Fasciation

This marsh marigold (Caltha palustris) flower caught my attention because it differed from the the usual structure.  Normally such flowers have a central globular cluster of between 5 and 25 carpels.

In this fower head, the number of carpels is much greater (over 30) and arranged in a linear fashion.  Such abnormal growth is called fasciation.

According to Wikipedia: "Any occurrence of fasciation has several possible causes, including hormonal, genetic, bacterial, fungal, viral and environmental causes".

That link has several examples at the bottom of the page.