formation. This may be due to the action of the substances on the virus agent itself or to their action on the tissues of the plants, whereby the normal response of the tissues to infection is prevented. It is not always clear which of the two effects is responsible for the suppression of the lesions in some instances. The purpose of the present paper is to set on record a method which, it is hoped, could be used in a chemical study of the nature of the virus agent, the agent not necessarily being freed from the plant materials with which it is associated. By the use of this method, it should be possible to determine whether an