Step 1: Understanding the Concept:
Agarwood oil (Oud) is one of the most expensive natural essential oils in the world, produced in response to a specific biological defense mechanism in the tree.
Step 2: Detailed Explanation:
1. Agarwood oil is obtained from the infected wood of Aquilaria agallocha (also known as Aquilaria malaccensis).
2. The healthy wood of this tree is light, soft, and odorless.
3. However, when the wood is infected by a specific mold (Phialophora parasitica), the tree produces a dark, aromatic, resinous defense compound in the heartwood.
4. Steam distillation of this dark, infected wood yields the highly fragrant and expensive agar oil (Oud).
Step 3: Final Answer:
Agar oil is obtained from the wood of Aquilaria agallocha.