Respected Sir, at a glance it looks like a member of Marchantiophyta(like Pellia sp.), but before conclusion can you send/upload images of internal tissue differentiation and reproductive characters are needed.
In fact, I should thank Mr.Richard Lansdown (Chair IUCN SSC Freshwater Plant Specialist Group www.ardeola-environmental.com), Prof C Thomas Philbrick (Connecticut State University Distinguished Professor) and Dr. Masahiro Kato (National Museum of Nature and Science Herbarium Japan) for the best answer. & please see Dr. Kato's comments: " Perhaps it is Polypleurum stylosum, Podostemaceae. (Reference: Mathew and Satheesh. 1997. Taxonomy and distribution of the Podostemaceae in Kerala, India. Aquat. Bot. 57: 243-274.) "
I was also curious about this plant because it looks so familiar and was regularly checking whether anybody answered it or not. And later just three days ago I have send this question along with the plant photo to Mr. Richard whom then circulated the question among the Freshwater Plant Specialists. So all credits to this plant id here must go to the three experts.