Between Alisma and Ludwigia, which genus is spread more widely around the world?
Alisma: Alisma is a genus of flowering plants in the family Alismataceae, members of which are commonly known as water-plantains. Alisma:  The genus consists of aquatic plants with leaves either floating or submerged, found in a variety of still water habitats around the world (nearly worldwide). Alisma:  The flowers are hermaphrodite, and are arranged in panicles, racemes, or umbels. "Alisma" flowers have six stamens, numerous free carpels in a single whorl, each with 1 ovule, and subventral styles. Ludwigia (plant): Ludwigia (primrose-willow, water-purslane, or water-primrose) is a genus of about 82 species of aquatic plants with a cosmopolitan but mainly tropical distribution.
Alisma