A thick carpet-like growth of Pistia on the lake surface is likely to prevent water evaporation. To discover the extent of this, 7 litres of tap water were put into each of two buckets and Pistia plants were introduced into one to cover the whole surface. Both buckets were kept in a glasshouse. Within 18 days, all the water in the control bucket evaporated, while in the experimental bucket, 1.43 litres of water remained. This observation was confirmed by a further measurement, indicating that a net 20% evaporation was prevented when the plants covered the water surface.