The experiments were made with tobacco mosaic virus in tobacco (Nicotiana
tubmum L. var. White Burley), using for inoculum a purified virus preparation
at a concentration of 1 g./l. Quantitative infectivity tests were made by the
local-lesion method, using
Nicotia?za glutinosa L. The tobacco leaves were
detached from the plants
a day after their inoculation and placed in distilled
water or
in solutions of the different substances being tested. This interval
between inoculation and treatment was adopted to avoid the diminution in
the
number of local infections produced by floating leaves immediately after
they
are inoculated, a phenomenon that has also been described by Yarwood,
Hall
& Nelson (1958). Two to four tobacco plants provided the six to twelve
leaves which were inoculated for every treatment; from these a total of
15-20 ml. of sap was extracted. The leaves were placed in shallow enamel