Without knowing what this plant is, it's only speculation as to what these little structures are. Without going on too thin of a limb, though, these are almost certainly seeds of some type. Perhaps this is some aquatic plant or semi-aquatic plant that utilizes water to spread its seeds downstream. It would make sense as these little things are too low to the ground for wind dispersal. There's no fruit or flower to tempt an animal. What are your thoughts?