In the stagnant water, we can find remains from the vessel elements from the lettuce. Vessel elements are cells bound together to transport water with dissolved minerals, from the root to the leaves. They are like are blood vessels, and they are a part of the xylem (a type of transport tissue in the plant). These cells are strengthened by lignin, the principal component of wood. The lignin is arranged in spiral which makes the vessel stronger.
Vessel elements examined with an optical microscope
What is left in the stagnant water is the hard lignin spiral, as the soft components of the cell have been eaten by microorganisms.