The Infectivity of Extracts made from Leaves at Intervals after Inoculation with Viruses

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Abstract
Rothamsted tobacco necrosis virus and 2 strains of tobacco mosaic virus were used in this study. Inoculations were made into the leaves of French bean and tobacco plants. In all cases the infectivity of successive leaf extracts decreased for some hours following inoculation. The first increase in infectivity could not be detected until after almost twice the time necessary for the virus to penetrate into the mesophyll. No evidence was obtained that this initial decrease was due to any change in the virus particles. Mesophyll cells of French bean leaves apparently synthesize new virus particles within 5 hours after becoming infected from the epidermis. The average number of virus particles per cell increases to 106, when the cells die. The virus, however, spreads to other cells and the infectivity of the leaf extract continues to increase for several days.
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