Step 1: Understanding the Question:
The question requires knowledge of natural occurrences of medicinal compounds. Step 3: Detailed Explanation:
Salicylic acid is a precursor to aspirin.
It was traditionally extracted from the bark of the Willow tree (genus Salix).
The active ingredient salicin is metabolized into salicylic acid in the body. Step 4: Final Answer:
The natural source of salicylic acid is the Willow plant.