The tree came first. He chose the room because the olive was already there, ancient, rooted, and instead of clearing it he built the walls around it and shaped the trunk into a bedpost. The roots stayed in the earth beneath the floor. [SPOILER: It became the secret only two people in the house knew, and twenty years later Penelope used it as a knife. Move the bed for the stranger, she said. He went rigid. The tree is still alive at the end of the book — under the floor, under the bed, under whatever came back of their marriage. It is the literal living thing the household is built on, and the only object in the palace the suitors never managed to touch.]
The Living Olive Tree
A living tree, rooted under the floor of the bedchamber. He built the room around it. He built the bed around it. Cut the tree, you cut the marriage.