The inhabitants of a planet called Iszm, a species known as the Iszic, have evolved the native giant trees into living homes, with all needs and various luxuries supplied by the trees' own natural growth. The Iszic maintain a jealously-guarded monopoly, exporting only enough trees to keep prices high and make a great profit. The protagonist, Ailie Farr, is a human botanist who goes to Iszm (like many others before him, of many species) to steal a female tree, which might allow the propagation of the species off world and break the monopoly.
