To all involved in this discussion. It may be my imagination, but the stem on that plant is entirely too thick for a Ludwigia, Alternanthera or other plant discussed here. I will go with my original suggestion of 1) Green Stricta (Gymnocromis spilanthoides) or 2) member of the Hygophila genus.
As for the term 'winter leaf', it refers to the growing season, in Asia, of this plant in emersed form. It is not generally used, except for us farmers. I'll see if I can dig up a pic. I know I had one somewhere because i used it to help identify this plant.