"I myself feel that our country, for whose Constitution I fought in a just war, might as well have been invaded by Martians and body snatchers."
- Kurt Vonnegut, Jr.

The House Trees were at the center of the novel's plot; it centers around attempts to steal the seeds for House Trees and take them off-planet to destroy the Iszic monopoly. This same theme of tree theft breaking a monopoly is found in Clifford Simak's 1944 story Ogre.

Zhde Patasz’s eyes snapped. “But this is crudity! You understand, do you not? A dwelling must be all of a unit—the walls, the drainage, the decor grown in! What use is our vast lore, our two hundred thousand years of effort, otherwise? Any ignoramus can paste up green floss, only an Iszic can grow it!”

“Yes,” said Farr. “I believe you.”

Zhde Patasz continued, passionately waving his viewer. “And if you stole a female house, and if you managed to breed a five-pod house, that is only the beginning. It must be entered, mastered, trained. The webbings must be cut; the nerves of ejaculation must be located and paralyzed. The sphincters must open and close at a touch.

“The art of house-breaking is almost as important as house-breeding. Without correct breaking a house is an unmanageable nuisance—a menace.”

“K. Penche breaks none of the houses you send to Earth.”

“Pahl Penche’s houses are docile, spiritless. They are without interest. They lack beauty, grace.” He paused. “I cannot speak. Your language has no words to tell what an Iszic feels for his house. He grows it, grows into it. His ashes are given it when he dies. He drinks its ichor; it breathes his breath. It protects him; it takes on the color of his thoughts. A spirited house will repel a stranger. An injured house will kill. And a Mad House—that is where we take our criminals.”

Farr listened in fascination. “That’s all very well—for an Iszic. An Earther isn’t so particular—at least, a low income Earther. Or as you would put it, a low-caste Earther. He just wants a house to live in.”

A sly story that made use of the idea that a tree could produce everything you might need The Pork Chop Tree (1965), a very clever story by James Schmitz that describes a tree that gives you just what you want - and makes you want to stay around.

(Pork Chop Tree by James Schmitz)

"...almost every part of the tree has a high nutritional value for the human organism..."

"And if you're thirsty," Trigger said, "you can drink sap from the creepers. Then, every few days, there's something new coming out, so you could get your meals from one tree all your life and never get tired of the diet!"

Mantelish grinned, said, "There are also other attractions."

"Yes. It's very decorative. It makes temporary hammocks out of its leaves for you, and the big trees even have cubbyholes in the trunk where you can sit when it's raining too hard outside..."

Thanks to writer and editor Dominic Brown for contributing the original quote for this item. (I've included another excellent quotation in the comments.

The Iszic see themselves as part of a long line of house-breeders:

We are Iszic and house-breeding is in our blood, a basic instinct. The line of planters began two hundred thousand years ago when Diun, the primordial anthrophib, crawled out of the ocean. With salt-water still draining from his gills he took refuge in a pod. He is my ancestor. We have gained mastery over houses; we shall not dissipate this accumulated lore, or permit ourselves to be plundered.”