Sentence:  Root and stem tumors from <WSD>sweet</WSD> clover plants infected with WTV were quick-frozen in liquid nitrogen , embedded in ice , and cut at 3 - 6 | m in a cryostat maintained at - 16 ` to - 20 ` . 
Meaning of the word marked with <WSD> tags:  erect annual or biennial plant grown extensively especially for hay and soil improvement