Salvinia natans is a tropical water fern. In Israel it is rare and sporadic. The leaves have special hairs on the upper surface, these hairs prevent water to accumulate on the leaf, thus enabling it to float.
2010.07.28 Vienna II. district (bayou in Vienna Prater - Mauthnerwasser 156 m).Species introduced in the habitat by (most likely) fishkeepers who disposed of their aquarium and its content there.German name: Gewhnlich-SchwimmfarmID: Fischer & al., Exkursionsflora A/FL/SdT (2008 3rd), who also mention the very same habitat where, it seems, this species is able to sustain itself
2010.07.28 Vienna II. district (bayou in Vienna Prater - Mauthnerwasser 156 m).Put on a twig to shoot leaves and roots: note that not two leaves are paired on stems (as it seems) but three: two ordinary ones (swimming leaves), and a third one which remains submersed and looks like it were a root.German name: Gewhnlich-SchwimmfarmID: Fischer & al., Exkursionsflora A/FL/SdT (2008 3rd); see also comments here
2010.07.28 Vienna II. district (bayou in Vienna Prater - Mauthnerwasser 156 m).Put on a twig to shoot leaves and roots.German name: Gewhnlich-SchwimmfarmID: Fischer & al., Exkursionsflora A/FL/SdT (2008 3rd); see also comments here