There are two similar plants in my aquarium, Echinodorus Tenellus and Helanthium Tenellum. But I don't know which one is Tenellus and which one is Tenellum. Google image result didn't that help much either.
The plant with longer leaves in the middle can not be grown emersed, while the smaller one can be grown emersed. It left outdoor with strong sunlight, the smaller plant will be in dark green color. The small plant can grow longer leaf too if they are in deeper water.
The smaller plant grow longer in the back of my aquarium. Algae and snail everywhere.
I remember I blog about they being the best low tech tank plant in 2015. Now, they are still in my low tech tank.