The experimental work was carried out on a lake-fringing papyrus swamp on the N-W shore of Lake Naivasha, Kenya (0°45'S, 36°20'E) at an altitude of 1890 m in the Eastern Rift Valley of Africa. The flora and ecology of the lake edge has been described by Gaudet (1977). The floating papyrus mat at the study site extended 500 m into the lake but it was grounded at the lake margin. During the period of measurements the water level of the lake fell by an unknown amount. The effect of this was to expose an increasing amount of the rhizome mat and associated detritus to the atmosphere, although the root systems appeared to remain continuously waterlogged.