Sentence:  Root and stem tumors from sweet clover plants infected with WTV <WSD>were</WSD> 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:  have the quality of being; (copula, used with an adjective or a predicate noun)