This plant grows rooted on the bottom of ponds that have water that is high in dissolved lime content. This lime is really limestone dissolved in the water. We can also say that the pond water is very 'hard' water. Stonewort has fine, straight leaves that fan out in a circle around the green stem at ring-like joints. Further out, smaller leaves also branch off the leaves which are attacked to the stems.
Stonewort secretes a hard lime coating and becomes brittle.
Some plants are covered with a brittle, limy crust. When you roll the plant in your hands it feels gritty. It seems to be a dead plant. That is why one name for this plant is 'stonewort'. It also has a garlic-like smell or odour, especially when crushed.