Abstract
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, which 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 concentrations 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 - e -ax ).
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