WHY IS THERE A "SIPEL ON THE TOER" IN DEINUM?

In a folk song from the beginning of the 20th century, one of the verses said, Yn Deinum was a siepel on the toer. From that time on Deinum has a Sipel (= onion) as the crowning glory of the church tower. Until then he was also called cabbage or pear.
 
Today we were sailing to Drachten... here on the Van harinxsmakanaal...
and it was foggy...