This section is from the book "Who's Who Among The Wild Flowers", by W. I. Beecroft. Also available from Amazon: A Field Guide to Edible Wild Plants.
Brassica nigra.
Mustard Family. June - Sept.
A plant 2 - 5 feet high, found in fields and roadsides. Naturalized from Europe. Common throughout United States.
Yellow, 4-parted, in loose clusters terminating branches. Seed pods about 1/2 inch long, 4-sided, lying close to the stem.
Variously lobed and toothed, terminal lobe largest.

Black Mustard.
Herb of St. Barbara: Yellow Bitter, or Rocket Cress.
Barbarea vulgaris.
Mustard Family. Apr. - June.
Flowers similar to those of preceding; seed pods long, curved. Upper leaves stemless, lower leaves usually in 5 divisions, terminal one largest. Lake Superior northward and westward; apparently introduced eastward.
 
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