DR. CALDWELL'S chief criticism is directed against a point which has already been proved1, namely, the ability of this virus to enter the roots of a plant without the aid of any known external agencies. Dr. Caldwell says that the term ” air-borne” implies ” that the virus agent can escape from the tissues of the infected plant into the air under natural conditions”. I fear this is wider than my conception of the term; I only meant that the virus was carried in the air, with the added implication that it was able to reach and infect virus-free plants by this means. It never occurred to me that anyone would suppose that the infected plant was discharging virus particles into the air after the manner of an influenza patient. Indeed, the following sentence from my letter makes my meaning quite clear: ” It is not perhaps surprising that such a minute virus and one capable of withstanding complete desiccation should be present in the air.”