SUMMARY

Two kinds of assay, particle counts by electron microscopy and infectivity for leaves ofPhaseolus vulgaris were used to estimate the amounts of virus in extracts made from tobacco leaves kept at 14 to 34 ° for I to 6 days after inoculation, and in extracts from uninoculated tip leaves sampled 14 days after the plants were inoculated. One, 2 and 6 days after inoculation, virus isolate PRN was obtained in largest amount from leaves at 22 °, 18 ° and 14 °, respectively. Isolate CAM showed the same trend, but equivalent temperatures were 4 to 6 ° higher. At 14 ° infectivity and particle numbers continued to increase between I and 6 days after inoculation but at 3 ° ° infectivity de-creased after i day. At some temperatures the number of particles first increased and then decreased. For instance, at 26 ° the number of particles of isolate CAM tripled between I and 2 days, and then decreased at least loo-fold between 2 and 6 days after inoculation. Temperature had only small effects on the ratio of long (c. 19oo ~) to short (mainly 45o to 9oo ~.) virus particles. Specific infectivity increased slightly when particle number was increasing and decreased greatly when particle number was decreas-ing. Specific infectivity sometimes decreased slightly before particle num-ber. As with some small isometric plant viruses, the amount of tobacco rattle virus extracted from leaves apparently represents he resultant between synthesis and degradation, and the two processes are differently affected by temperature. At 14 to 22 ° but not at 26 °, isolate PRN produced necrotic lesions in inoculated leaves, whereas isolate CAM did not cause visible lesions. This difference seemed to have little effect on the changes in virus titre but although isolate PRN multiplied optimally at a lower tempera-ture than CAM, it was more stable in vivo at 26 °. Isolate PRN invaded un-inoculated leaves only sporadically and accumulated in only small amounts. Isolate CAM readily invaded uninoculated leaves, in which it reached its greatest concentration at22 °