Which aquatic plant is commonly referred to as stonewort?

Stonewort is a common name used to refer to Chara, a group of green algae found in freshwater environments. Chara is distinguished by its calcareous deposits, giving it a hardened appearance that resembles that of a stone, which is how it acquired its nickname. This plant often forms dense underwater meadows and plays a vital ecological role by providing habitat and food for various aquatic organisms.

In contrast, Elodea, commonly known as waterweed, is a submerged aquatic plant that has a different morphology and does not have the hard, stone-like structure typical of Chara. Duckweed is a free-floating plant that consists of tiny leaves and does not resemble stonewort at all. Milfoil, often referred to in the context of invasive aquatic plants, has a feathery appearance and is quite distinct from the rigid structure of Chara. Thus, the identification of stonewort as Chara is well-founded in the characteristics that define this specific group of algae.