Do you think this a Myriophyllum tuberculatum?
So at least should it be classified as "amphibious plant"?Hola Juan,
there are many plants predominantly occuring submerged in the nature, but being able to develop a terrestrial form when they fall dry. They don't reach their growth optimum in the emersed form. This is also the case with Myriophyllum verticillatum or aquatic Ranunculus species in Europe - large flowering plants in the water and little stout emersed plants when fallen dry.
But there is a smooth transition from water plants to amphibious and bog plants, and the definitions differ between the authors, anyway. C.D.K. Cook, for example, quotes many emergent species in his "Aquatic plant book".
Understood.Maybe... but it depends on its ecology in the natural habitats. If it occurs mostly submerged there, it is rather a "true" water plant. Typical amphibious plants undergo a rather regular (mostly seasonal) change between emersed and submerged stage. E.g. many species of Ludwigia, Bacopa, Sagittaria or Echinodorus.