Helanthium tenellum "parvulum" is a light green, usually smaller and wider-leaved form of Helanthium tenellum (= Echinodorus tenellus). It was probably first imported from North America. Decades ago, this was the aquarium plant generally known as Echinodorus tenellus, today it is only rarely cultivated any more. Its submersed leaves are always light green, never reddish, up to a little over 2 mm wide and widest below the middle.
The plant usually sold under the name H. tenellum today was imported from the American tropics (Brazil?), and its submersed leaves are generally of an olive green to reddish colour, longer and narrower (up to around 1.5 mm) that are widest above the middle (see Helanthium tenellum). Its emersed, flowering form also differs clearly from H. tenellum "parvulum".
A well-growing clone of H. tenellum "parvulum" has spread in the hobby, under the erroneous name "Ranalisma rostrata". True Ranalisma rostratum from Asien indeed looks rather like the submersed form of Helanthium tenellum, its emersed form, however, differs clearly (see Ranalisma rostratum).
To be continued...
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