On Thu, 27 Jun 2013 04:32:27 +0000 (UTC), Danny D. wrote:
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?

Looks more like a petal that hasn't completely colored up yet. But that's
really hard to tell in a photo.

If you look at sepals and petals on almost all flowers, you'll find
that each series of flower parts are in whorls -- multiple parts
all coming out at the same level. So the lowest series is the
sepals. Let's say in an opened out mustard flower, they're laid
out like a + sign. The next whorl is petals, and if the sepals are
laid out like a + sign, the petals will be an X. Then there will be
4 stamens, laid out like a + and two more like two arms of the X,
and finally in the middle, the two chambers of the ovary, the two
carpels joined together to form a single fruit.

http://www.discoverlife.org/mp/20p?i...WS95452&res=mx is a pretty
good view of the flower of a different species of mustard from the side...
in this case, both the sepals and petals are yellow, but different shapes,
and you can also see the four long stamens and two shorter ones most of
the mustards have.
http://www.plantbiology.siu.edu/PLB3...werGeneral.jpg and http://www.tsflowers.com/lilyphotos/Lily_Stargazer2.jpg is a lily
flower straight on, and you can see the different shapes of the
sepals, the narrower "petals" and the wider petals. Unfortunately, it's
not a good photo of the stamens or ovary, but I'll take what I can
get here. g

And yes, I can rattle on for hours about flower parts and how to
interpret what you're seeing... it's one of the major tools for plant
identification, once you get beyond pure recognition of different species.

Kay