What flowers have no petals?

Well, perhaps flowers of sulfur or frost flowers. But the presence of petals is part of the usual definition of a flower. There may be flowers without petals, but I can't think of any.

Glumiflorae flower has no petals. This family which includes grasses, sedges, rushes and cattails have very small, almost unnoticeable petals or no petals at all. The corn, rice, wheat, barley, and sugar-cane are included in this class of plants.