Sentence:  Sections of sweet clover stem and root tumors were treated with <WSD>1</WSD> : 10 solution of * * f for 30 minutes , washed in buffered saline for 15 minutes , stained with * * f for 30 minutes , and washed for 15 minutes in PBS . 
Meaning of the word marked with <WSD> tags:  used of a single unit or thing; not two or more