Sentence:  The specificity of staining in WTV tumors with * * f and * * f but not with NS and * * f or with <WSD>antiserum</WSD> to potato yellow-dwarf virus and * * f , and the absence of such staining in crown gall tumor tissue from sweet-clover , indicate that an antigen of WTV was being stained . 
Meaning of the word marked with <WSD> tags:  blood serum containing antibodies against specific antigens; provides immunity to a disease