The houses if Iszm. The people of Iszm live in houses created from living tree-like plants and they alone know the secret of cultivating these plants. Aliens from other worlds have been trying for decades to break the monopoly by stealing a female house-seed.
Son of the tree. When Joe Smith arrives on Kyril he finds a society dominated by "druids" who control the worship of a huge tree with a trunk five miles in diameter twelve miles high. Joe is taken for a spy and must flee Kyril, but at the same time he discovers the true nature of the tree.