Picture was taken Feb. 22, 2004. The plants were invading a ditch recently created by some road work. I think the sun, the open water, uncrowded by plants, and perhaps the cold temperatures influenced the leaf shape. Other puddles that were more crowded with Ludwigia did not have the elongated leaves.
I didn't collect a stem to try out in an aqarium. I wish I had. I would have done it this spring (2007) but it has been an exceptionally dry spring and all the ditches have been dry most of the time.