Step 1: Understanding the Concept:
Volatile oils in plants are stored in specialized secretory structures. These structures are used as diagnostic microscopic features in pharmacognosy.
Step 2: Detailed Explanation:
1. Fennel (Foeniculum vulgare): As an umbelliferous fruit, fennel contains specialized oil canals called vittae. These are schizogenous (formed by the separation of cells) oil cavities located in the mesocarp of the fruit.
2. Microscopy: Under the microscope, these vittae appear as brown, elongated secretory ducts that contain the aromatic volatile oil.
3. Others: Clove has schizolysigenous oil glands. Ginger has secretory cells (oleoresin cells). Asafoetida is an exudate stored in the rhizome's schizogenous ducts but is usually identified in its mass form.
Step 3: Final Answer:
The presence of vittae (schizogenous oil cavities) is the classic microscopic hallmark of umbelliferous fruits like Fennel.