On Wed, 26 Jun 2013 02:01:03 +0000, Kay Lancaster wrote:

Turn the flower upside down and you'll find there are 4 green sepals,
then the four yellow petals.

Thanks for that information.

Here's a picture of the underside of the wild mustard flower:
http://www2.picturepush.com/photo/a/...g/13419875.jpg

Is the green arrow pointing to a (football-shaped) sepal?