Aeschynomene elaphroxylon

Aeschynomene elaphroxylon, also known as an ambatch, pith-tree, balsa wood tree, or umburu, is a common large shrub to small tree in the genus Aeschynomene in the family Fabaceae, growing in swamps, lakes and rivers in Tropical Africa. This is an unusual leguminous tree in that it grows in water as a freshwater mangrove, with an extremely lightweight wood acting as a float and a specialised root system sprouting from the trunk which forms a tangled web hanging through the water and sprawling through the mud. It has adventitious roots and roots which are differentiated into special structures adapted to the swamp environment. It can even grow as floating islands of drifting forests. The name 'ambatch' derives from the name of the plant in the Arabic dialect spoken in Nigeria known as Shuwa by the British. It is called Nile pith tree in Sri Lanka, ambach there and in Malaysia, or ambatsch in Malaysia. The extremely light, spongy, but rot-resistant wood is traditionally used to make float......read more on Wikipedia.