Tobacco mosaic virus particles were found in small packets and in small numbers, with the electron microscope, in necrotic leaf cells of Nicotiana glutinosa when the samples were fixed in glutaraldehyde and postfixed in OsO4, and the sections were stained with heavy metals. The numbers and size of the virus packets were increased greatly when the leaves were detached from the plant after inoculation Assay of concentration showed that detachment resulted in a 30-fold increase of virus. A similar increase in the number of virus particles detected by electron microscopy was produced by keeping inoculated plants at an air temperature of 26°C. A still greater increase in concentration was effected by incubating detached inoculated leaves at 26°C. Moreover the arrangement of virus particles in these cells resembled that of a systemic virus infection. Cells in local lesions of Chenopodium amaranticolor contained large numbers of virus particles both as packets and in the loose arrangement characteristic of systemic infection. Neither the number of particles nor their arrangement was affected in this host by detaching the leaf or by changing the air temperature. It is suggested that there may be two types of localized virus infections, one of which produces virus in low concentration and is amenable to changes in virus concentration and arrangement as a result of environmental manipulation.

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