premise: The specificity of staining in WTV tumors with * * f and * * f but not with NS and * * f or with antiserum to potato yellow-dwarf virus and * * f , and the absence of such staining in crown gall tumor tissue from sweet-clover , indicate that an <mask> of WTV was being stained .
hypothesis: the proper context for <mask> here should be any substance (as a toxin or enzyme) that stimulates an immune response in the body (especially the production of antibodies)