Sentence:  Sections of sweet clover stem and root tumors were treated with 1 : 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 <WSD>PBS</WSD> . 
Meaning of the word marked with <WSD> tags:  a solution containing a phosphate buffer