Step 1: Source of Cocaine.
Cocaine is an alkaloid drug obtained from the leaves of the Erythroxylum coca plant, commonly known as the coca plant. The leaves are harvested, and cocaine is extracted from them.
Step 2: Conclusion.
Therefore, cocaine is derived from the Erythroxylum coca plant.
Final Answer:
\[
\boxed{\text{Cocaine is obtained from the Erythroxylum coca plant.}}
\]