Many of you may have seen (at least before the current spate of gales) seed-like structures hanging on stands of Japanese knotweed Fallopia japonica. What you are seeing is the embryonic hybrid between that species and Russian vine F. baldschuanica, a not uncommon (if excesively rampant) garden climber.
Some at least of these seeds are fertile, and if sown will give you the hybrid Fallopia x conollyana (`the railway-yard knotweed`). I have grown this hybrid from seed collected from both Pwll (Llanelli) and Gelli Aur (near Llandeilo), but these plants (which were grown in 2007-08), were rather floppy in growth form and did not prove to be hardy in one of our more ordinary winters, let alone the more severe ones that we subsequently experienced in recent years.
The hybrid has, however, been found growing wild elsewhere in Britain and Europe, albeit rather rarely. An informative article, written by JP Bailey, is found in Watsonia 23: 539-541 (2001) for those who are interested.