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 <WSD>15</WSD> minutes , stained with * * f for 30 minutes , and washed for 15 minutes in PBS . 
Meaning of the word marked with <WSD> tags:  a quarter of an hour