Growing at Bluebonnet Swamp in Baton Rouge, LA.
Small yellow flowers (about 3/8 inch diameter)
On a stalk that is 8 to 9 feet tall.
Stem in about 3/4 inch in diameter near the btm.
Growing in a sunny meadow area at the edge of a swamp.
On high ground (not wet).
Photos taken on 6/29/12.
I just got back from another visit to the plant.
Too dark for pictures, but...
Cut a leaf from the stem.
A milky sap oozed from the cut, as it should for lactuca serriola.
I did NOT find prickles on the underside of the mid veins.
I did NOT find prickles on the stem.
There are a few prickles on the outer edges of some leaves.
It may not be "prickly lettuce" (lactuca serriola), but maybe something closely related (???).
Maybe some plants of "prickly lettuce" don't have many prickles (???).
Thanks for the lead and the good link.
PlantFiles: Canada Lettuce, Tall Lettuce, Tall Wild Lettuce - Lactuca canadensis
I think meltonw has got it!
Thanks to all who commented.
Fun, as always.
I found this to be a helpful reference for this plant: