A man buys a tree in a pot and carries it home to fill a dim corner of his flat, and almost at once the purchase begins to feel wrong. The speaker looks at the roots and sees damaged bodies: a club foot, a wrist with no hand, a fist that doesn't know it is clenched. Confined to the pot, they can only keep curling back on themselves, "open, shut," like something running on habit. The comparison to "a dead brain in a body automatically fed" is the bleakest moment, since the tree is alive but has lost any purpose for its life.
The last stanza widens the picture. The tree's branches have no birds, and the birds outside have no branches, so each is missing the other. The cat on the windowsill seems to share the longing. The speaker's tone is uneasy and quietly sad, and the final lines hint that this isn't only about a plant. It may be about all of us, living fed and sheltered but cut off from whatever once gave life its point.