Shellflower

Pistia stratiotes is a free-floating perennial aquatic herb of the Araceae family, forming rosettes of soft, light-green leaves that resemble a small lettuce, 5 to 15 centimetres across, supported on the water by spongy, air-filled tissue. The leaves are obovate to spathulate, 2 to 15 centimetres long, broadly rounded and notched at the apex, with deeply impressed parallel veins and a dense covering of fine velvety hairs that repel water and trap a thin air layer. Long, feathery, dark roots hang freely below the rosette, anchoring the plant and absorbing nutrients directly from the water. Reproduction is mainly vegetative, by short stolons that produce daughter rosettes; flowering is inconspicuous, with a small pale-green spathe up to 1.5 centimetres long enclosing a reduced spadix near the centre of the rosette. The species is pantropical in distribution, colonising still and slow-moving fresh waters such as ponds, ditches, marshes and slow rivers, often forming dense mats that affect navigation.