One of the books that first introduced me to SF, Aldiss' tale of a distant future Earth where humanity - a much changed, reduced species - lives a precarious life in the high branches of the continent-spanning banyan tree, until a member of one of the tiny tribes in which they live is infested with morel, an intelligent parasitic fungus. Gren and his tribe find their way to a new home, battling dangers, mostly of a mobile, predatory vegetable nature, which has become the dominant form under the bloated red sun. It suffers a little from the aging of the language used (it was first published in 1961) and a somewhat hallucinatory feeling to some of the descriptions and action - again, possibly to do with the era in which it was written - but is a classic of SF.