* The tree itself: The poem describes a tree that grows from the speaker's anger towards a foe. It thrives on his suppressed feelings, becoming poisonous and dangerous.
* Poison as a metaphor: Poison is a potent symbol of harm, destruction, and slow, insidious death. It's not an immediate, violent reaction, but a gradual, insidious process that ultimately leads to ruin.
* The tree's fruit: The poisonous fruit represents the consequences of unaddressed anger. It ultimately leads to the speaker's own downfall, as he is the one who ends up being harmed by the tree.
* The contrast with the "friendly tree": The poem juxtaposes the poisonous tree with a friendly tree, symbolizing the healthy and positive outcome of forgiveness and compassion.
In essence, "A Poison Tree" is a cautionary tale, warning against the destructive power of holding onto anger and resentment. The title itself is a stark reminder of the dangerous consequences that can result from unchecked negativity.