The green plant in post #14 has leaf veination consisting of parallel main veins going back to the base of the leaf and leaf margins that exclude it as a Ludwigia, even the woody ones like L. alternifolia (which does not grow submersed). L. repens doesn't grow that rigidly or have such long internodes either. You're right that many plants look drastically different above and below water, but that does have limits. The reddish plant on the right could well be a Ludwigia, but they aren't the same plant.
As far as the difference between L. palustris and L. repens goes, the former usually has longer petioles, somewhat narrow leaves and flowers with no petals. Leaf texture is a tad different too, but that's a bit tough to describe. Finally, if you've seen enough of it enough times out collecting when you were hoping to see something else for a change...
As far as the difference between L. palustris and L. repens goes, the former usually has longer petioles, somewhat narrow leaves and flowers with no petals. Leaf texture is a tad different too, but that's a bit tough to describe. Finally, if you've seen enough of it enough times out collecting when you were hoping to see something else for a change...