The sections were fixed in acetone for 15 minutes and dried at 37 ` for 30 minutes. Some of them were then covered with a drop of ** f in a moist chamber at 24 ` for 30 -- 40 minutes. As controls other sections were similarly covered with NS. Sections were then washed with PBS for 15 -- 30 minutes. After blotting out most of the saline around the sections, a drop of ** f was layered over each of the sections, allowed to react for 30 minutes, and then washed with PBS for 15 -- 30 minutes. After blotting out most of the liquid around the sections, the latter were mounted in buffered glycerine (7 parts glycerine to 3 parts of PBS).

After drying the sections under the fan, fixing in acetone, and drying at 37 ` as in the indirect method, the sections were treated with conjugated ** f or ** f (undiluted unless mentioned otherwise) for 5 -- 30 minutes. As controls, other sections were similarly treated with ** f or conjugated antiserum to the New York strain of potato yellow-dwarf virus (Wolcyrz and Black, 1956). The sections were then washed with PBS for 15 -- 30 minutes and mounted in buffered glycerine.

Stained or unstained sections were examined under dark field illumination in a Zeiss fluorescence microscope equipped with a mercury vapor lamp (Osram HBO 200). The light beam from the lamp was filtered through a half standard thickness Corning 1840 filter. In the eyepiece a Wratten 2 B filter was used to filter off residual ultra-violet light. A red filter, Zeiss barrier filter with the code (Schott) designation BG 23, was also used in the ocular lens assembly as it improved the contrast between specific and nonspecific fluorescence.