Summary: By treating leaves of French bean ( L.) in various ways at intervals after they were inoculated with the Rothamsted tobacco necrosis virus, a series of events in infected cells was detected and approximately timed. Treating leaves with ribonuclease, or floating them in water, in the first hour after inoculation decreases the number of infections; these treatments fail to do so later, perhaps because the virus has become firmly attached to some host component, or because cells injured at inoculation have healed and no longer allow virus particles to diffuse out or ribonuclease in.

After 2 hr. at 25° the virus is less readily inactivated by ultraviolet radiation than previously, suggesting that infected cells now contain more of substances that absorb radiation of 2537A. After 4 hr. the inactivation curve with ultraviolet radiation deviates from the course of a first-order reaction, probably because some infected cells now contain more than one potentially infective particle. After 6 hr. the formation of some lesions cannot be prevented by irradiation, suggesting that newly formed virus has spread from initially infected cells to deeper tissues, where it is protected from inactivation. At 16° the mean time for virus to spread from the initially infected cells is 12·5 hr. and it probably spreads from some cells by 8 hr. and from most by 15 hr.

Newly formed virus is not detectable in extracts of inoculated leaves until after it has spread from the epidermis to deeper tissues. Up to 20 hr. after inoculation most of the extractable virus seems to come from the epidermis, but later it comes predominantly from other tissues.


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