Your plant does not have opposite leaves. I think the leaves at the tip are alternate - the stem has not elongated enough to make alternate real apparent. Some pondweeds can have alternate leaves and some will have opposite leaves near the ends. I know of no plant that has opposite and alternate leaf arrangement on the same plant. It is almost always one or the other, not both.
Very good pictures of the seeds. Nice photography. You are correct the seeds are not correct for small pondweed. The narrow leaved pondweeds are sort of difficult to separate due to several of them look very similar in casual pictures, plus the narrow leaved pondweeds can be fairly variable with several varieties of each species. I think you might have leafy pondweed Potamogeton foliosus especially if the seeds are on stalks not at the end of the plant but originating in the leaf axils (angles) back from the tips or ends. Mature seeds are supposed to have small keels with a wavy margin. Leafy pondweed occurs in all states of the US.
Last edited by Bill Cody; 04/02/17 03:31 PM.
