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Five Nicotiana species were found to develop necrotic lesions wherever virus of the common field type of tobacco mosaic successfully entered leaf tissues. These species were N. rustica, N. langsdorffii, N. sanderae, N. acuminata, and N. glutinosa. 2. The local lesions developing in N. rustica can be used to measure accurately the potency of tobacco mosaic virus. 3. The local lesions in N. glutinosa are exceptional in the rapidity of their development. They sometimes begin to appear thirty hours after inoculation. In four or five days they are well developed. Large numbers of them may be distinguished on a single plant. This allows comparisons to be made between virus samples, since the number of lesions developing depends on the virus concentration of the inoculum. 4. A standardized method for using N. glutinosa as a test plant for measuring the concentration of mosaic virus gives as rapid and as accurate results as the determination of bacterial numbers by plating methods.
Francis O. Holmes (Fri,) studied this question.