The Tree of Foreknowledge is a legendary plant grown from four pieces of fluff. If placed in a sauna it will grow from nothing to fruit-bearing size within hours. Eating its fruit allows one to see an important event in the near future. It is the only known organism to have survived the destruction of the tropical planet Fuzzbol, probably due to the prevalence of fluff throughout the universe.
The tree was thought to be only a legend until one was grown by Arthur Dent using fluff from his nightgown pocket, his jacket, the seat cushion of Zaphod Beeblebrox's cruiser, and Ford Prefect's satchel. Whether these particular types of fluff are necessary to grow the tree or whether others will work is unknown.